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Review

A Review of Additively Manufactured Iron-Based Shape Memory Alloys

by
Qian Sun
1,
Xiaojun Tan
1,
Mingjun Ding
1,
Bo Cao
1,* and
Takeshi Iwamoto
2,*
1
School of Civil Aviation, Northwestern Polytechnical University, Xi’an 710072, China
2
Academy of Science and Technology, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
*
Authors to whom correspondence should be addressed.
Crystals 2024, 14(9), 773; https://doi.org/10.3390/cryst14090773
Submission received: 8 August 2024 / Revised: 20 August 2024 / Accepted: 28 August 2024 / Published: 29 August 2024
(This article belongs to the Special Issue Shape Memory Alloys: Recent Advances and Future Perspectives)
Browse Figures
Versions Notes

Abstract

:
Iron-based shape memory alloys (Fe-SMAs), traditionally manufactured, are favored in engineering applications owing to their cost-effectiveness and ease of fabrication. However, the conventional manufacturing process of Fe-SMAs is time-consuming and raw-material-wasting. In contrast, additive manufacturing (AM) technology offers a streamlined approach to the integral molding of materials, significantly reducing raw material usage and fabrication time. Despite its potential, research on AMed Fe-SMAs remains in its early stages. This review provides updated information on current AM technologies utilized for Fe-SMAs and their applications. It provides an in-depth discussion on how printing parameters, defects, and post-printing microstructure control affect the mechanical properties and shape memory effect (SME) of AMed Fe-SMAs. Furthermore, this review identifies existing challenges in the AMed Fe-SMA approach and proposes future research directions, highlighting potential areas for development. The insights presented aim to guide improvements in the material properties of AMed Fe-SMAs by optimizing printing parameters and enhancing the SME through microstructure adjustment.

1. Introduction

Shape memory alloys (SMAs) are a class of materials characterized by their unique ability to “remember” and recover their original shape after deformation [1,2,3,4]. The defining characteristics of SMAs include the shape memory effect (SME) [4,5,6,7,8,9,10] and superelastic behavior [9,11,12,13,14,15]. Extensive research has detailed these phenomena, revealing the enhanced SME from micro- to macroscopic [10,11], quasi-static to impact loading [1,2,7], and low to high temperature [13,14], and their mechanisms, which highlight the significant potential of SMAs in various applications. The SME enables these alloys to revert to a predetermined shape when subjected to certain stimuli, such as heat or stress removal, after being deformed into a temporary shape. SMAs are extensively utilized across a range of industries due to their unique properties and functional versatility as shown in Figure 1. The most common types of SMAs include Ni-Ti SMAs [14,16,17,18,19], Cu-based SMAs [5,13,20], and Fe-based SMAs (Fe-SMAs) [1,2,12,21,22,23,24]. Ni-Ti SMAs are renowned for their superior SME and superelasticity, coupled with excellent thermodynamic stability and corrosion resistance, making them widely applicable in high-demand fields such as medicine, aerospace, and robotics. Cu-based SMAs, conversely, are favored for their lower cost and faster response times, primarily finding use in refrigeration and various engineering applications. In contrast, Fe-SMAs, while exhibiting somewhat less pronounced SME and superelasticity, offer advantages in terms of cost, machinability, and high-temperature applications, positioning them as promising materials, particularly in the fields of civil engineering and structural repair. In Figure 1, the inner section highlights the dominant applications of Ni-Ti SMAs, while the outer section showcases the current applications of Fe-SMAs. For instance, a smart antenna made of Ni-Ti SMA was used on the Apollo 11 lunar module to effectively minimize the space occupied by the antenna, which was restored to its original shape through heating upon arrival at its designated location. The FlexShapeGripper [25] developed by Festo utilizes Ni-Ti SMA wires as the driving element, where electrical heating controls the shape changes for flexible gripping. Additionally, Ni-Ti SMAs can mimic biological muscle movements in bionic robots. SoFi (Soft Robotic Fish) [26], a bionic fish robot, uses Ni-Ti SMA wires to replicate the muscle movements of fish, allowing it to swim flexibly through the water. Conversely, Fe-SMAs are more predominantly used for structural strengthening [27,28,29,30,31,32,33,34], construction self-healing [35,36,37], and pipe connection [12,21,22,38], among other applications. For example, fatigue in large steel girders can be effectively solved by using Fe-SMA strips and adhesive bonding [30].
Compared to traditional Ni-Ti SMAs, the lower cost of Fe-SMAs [2,3,46,47] makes them economically appealing for large-scale production, especially in additive manufacturing (AM) applications [48,49]. Additively manufactured (AMed) Fe-SMAs present significant opportunities due to their cost-effectiveness. AM technology, which constructs materials layer by layer, contrasts sharply with traditional material processing methods, enabling rapid and unrestricted fabrication of three-dimensional structures. This technology offers novel opportunities to transcend the limitations of conventional manufacturing, particularly in terms of complexity, thereby fully utilizing the exceptional functionalities of these SMAs. A comparison of the detailed steps involved in conventional and additive manufacturing is given in Figure 2. Traditionally, Fe-SMAs are produced through melting and casting [50], which typically necessitate subsequent machining processes, such as hot forging, cold rolling, and cutting, to achieve the final shape. This limits the application of Fe-SMAs to more complex structures, whereas only simple geometries, such as strips or bars are preferred to be manufactured on a large-scale to reduce costs. In contrast, the cost of powders for AMed Fe-SMAs is significantly lower than those of AMed Ni-Ti SMAs, costing only about one-fifteenth to one-twentieth as much according to material suppliers (POWDMAX, Ningbo Zhongyuan Advanced Materials Technologies Co., Ltd. http://powdmax.com). Moreover, the machinability of Fe-SMAs surpasses that of Ni-Ti SMAs in both conventional machining and AM processes. The economics of AMed Fe-SMAs are more favorable for small batches and complex shapes compared to conventional machining methods. Figure 3 presents examples of AMed Fe-SMAs with small complex structures. A delicate flower-like sample of an Fe-SMA (Figure 3a) was fabricated using AM technology and can open (Figure 3c,d) or close (Figure 3e,f) its leaves when heated [51]. A spring-like Fe-SMA sample (Figure 3b) was also prepared, which demonstrates shape morphing behavior upon heating, as well as spider-like sample (Figure 3g) exhibiting the artificial muscle effect [51].
Although the conventional processing of Fe-SMAs is well established, AMed Fe-SMAs have begun to evolve in recent years [51,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75] through composition optimization, microstructure control, and AM-technique choice and innovative applications have been developed, which will be discussed in detail later. In addition, current studies are primarily focusing on investigating the mechanical properties and underlying mechanisms of AMed Fe-SMAs. A limited number of studies have examined the performance of AMed Fe-SMAs in complex, small-scale structures [51]. Some research has explored their seismic damping effects in construction, particularly in bridges [76]. However, the variety of Fe-SMAs available for AM is currently too limited, and there are few examples of their application across different fields, which contradicts the initial purpose of AM technology. It is crucial to conduct more detailed and systematic studies on the various parameters affecting the mechanical properties of AMed Fe-SMAs. This would enable their application in more practical, large-scale uses, rather than being confined to the creation of small models and simple deformations. Although different intricate structural models, such as those shown in Figure 3, can be fabricated, it is imperative to address the pressing issue of their actual applicability.
In this review, a comprehensive overview of current AM technologies, with a detailed focus on the characteristics and applications of Fe-SMAs, is provided, as illustrated in Figure 4. First, the specific AM technologies employed for Fe-SMA will be elucidated. Next, the mechanical properties, shape memory behavior, and fatigue characteristics of AMed Fe-SMAs without any internal structure such as a designedly porous or lattice structure will be discussed in relation to various printing parameters. This includes an examination of the control of defects and microstructure post-printing. Finally, the existing challenges associated with AMed Fe-SMAs will be addressed and future research directions along with potential areas for development will be highlighted. The objective of this review is to assess the current status of AMed Fe-SMAs and provide possible directions for their future development, with an emphasis on enhancing material properties and shape memory behavior.

2. Overview of Additive Manufacturing Technology for Fe-SMAs

Studies focusing on the AM of Fe-SMAs are notably limited despite the extensive application of AM techniques in metallic materials, particularly Ni-Ti-based alloys [81,82,83,84,85,86,87,88] as shown in Figure 5. Among the current studies on AMed Fe-SMAs, laser powder bed fusion (LPBF) [51,61,64,65,66,68,69,70,71,74,89,90,91,92] and direct energy deposition (DED) [75,93] are preferred, rather than selective laser melting (SLM) [94], due to severe surface cracking and low manufacturing efficiency associated with SLM when applied to AMed Fe-SMAs. The following subsections provide a brief overview of the LPBF and DED technologies, both of which have been used for fabricating Fe-SMAs. These methods are major branches of metal AM technology. While they share some similarities, there are significant differences in material supply methods, energy sources, etc. For instance, the energy source for DED is typically a laser or electron beam, whereas wire arc AM (WAAM) primarily utilizes an electric arc as its energy source.

2.1. Laser Powder Bed Fusion (LPBF) Process

LPBF, widely known as direct metal laser sintering (DMLS) or metal LPBF (M-LPBF), begins with a digitized model segmented into layers. Figure 6 illustrates a schematic diagram of the LPBF process, alongside LPBF-fabricated Fe-SMA samples showcasing their microstructure and mechanical properties. The distinct advantages of LPBF technology make it a promising fabrication method for addressing challenging issues in SMA applications [95]. Notably, LPBF enables the production of near-net-shape complex geometries without requiring additional machining, significantly enhancing the geometric complexity of the AMed samples [96] and reducing costs. Additionally, LPBF provides superior surface finishes that limit the release of specific elements [97] and offers a high intrinsic cooling rate that aids in grain control [98]. Patriarca et al. [74] prepared an LPBF-fabricated lattice specimen (Figure 6b), which demonstrated a promising pseudoelasticity (PE) (Figure 6c,d), and exhibited approximately 2% strain recovery. Ferretto et al. [71] manufactured various shapes (Figure 3 and Figure 6e) of Fe-SMA samples by LPBF, which also showed good performance, but rough surfaces could still be observed. The potential for further performance improvements through additional steps, such as polishing to reduce the surface roughness of the AMed Fe-SMAs, is therefore a topic worth exploring. Other instances of LPBF-fabricated Fe-SMAs are also summarized in Table 1, highlighting the dominance and development of LPBF methods over the past three years.

2.2. Direct Energy Deposition (DED) Process

Although most research on AMed Fe-SMAs is based on LPBF, the primary applications for Fe-SMAs are currently in the construction sector, including the manufacture of pre-stressed concrete [102] and seismic isolation systems [103]. These applications typically involve larger parts with relatively simple shapes, making the DED process particularly suitable. Unlike the LPBF process, the DED process can create large components more efficiently and cost-effectively, as it does not require complex post-processing steps. Furthermore, DED is close to 10 times faster than LPBF in terms of printing speed. A schematic diagram of the DED process and an AMed Fe-SMA sample produced by DED are shown in Figure 7. In this process, metal material is deposited and simultaneously fused by intense energy delivery.
Felice et al. [75] investigated microstructure evolution and the mechanical/functional response of WAAM-fabricated Fe-17Mn-5Si-10Cr-4Ni-1(V,C), which presented negligible porosity and high deposition efficiency (Figure 7c,d), thereby leading to excellent mechanical performance including yield strength (~472 MPa), fracture stress (~821 MPa), and elongation (~26%). Additionally, cyclic testing (Figure 7e) demonstrated repeatable mechanical response and high absorbed energy, maximum stress, and low irrecoverable strain [75]. Nonetheless, key drawbacks of DED include the development of residual stresses [93], microstructural changes due to rapid solidification [93], and rough surfaces. These factors can significantly change the properties of Fe-SMAs, affecting SME, superelasticity, mechanical strength, and phase transformation behavior.

3. Parameters of AM Process for Fe-SMAs

3.1. Chemical Composition

The basic chemical composition of Fe-SMAs typically includes Fe as the primary element, with additions of elements such as Mn, Ni, Si, Cr, and others to tailor the SME of the alloy. Figure 8 shows the major elements added to the Fe-Mn-Si SMA system with their advantages and disadvantages. The representative Fe-Mn-Si SMA systems developed in recent years with detailed information are summarized in Table 2. The addition of specific elements can enhance the SME of AMed Fe-SMAs; for instance, the SME can be modified by a small change in Mn content [61]. The shape memory characteristics are larger for an alloy with 23.6 wt% Mn than for an alloy with 28.5 wt% Mn [61], while the effect of other elements such as Ni, Si, and Cr on the SME of AMed Fe-SMAs is still unknown. The aging-induced precipitates (V,C) and related stacking faults account for a significant improvement in SME of Fe-SMA during the AM process [92]. However, certain elements may introduce brittleness or reduce mechanical strength; for example, excessive Si can increase hardness but decrease toughness [75]. As can be seen in Table 1, the most popular chemical composition for AMed Fe-SMA is Fe-17Mn-5Si-10Cr-4Ni, which may be due to the economy of its processing. Although there may be some compositions that are not suitable for printing (yet to be reported), AMed Fe-SMAs with different compositions are worth exploring.

3.2. Fe-SMA Powder

For the LPBF technique, the preparation of suitable Fe-SMA powders is necessary. One common method for preparing Fe-SMA powders for AM processing is gas atomization [51,100,107]. This technique involves rapidly solidifying molten metal by spraying it into a gas environment, typically using an inert gas such as argon or nitrogen. The molten metal is forced through a nozzle where it breaks up into fine droplets due to the high velocity of the gas stream. As these droplets cool and solidify, they form spherical or nearly spherical particles with a controlled size distribution.
Gas atomization yields powders with uniform particle shapes and sizes, making them suitable for the SLM or EBM [108] processes. To be specific, uniform particle size distribution [109,110] ensures consistent layer deposition, promoting uniform melting and solidification essential for dimensional accuracy and mechanical integrity in an alloy. A spherical or near-spherical particle shape [111,112] enhances flowability and packing density, facilitating smooth powder distribution and uniform layer deposition during printing. Good powder flowability [111,113] is critical for stable and reliable printing, as it prevents powder agglomeration and ensures consistent layer thickness. Figure 9 shows SEM images of various powders for both AMed Ni-Ti SMAs and Fe-SMAs. The powder particles are all spherical, or nearly spherical in shape, with a medium particle size of around ~30 μm mostly for AMed Fe-SMAs (Figure 9i–k).
It is currently quite hard to find relevant studies on the effect of powder including powder-mix process, powder size effect, etc., on the properties of AMed Fe-SMAs. However, for AMed Ni-Ti SMAs, a number of studies [82,110,114] have shown that a smaller particle size (Figure 9a–f) and Gaussian size distribution can improve the surface quality and material properties of the formed components, as well as SME. It is questionable whether smaller and more homogeneous powder particles can improve the mechanical properties and SME for AMed Fe-SMAs. Hence, a systematic and precise experimental evaluation is required.
Crystals 14 00773 g009
Figure 9. SEM images of (a–f) Ni-Ti SMA powder and (g) schematic to-scale representation of melt pool size formed under SLM and DED processes [82]; (h) Ni-Ti SMA powder and its particle size distribution under LPBF process [115]; (i) Fe, (j) Mn, and (k) Si powders for Fe-30Mn-6Si [90]. The yellow dash bracket lines in (a–c) indicate powder agglomerations. The red circle in (f) illustrates the length scale for SLM melt pool size. Reprinted from [82,90,115] by permission of Elsevier and OAE Publishing Inc.
Figure 9. SEM images of (a–f) Ni-Ti SMA powder and (g) schematic to-scale representation of melt pool size formed under SLM and DED processes [82]; (h) Ni-Ti SMA powder and its particle size distribution under LPBF process [115]; (i) Fe, (j) Mn, and (k) Si powders for Fe-30Mn-6Si [90]. The yellow dash bracket lines in (a–c) indicate powder agglomerations. The red circle in (f) illustrates the length scale for SLM melt pool size. Reprinted from [82,90,115] by permission of Elsevier and OAE Publishing Inc.
Crystals 14 00773 g009

3.3. Printing Processing

There are many process parameters in the AM process that affect the quality and material properties of the final print, such as volumetric energy density (VED, E), scanning speed (v), layer thickness (t), laser power (LP, P), hatch distance (h), etc., as shown in Figure 10.
The VED, E, can be expressed as a function of the previously mentioned parameters in accordance with the following Equation (1). The details of parameter settings for AMed Fe-SMAs are also summarized in Table 1.
E = P v · h · t
Yang et al. [65] found that E significantly affects the microstructure of Fe-21Mn-5Si-9Cr-5Ni fabricated by LPBF. An increase in E promotes the formation of primary γ-austenite and the solid-phase transformation from primary δ-ferrite to γ-austenite, resulting in the formation of an almost complete γ-austenite microstructure. The residual δ-ferrite decreases with increasing E, resulting in finer grains (~6.19 µm) and reduced thermal cracking susceptibility, thus enhancing mechanical properties. The phase composition shifts towards a single-phase austenitic microstructure with higher E values, which affects the strength and ductility of the material. Kim et al. [70] reported that LP variations/E values lead to different microstructures. For example, at 420 W with an E value of 105 J/mm3, the observed microstructure is almost exclusively face-centered cubic (fcc)-γ, whereas at 380 W with an E value of 95 J/mm3, bcc-δ grains are predominantly present. Additional laser profile scanning during LPBF also affects the microstructure [70]. Due to the bcc-δ-to-fcc-γ phase transition induced by the scanning effect, the contour scanning region shows a smaller bcc-δ phase fraction [70]. Dela Cruz et al. [90] determined that different laser powers, scanning speeds, and rescanning strategies in LPBF affect the microstructure evolution. Adjustments in laser power and scanning speed affect grain size and texture, and rescanning can lead to microstructural inhomogeneities. Furthermore, Niendore et al. [94] also found that other parameters such as the thermal gradient and solidification rate determine the transition region from columnar to equiaxed, affecting the grain structure and orientation. Meanwhile, other authors demonstrated that martensitic transformation and SME of Fe-SMAs can be affected by grain orientation [116]. Therefore, a careful selection of thermal gradient and solidification rate during the AM process could contribute to producing fabricated Fe-SMAs with excellent SME. In addition, high thermal gradients and cooling rates during SLM produce specific microstructures, such as the α-phase body-centered cubic (bcc) structure in the as-built state [94]. Unfortunately, currently, it is quite hard to find studies on the effect of layer thickness on the properties of AMed Fe-SMAs. A more comprehensive investigation into the effects of printing parameters on the properties of AMed Fe-SMAs is urgently needed.

4. Microstructure of AMed Fe-SMAs

4.1. Defects during the AM Process

The aforementioned AM techniques such as LPBF and SLM can exhibit various defects during the printing process that affect the quality and performance of Fe-SMAs. Common defects as shown in Figure 11 include porosity [117], cracking [89], surface roughness (Figure 3a and Figure 7b), lack of fusion [118], and others. These defects are caused by a variety of factors, for example, evaporation of Mn content [64,68], too low or high energy input [69], short phase transformation duration [70], low E [71], crack formation and grain structure [89], scanning speed and rescanning strategy [90], repeated heating can cooling during LPBF [92], rapid cooling rate during LDED [93], residual stress and high brittleness [94], rapid passage of the α → γ phase transformation region [72], and others.
Felice et al. [75] obtained low porosity (~1.044%) in an as-deposited Fe-17Mn-5Si-10Cr-4Ni-1(V,C) sample, which could help in reducing the occurrence of such imperfections in components manufactured via the WAAM process. Cruz et al. [90] found heat treatment increases the hardness of Fe-30Mn-6Si and optimizes the microstructure. Meanwhile, post-treatment steps reduce porosity and improve surface quality and density [90]. Niendore et al. [94] employed heat treatment steps such as dissolution at 1473 K to build up the coarse-grained microstructure that is essential for pseudoelastic behavior. They also found that annealing and aging treatments reduce internal stresses, optimize phase transition behavior, and increase material strength and hardness. Viebranz et al. [72] created a narrow heat-affected zone (HAZ) by causing rapid passage of the α-γ phase transition through direct conduction heat dissipation during the welding of the first layer. As the height of the wall structure increases, heat dissipation decreases, leading to heat buildup and a wider HAZ. Each new weld layer involves epitaxial growth and grain-selective solidification, which favors grain orientation relative to heat flux. These detailed heat treatment steps [72] illustrate the complex interplay between heat dissipation, solidification, and grain selection in Tungsten insert gas wire and arc AM, affecting the microstructure and properties of Fe-34Mn-15Al-7.5Ni.

4.2. Microstructural Control of AMed Fe-SMAs

Dela Cruz et al. [90] found the microstructure was a response to the processing conditions. For example, the microstructure of the parts produced by a high LED had a columnar and strong crystallographic texture, while in a low LED, the parts were almost equiaxed and had a weak texture. Niendore et al. [94] controlled parameters such as E (65 J/mm3), LP (183 W), and cooling rate during the SLM process, which led to a bcc-α microstructure of Fe-SMA in the as-built state, whereas the aging treatment at 473 K leads to the evolution of coherent nanoscale β-phase precipitates. At the same time, this microstructural evolution leads to changes in grain size. Initially, the fine-grained microstructure transforms into a coarse-grained structure upon dissolution, exhibiting abnormal grain growth (AGG). The grain size increases significantly and after several heat treatment cycles, the grains grow to sizes in the centimeter range. Viebranz et al. [72] ensured that grain growth aligned parallel to the building direction by bidirectional welding. Additionally, to ensure a consistent temperature–time trajectory during cooling, the structure was fully cooled to room temperature (298 K) after each welding cycle. This method enabled the development of an anisotropic microstructure characterized by a grain length of up to 8 mm along the major axis. Figure 12 shows some results of electron backscatter diffraction (EBSD) maps with respect to (a–i) scanning speed, and (j–m) E, respectively. Finer austenite grains can be obtained at low and intermediate scanning speeds (Figure 12b,c,e,f), while grains are coarser at high scanning speeds [64]. A further investigation [65] based on a transmission electron microscope (TEM) equipped with scanning TEM (STEM) indicated that an increase in E facilitates the formation of the primary γ austenite as well as the solid phase transformation from primary δ-ferrite to γ-austenite in an LPBF-fabricated Fe-Mn-Si-CrNi SMA (Figure 13). An E higher than 194 J/mm3 is needed to achieve a nearly complete γ-austenitic microstructure. From the above, adjusting the printing parameters can easily change the microstructure of AMed Fe-SMAs, and proper printing parameters are the key to obtaining a fine and uniformly distributed microstructure.

5. Material Properties of AMed Fe-SMAs without Any Internal Structures

5.1. Mechanical Properties

Table 3 summarizes the mechanical properties of AMed Fe-SMAs. Figure 14 shows a comparison between stress–strain curves of AMed Fe-SMAs and conventionally manufactured Fe-SMAs. For AMed Fe-SMAs with a composition of Fe-17Mn-5Si-10Cr-4Ni-1(V, C) [75], both stress and strain are lower than those of a conventionally manufactured material [119]. However, for AMed Fe-17Mn-5Si-10Cr-4Ni [51], both stress and strain are higher compared to those of a conventionally manufactured material [120]. A significant difference between these results can be seen clearly. It is challenging to assert that AMed Fe-SMAs exhibit superior mechanical properties, as these properties are highly dependent on the specific AM techniques employed, including the selection of printing parameters. At the same time, it is possible to achieve improved mechanical properties by optimizing the printing parameters. This is supported by the observation that most AMed Fe-SMAs exhibit higher stresses and larger strains, as illustrated in Figure 14. Normally, the relationship between Young’s modulus and tensile strength is better for understanding the mechanical properties of materials. Unfortunately, the present studies on AMed Fe-SMAs rarely present any data on these two properties. Therefore, the relationships between Young’s modulus and yield stress of AMed Fe-SMAs, as compared with conventionally manufactured Fe-SMAs are illustrated in Figure 15. The details of the mechanical properties of conventionally manufactured Fe-SMAs can be found in Table A1 in the Appendix A. Although limited research has been conducted on AMed Fe-SMAs, existing data indicate that these materials exhibit slightly higher stiffness and strength compared to conventionally manufactured Fe-SMAs. It suggests that AMed Fe-SMAs may be more suitable for applications demanding high stiffness and load-carrying capacity, such as aerospace and automotive structural components. Additionally, as indicated in Table 3, the ultimate tensile strength of AMed Fe-SMAs is generally lower than that of a conventionally manufactured material (Table A1 in the Appendix A), except in the case of Fe-21Mn-5Si-9Cr-5Ni [65]. This is because a higher E value facilitates the fabrication of Fe-SMAs with excellent mechanical properties, including high yield strength (>480 MPa), and ultra-high ultimate tensile strength (>1 GPa) [65]. Due to various strengthening mechanisms [65] such as grain refinement, defect density, solute segregation, and phase particle precipitation, the LPBF-fabricated Fe-21Mn-5Si-9Cr-5Ni alloy exhibited excellent overall mechanical properties.
Moreover, AMed Fe-SMAs typically exhibit higher hardness (Table 3). Notably, the studies that show elevated maximum strain and hardness values utilized higher energy densities, as indicated in Table 1. However, it is quite difficult to determine the relationship between E values, maximum strain, and hardness from the limited literature because the effect on material properties is not solely due to individual processing parameters and a linear regression model might not be the ideal choice. The value of LP, however, has a positive effect on the hardness [69].
Table 3. Mechanical properties of AMed Fe-SMAs.
Table 3. Mechanical properties of AMed Fe-SMAs.
Chemical CompositionYoung’s Modulus (GPa)Maximum Strain (%)Ultimate Tensile Strength (MPa)Density
(g/cm3)		Yield Strength
(MPa)		Hardness (Hv)Processing MethodRef.
Fe-17Mn-5Si-10Cr-4Ni-54840 230-LPBF[51]
Fe-21Mn-5Si-9Cr-5Ni-0.6658.67.491--LPBF[65]
2.5829.9609.5
10.6946.8553.4
18.81014.1519.6
28.61068.6494.9
29.91036.5480.9
Fe-17Mn-5Si-10Cr-4Ni-(V, C)141---664-LPBF[66]
182457
198322
Fe-17Mn-5Si-10Cr-4Ni-(V,C)----750460LPBF[68]
Fe-34Mn-15Al-7.5Ni-----450LPBF[69]
Fe-17Mn-5Si-10Cr-4Ni-47.869407.491320.38-LPBF[71]
Fe-34Mn-14Al-7.5 Ni-----627LPBF[89]
Fe-30Mn-6Si---7.408-292LPBF[90]
Fe-17Mn-5Si-10Cr-4Ni-1(V, C)200--7.48400-LPBF[92]
Fe-20Mn-6Si-9Cr-5Ni-43933---LDED[93]
Fe-17Mn-5Si-10Cr-4Ni-1(V, C)-268217.4472-DED[75]
Fe-34Mn-14Al-7.5Ni-----420SLM[94]
Fe-17Mn-5Si-10Cr-4Ni-1(V, C)166---408-LPBF[107]
Fe-17Mn-5Si-10Cr-4Ni-43.3928.5-297-LPBF[101]
Crystals 14 00773 g014
Figure 14. (a) Comparison between stress–strain curves of AMed Fe-SMAs and conventionally manufactured Fe-SMAs; (b) AMed Fe-SMA sample [75]; (c) AMed with different directions, where the vertical result is used and shown in figure (a) [51]; (d) photos of Fe-SMA fixed on the testing machine with heating system [119]. Reprinted from [51,75,119,120] by permission of Elsevier.
Figure 14. (a) Comparison between stress–strain curves of AMed Fe-SMAs and conventionally manufactured Fe-SMAs; (b) AMed Fe-SMA sample [75]; (c) AMed with different directions, where the vertical result is used and shown in figure (a) [51]; (d) photos of Fe-SMA fixed on the testing machine with heating system [119]. Reprinted from [51,75,119,120] by permission of Elsevier.
Crystals 14 00773 g014
Crystals 14 00773 g015
Figure 15. Relationships between Young’s modulus and yield stress of AMed Fe-SMAs [66,92,107] compared with conventionally manufactured Fe-SMAs. The circles with different colors indicate the possible ranges of the data. The details of the mechanical properties of the conventionally manufactured Fe-SMAs can be found in Table A1 in the Appendix A.
Figure 15. Relationships between Young’s modulus and yield stress of AMed Fe-SMAs [66,92,107] compared with conventionally manufactured Fe-SMAs. The circles with different colors indicate the possible ranges of the data. The details of the mechanical properties of the conventionally manufactured Fe-SMAs can be found in Table A1 in the Appendix A.
Crystals 14 00773 g015
Fatigue resistance is also one of the important properties of AMed Fe-SMAs. However, there is a scarcity of studies investigating the fatigue properties of AMed Fe-SMAs. Nonetheless, as mentioned above, Viebranz et al. [72] obtained a grain growth parallel to the building direction by bidirectional welding. However, this enhanced grain growth reduces the grain boundary surfaces, lowering susceptibility to functional fatigue during loading tests. The competitive growth of individual grains therefore leads to the formation of high-angle grain boundaries when grains with high misorientations collide, which also reduces fatigue resistance. Other studies on the fatigue resistance of AMed Ni-Ti SMAs [121] and austenitic stainless steel [122] have demonstrated that microstructural defects significantly reduce the fatigue life of the AMed samples. Therefore, it can be inferred that microstructural defeats may also affect AMed Fe-SMAs. The location, shape (irregular or spherical shape), and size of the defects were the major factors affecting the fatigue resistance [121] of AMed samples. Unfortunately, there is no specific data available for AMed Fe-SMAs. As a next step, the fatigue properties of AMed Fe-SMAs, as a crucial characteristic, should be investigated. The influence of various factors, including printing parameters and microstructure, on these fatigue properties has to be thoroughly elucidated.

5.2. Shape Memory Behavior

For AMed Fe-SMAs, shape memory behavior such as recovery strain or recovery stress is a necessary characteristic to examine. Figure 16 shows schematic diagrams of (a) recovery strain behavior, and (b) recovery stress behavior of Fe-SMAs. As shown in Figure 16a, after being loaded with a pre-strain and unloaded, the sample is then heated up to a high temperature to activate reverse martensitic transformation. The red arrow in the figure shows the so-called recovery strain. For recovery stress behavior, as shown in Figure 16b, the sample is loaded to a pre-strain (εpre) and then unloaded. In the next step, the sample is heated while it is restrained at both ends (path 3). The heating process first produces a compression in the restrained Fe-SMA as thermal expansion dominates initially and is then surpassed by reverse martensitic transformation. Upon cooling, tensile stresses are further generated in the sample because of the thermal contraction. Finally, a recovery stress is generated in the sample.
Table 4 shows the shape recovery properties of AMed Fe-SMAs. Shape recovery properties of representative conventionally manufactured Fe-SMAs are also summarized in Table A2 in the Appendix A. For a more visual comparison, recovery strain with respect to different pre-strains of AMed Fe-SMAs and conventionally manufactured Fe-SMAs are plotted in Figure 17. As previously discussed, better shape recovery behavior was also attributed to different factors. For example, higher laser power leads to an increase in the volume fraction of the fcc-γ phase, which enhances SME [70]. Liu et al. [93] attributed the improvement in SME to the large number of stacking faults distributed in an LDED Fe-SMA. The larger the stacking fault density, the smaller the driving force required for stress-induced martensite. The more ε-martensite is produced under the same stress conditions, the more it benefits the improvement in shape memory performance [93,124]. In addition, recovery strain can be improved when the loading direction is parallel to the print direction [71].
Furthermore, proper heat treatment and aging time could also enhance SME [91,92]. Kim et al. [67] found that heat treatment temperatures lower than 1073 K are not sufficient to completely undergo phase transformation from bcc-δ to fcc-γ, whereas temperatures higher than 1073 K lead to grain growth and thickening of hexagonal close-packed (hcp)-ε phases. The latter has a severely negative effect on shape memory. Furthermore, heat treatment at 1073 K for more than 3 h leads to σ-phase formation, which also negatively affects the SME by changing the chemical composition of the fcc-γ matrix [67]. The SME along the building direction (BD), is higher than that along the laser scanning direction (SD) and at 45° to the SD (45 SD). The higher SME along the BD is attributed to the {110} texture in the plane normal to the BD, whereas almost random but weak {111} and {100} textures are observed in the SD and 45 SD. Furthermore, TEM images revealed that a high stacking fault density and very thin hcp-ε phase are formed in the heat-treated sample [67], which explains the higher SME regardless of direction, compared with conventionally fabricated Fe-SMAs with a similar composition. Additionally, for conventionally manufactured Fe-SMAs, SME is greatly improved by the shape memory training process. Therefore, it is possible that the higher SME of AMed Fe-SMAs could be similarly enhanced by a training process.

6. Applications of AMed Fe-SMAs

Wang et al. [125] summarized the application of Fe-SMAs in smart steel structures, including the applications in AM fields. Conventional processed Fe-SMAs are commonly used for bridge reinforcement, pipe connections, and building vibration damping. In all of these applications, conventionally processed Fe-SMAs could potentially be replaced by AMed Fe-SMAs owing to advantages such as better SME and ease of manufacture. Recently, Fe-SMA walls [62] have been fabricated using the WAAM process as shown in Figure 18. A mono-stable architecture combined with an AMed Fe-SMA can result in advanced functionalities in terms of recoverability and energy absorption and dissipation [62].

7. Current Issues and Future Expectations

7.1. Current Issues and Challenges

AMed Fe-SMAs present several notable technical challenges and material issues that impede their widespread adoption and optimized performance. One key challenge lies in achieving precise control over the microstructure and phase transformation behavior during the AM process. Fe-SMAs are sensitive to thermal history and cooling rates, which can influence their transformation temperatures and mechanical properties. Inaccurate control of printing parameters such as laser power, scanning speed, and layer thickness can result in undesirable microstructures and phase compositions, leading to compromised shape memory and fatigue properties. Moreover, it remains unclear whether optimization efforts will compromise the current mechanical properties of AMed Fe-SMAs.
Another significant challenge is the limited availability of Fe-SMA powders with tailored compositions suitable for AM techniques. The availability of high-quality and atomized Fe-SMA powders with controlled particle size distribution and chemical composition is crucial for achieving consistent printing outcomes.
Furthermore, as mentioned above, AMed Ni-Ti SMAs were developed a long time ago and are widely used in the biomedical, aerospace, and other fields, whereas, AMed Fe-SMAs are still in the early stages of development, while AMed Cu-SMAs have not been studied much. Thus, the comparison of physical and mechanical characteristics of only AMed Fe-SMAs and Ni-Ti SMAs is summarized in Table 5. Figure 19 presents a comparison of the various properties of AMed Fe-SMAs and Ni-Ti SMAs after quantification according to Table 5. Higher density, higher negative Poisson’s ratio, and higher Young’s modulus of AMed Fe-SMAs have led to their limited applications. Although the enhanced SME of AMed Fe-SMAs is still not comparable to that of Ni-Ti SMAs, their cost is much lower. Therefore, it is reasonable to ask whether the application of existing AMed Fe-SMAs will increase. Alternatively, the focus may involve the exploration of novel compositions of AMed Fe-SMAs to enhance overall performance.

7.2. Future Expectations

The development of AMed Fe-SMAs is poised for significant advancement and diversification across various industries. One crucial area of focus is the diversification of AMed Fe-SMA types. As indicated in Table 1, Table 3, and Table 4, the range of available AMed Fe-SMA compositions remains limited. Therefore, it is imperative to expand the scope by exploring additional Fe-SMA compositions with varying alloy compositions to investigate their unique properties, including shape memory characteristics and fatigue properties. In addition, in larger-scale applications such as architecture, further research should be conducted on the fabrication of Fe-SMAs based on DED technology.
The second notable trend is the refinement of printing processes and material compositions tailored specifically for AMed Fe-SMAs. Research efforts should focus on optimizing printing parameters, such as laser power settings and scanning strategies, to achieve precise control over microstructure and phase transformations. Furthermore, advancements in alloy design and powder metallurgy techniques will enable the development of novel Fe-SMA compositions with enhanced mechanical properties and tailored functionalities optimized for AM processes. Considering various application environments, for example, as an alternative to car bumpers and airplane tail fins, bridge reinforcement, etc., the mechanical properties and SME of AMed Fe-SMAs under impact loading should also be further elucidated [2,133,134,135]. As reported in a previous study, it is found [136] that the impact toughness and mechanical strength of AMed stainless steel 316L are heavily dependent on layer thickness. Optimal results are achieved with thinner layers, though printing orientation can adversely affect these properties. Therefore, it is crucial to carefully optimize both layer thickness and printing orientation to maximize the performance of AMed Fe-SMAs.
Moreover, advancements in post-processing methodologies, such as heat treatment and surface finishing techniques, will play a pivotal role in enhancing the mechanical and functional properties of AMed Fe-SMAs. Tailored heat treatment processes can optimize microstructures and phase compositions, while surface treatments can improve wear resistance and biocompatibility. Combining this with the shape memory training process is expected to further improve the SME of AMed Fe-SMAs.
Furthermore, the fabrication of complex structures with optimal mechanical properties, which are challenging to achieve through conventional manufacturing, will be enabled by the use of AMed Fe-SMAs. Additionally, the development of structures with negative Poisson’s ratio [137] and other metamaterial properties [138,139] will pave the way for advanced shock absorbers, protective equipment, and more. After optimizing the performance of AMed Fe-SMAs, it is competitive to employ AMed Fe-SMAs with a lower price instead of conventionally manufactured or even AMed Ni-Ti SMAs. To date, it is still difficult to replace Ni-Ti SMAs with AMed Fe-SMAs; with more in-depth exploration and improvement, it is anticipated that it will be possible to apply AMed Fe-SMAs in aviation in the future. For example, some possible applications of AMed Fe-SMAs in airplanes as shown in Figure 20 will be expected.

8. Conclusions

This work discussed the characteristics and applications of AMed Fe-SMAs. The specific AM technologies employed for Fe-SMAs, including LPBF and DED, were introduced. The mechanical properties, shape memory behavior, and fatigue characteristics of AMed Fe-SMAs in relation to varying printing parameters, as well as the defects and microstructure control after printing were discussed. Although research on AMed Fe-SMAs remains in its nascent stages, preliminary findings suggest that they exhibit marginally superior mechanical properties and shape memory performance relative to conventionally manufactured Fe-SMAs. Notably, the potential for enhancement through compositional adjustments or application of the shape memory training process is promising. However, the fatigue properties of AMed Fe-SMAs have yet to be thoroughly investigated, indicating existing limitations in their application. Finally, the current problems of AMed Fe-SMAs, the future research direction of AMed Fe-SMAs, and the areas that may be developed were discussed.

Author Contributions

Conceptualization, methodology, Q.S. and B.C.; data curation, Q.S.; validation, B.C.; writing—original draft preparation, Q.S., B.C., M.D. and X.T.; writing—review and editing, Q.S., B.C., X.T. and T.I. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 12302190), the Fundamental Research Funds for the Central Universities (No. G2022WD01028), (No. G2024KY05105), and the Science Foundation of the National Key Laboratory of Science and Technology on Advanced Composites in Special Environments (No. JCKYS2023603C018).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Appendix A.1. Mechanical Properties of Conventionally Manufactured Fe-SMAs

In Table A1, the chemical composition of conventionally manufactured Fe-SMAs and their mechanical properties including Young’s modulus, maximum strain, ultimate tensile strength, density, yield stress, and hardness are provided as a comparison.
Table A1. Mechanical properties of conventionally manufactured Fe-SMAs.
Table A1. Mechanical properties of conventionally manufactured Fe-SMAs.
Chemical CompositionYoung’s Modulus (GPa)Maximum Strain (%)Ultimate Tensile Strength (MPa)Density
(g/cm3)		Yield Strength
(MPa)		Hardness (Hv)Ref.
Fe-17Mn-5Si-10Cr-4Ni-1(V, C)20015.4993-310316[140,141]
Fe-Mn-Si system17016–30680–10007.2–7.5200–300190–220[142]
Fe-17Mn-5Si-10Cr-4Ni-1(V, C)19230900-403-[143]
Fe-17Mn-5Si-10Cr-4Ni-1(V,C)17354.91015-546-[119]
Fe-28Mn-6Si-5Cr170---250-[144]
Fe-28Mn-6Si-5Cr-0.5NbC193---270-[145]
Fe-17Mn-5Si-10Cr-4Ni-1(V,C)16040–501000-500-[123]
Fe–17Mn–5Si–10Cr–4Ni–1(V,C)16053.471000.75-538-[146]
Fe-15Mn-5Si-9Cr-5Ni123.616880-420-[147]
Fe-17Mn-5Si-10Cr-4Ni-1(V,C)200---350-[148]
Fe-17Mn-5Si-10Cr-4Ni-1(V,C)160 530 [50]
Fe-14Mn-6Si-9Cr-5Ni17870.5788-258 [149]

Appendix A.2. Shape Recovery Properties of Conventionally Manufactured Fe-SMAs

Table A2 shows the shape recovery properties such as recovery strain and recovery stress of various conventionally manufactured Fe-SMAs with different pre-strain and loading modes.
Table A2. Shape recovery properties of conventionally manufactured Fe-SMAs.
Table A2. Shape recovery properties of conventionally manufactured Fe-SMAs.
Chemical CompositionPre-StrainLoading ModeRecovery Strain (%)Recovery Stress (MPa)Ref.
Fe–19Mn–5Si–8Cr–5Ni4.5Bending-460[150]
Fe–16Mn–5Si–10Cr–4Ni–1(V, N)4Bending-500[151]
Fe–15Mn–4Si–8Cr–4Ni–0.18C8Bending2.3565[152]
Fe–13Mn–5Si–9Cr–7Ni–0.03C6.5Bending4.5-[153]
Fe–17Mn–5Si–10Cr–4Ni–1(V,C)4Tension1.2580[50]
Fe-17Mn-5Si-10Cr-4Ni-1(V,C)2Tension-372[119]
Fe-28Mn-6Si2.5Tension0.47-[154]
Fe-30Mn-6Si1.08
Fe-32Mn-6Si1.17
Fe-26Mn-6Si-5Cr0.79
Fe-28Mn-6Si-5Cr1.26
Fe-30Mn-6Si-5Cr1.25
Fe-25Mn-6Si-7Cr0.89
Fe-27Mn-6Si-7Cr1.18
Fe-29Mn-6Si-7Cr1.06
Fe-18Mn-5Si-8Cr-5Ni1.11
Fe-20Mn-5Si-8Cr-5Ni2.05
Fe-22Mn-5Si-8Cr-5Ni1.41
Fe-14Mn-6Si-9Cr-5Ni0.97
Fe-15Mn-6Si-9Cr-5Ni1.11
Fe-16Mn-6Si-9Cr-5Ni1.55
Fe-11Mn-5Si-12Cr-7Ni1.35
Fe-13Mn-5Si-12Cr-7Ni1.64
Fe-16Mn-5Si-12Cr-5Ni1.45
Fe–17Mn–5Si–10Cr–4Ni–1(V,C)2Tension0.39392.7[146]
Fe-29.9Mn-6Si3Tension2.2-[155]

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Figure 1. Industrial applications of SMAs. The inner section illustrates the primary applications for Ni-Ti SMAs, while the outer section depicts the current applications for Fe-SMAs. Reprinted from [32,39,40,41,42,43,44,45].
Figure 1. Industrial applications of SMAs. The inner section illustrates the primary applications for Ni-Ti SMAs, while the outer section depicts the current applications for Fe-SMAs. Reprinted from [32,39,40,41,42,43,44,45].
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Figure 2. A comparison of detailed typical steps between conventional and additive manufacturing for producing an Fe-SMA bar. Retrieved from [52,53,54,55,56,57,58,59,60].
Figure 2. A comparison of detailed typical steps between conventional and additive manufacturing for producing an Fe-SMA bar. Retrieved from [52,53,54,55,56,57,58,59,60].
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Figure 3. Examples of AMed Fe-SMA samples with various designs [51]. (a) Appearances of samples in the as-built state. (b) Preparation of the sample with CAD and LPBF printing and its shape morphing behavior upon heating. The processes corresponding to the changes with temperature corresponding to the (c–f) samples in (a) are also described in detail. (c–f) Shape morphing upon the heating of samples with (c,d) open leaves and (e,f) closed leaves. (g) Artificial muscle effect due to the shape recovery of a spider-like LPBF-fabricated Fe-SMA sample. The arrows in the figure indicate temperatures from low to high. Reprinted from [51] by permission of Elsevier.
Figure 3. Examples of AMed Fe-SMA samples with various designs [51]. (a) Appearances of samples in the as-built state. (b) Preparation of the sample with CAD and LPBF printing and its shape morphing behavior upon heating. The processes corresponding to the changes with temperature corresponding to the (c–f) samples in (a) are also described in detail. (c–f) Shape morphing upon the heating of samples with (c,d) open leaves and (e,f) closed leaves. (g) Artificial muscle effect due to the shape recovery of a spider-like LPBF-fabricated Fe-SMA sample. The arrows in the figure indicate temperatures from low to high. Reprinted from [51] by permission of Elsevier.
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Figure 4. Overall structure of this review comprising AM process [77,78,79], mechanical properties [80], microstructure [61,65] and shape memory behavior. Reprinted from [61,65,77,78,79,80] by permission of Elsevier, Springer.
Figure 4. Overall structure of this review comprising AM process [77,78,79], mechanical properties [80], microstructure [61,65] and shape memory behavior. Reprinted from [61,65,77,78,79,80] by permission of Elsevier, Springer.
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Figure 5. The dominant methods, namely, (a) DED, (b) SLM, (c) electron beam melting (EBM), and (d) WAAM, used for the AM of Ni-Ti SMAs [82,84]. Reprinted from [82,84] by permission of Elsevier.
Figure 5. The dominant methods, namely, (a) DED, (b) SLM, (c) electron beam melting (EBM), and (d) WAAM, used for the AM of Ni-Ti SMAs [82,84]. Reprinted from [82,84] by permission of Elsevier.
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Figure 6. (a) A schematic diagram of LPBF process [99], (b) LPBF-fabricated Fe-SMA samples [74] produced with (c) microstructure of the lattice specimen and its axial stress-strain behavior under compression test. (d) Stress-strain response of an AMed FeMnNiAl compression sample. The EBSD maps show the predominance of austenitic phase with a crystal orientation parallel to <001> along the loading direction. Traces of martensite were observed at the grain boundaries. (e) Other LPBF-fabricated samples [71]. Reprinted from [71,74,99] by permission of Elsevier and Light: Advanced Manufacturing.
Figure 6. (a) A schematic diagram of LPBF process [99], (b) LPBF-fabricated Fe-SMA samples [74] produced with (c) microstructure of the lattice specimen and its axial stress-strain behavior under compression test. (d) Stress-strain response of an AMed FeMnNiAl compression sample. The EBSD maps show the predominance of austenitic phase with a crystal orientation parallel to <001> along the loading direction. Traces of martensite were observed at the grain boundaries. (e) Other LPBF-fabricated samples [71]. Reprinted from [71,74,99] by permission of Elsevier and Light: Advanced Manufacturing.
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Figure 7. (a) A schematic diagram of DED process [98], (b) DED-fabricated Fe-SMA sample [75], (c,d) its microstructure, and (e) cycling behavior. Reprinted from [75,98] by permission of Elsevier.
Figure 7. (a) A schematic diagram of DED process [98], (b) DED-fabricated Fe-SMA sample [75], (c,d) its microstructure, and (e) cycling behavior. Reprinted from [75,98] by permission of Elsevier.
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Figure 8. The major elements added to Fe-Mn-Si SMA system with their advantages and disadvantages.
Figure 8. The major elements added to Fe-Mn-Si SMA system with their advantages and disadvantages.
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Figure 10. Important parameters of the printing process that can affect the microstructure and shape recovery behavior of AMed Fe-SMAs.
Figure 10. Important parameters of the printing process that can affect the microstructure and shape recovery behavior of AMed Fe-SMAs.
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Figure 11. Common defects such as (a) porpsity, (b) cracking, and (c) lack of fusion during the printing process that affect the quality and performance of Fe-SMAs. The yellow circle in (c) indicate the lack of fusion. Reprinted from [89,117,118] by permission of Elsevier, and MDPI.
Figure 11. Common defects such as (a) porpsity, (b) cracking, and (c) lack of fusion during the printing process that affect the quality and performance of Fe-SMAs. The yellow circle in (c) indicate the lack of fusion. Reprinted from [89,117,118] by permission of Elsevier, and MDPI.
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Figure 12. EBSD maps with (a,d,g) phase, (b,c,e,f,h,i) inverse pole figure (IPF) coloring for AMed samples with different scanning speeds [64]; (j–m) EBSD phase maps containing grain boundary characters and IPF maps of LPBF-fabricated Fe-Mn-Si-Cr-Ni SMAs fabricated with different E [65]. Reprinted from [64,65] by permission of Elsevier.
Figure 12. EBSD maps with (a,d,g) phase, (b,c,e,f,h,i) inverse pole figure (IPF) coloring for AMed samples with different scanning speeds [64]; (j–m) EBSD phase maps containing grain boundary characters and IPF maps of LPBF-fabricated Fe-Mn-Si-Cr-Ni SMAs fabricated with different E [65]. Reprinted from [64,65] by permission of Elsevier.
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Figure 13. Typical STEM bright-field images of the δ ferrite and selected area electron diffraction image of the area within orange circle 1 of LPBF-fabricated Fe-SMAs with an E of (a) 111 J/mm3 and (b) 250 J/mm3 [65]. A high density of dislocations is indicated by yellow arrows. The epitaxial growth of the columnar δ ferritic grains is indicated by blue arrows. Reprinted from [65] by permission of Elsevier.
Figure 13. Typical STEM bright-field images of the δ ferrite and selected area electron diffraction image of the area within orange circle 1 of LPBF-fabricated Fe-SMAs with an E of (a) 111 J/mm3 and (b) 250 J/mm3 [65]. A high density of dislocations is indicated by yellow arrows. The epitaxial growth of the columnar δ ferritic grains is indicated by blue arrows. Reprinted from [65] by permission of Elsevier.
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Figure 16. Schematic diagrams of (a) recovery strain behavior, and (b) recovery stress behavior [123] of Fe-SMAs. Reprinted from [123] by permission of Elsevier.
Figure 16. Schematic diagrams of (a) recovery strain behavior, and (b) recovery stress behavior [123] of Fe-SMAs. Reprinted from [123] by permission of Elsevier.
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Figure 17. Recovery strain with respect to different pre-strains of AMed Fe-SMAs and conventionally manufactured Fe-SMAs.
Figure 17. Recovery strain with respect to different pre-strains of AMed Fe-SMAs and conventionally manufactured Fe-SMAs.
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Figure 18. Illustration of a few real-world applications of AMed Fe-SMAs, such as (a) vibration isolation for resilient buildings and construction, and (b) passive dampers for wind turbine nacelles, as well as (c) an actual AMed sample. Reprinted from [62] by permission of Elsevier.
Figure 18. Illustration of a few real-world applications of AMed Fe-SMAs, such as (a) vibration isolation for resilient buildings and construction, and (b) passive dampers for wind turbine nacelles, as well as (c) an actual AMed sample. Reprinted from [62] by permission of Elsevier.
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Figure 19. Comparison of various properties of AMed Fe-SMAs and Ni-Ti SMAs. The level of quantification is based on the ratios presented in Table 5.
Figure 19. Comparison of various properties of AMed Fe-SMAs and Ni-Ti SMAs. The level of quantification is based on the ratios presented in Table 5.
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Figure 20. Some possible applications of AMed Fe-SMAs in airplanes [3]. Reprinted from [3] by permission of Elsevier.
Figure 20. Some possible applications of AMed Fe-SMAs in airplanes [3]. Reprinted from [3] by permission of Elsevier.
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Table 1. A summary of present AMed Fe-SMAs with their AM techniques used and chemical composition, as well as printing parameters including laser power, volumetric energy density, scanning speed, and layer thickness.
Table 1. A summary of present AMed Fe-SMAs with their AM techniques used and chemical composition, as well as printing parameters including laser power, volumetric energy density, scanning speed, and layer thickness.
AM TechniquesChemical Composition of Fe-SMALaser Power (W)Volumetric Energy Density (J/mm3)Scanning Speed (mm/s)Layer Thickness (μm)YearRefs
LPBFFe-17Mn-5Si-10Cr-4Ni4201058002002022[51]
LPBFFe-26Mn-6Si-5Cr-0.5Ni-(Nb,C)200–500-400–1200502023[61]
Fe-28Mn-6Si-5Cr-0.5Ni-(Nb,C)
LPBFFe-17Mn-5Si-10Cr-4Ni-1(V,C)-144400302022[64]
LPBFFe-21Mn-5Si-9Cr-5Ni100–36042–300500–1000302023[65]
160–360111–250600
LPBFFe-17Mn-5Si-10Cr-4Ni-(V, C)420105800502023[66]
LPBFFe-17Mn-5Si-10Cr-4Ni130–17572–583100–600302023[68]
Fe-17Mn-5Si-10Cr-4Ni-(V,C)
LPBFFe-34Mn-15Al-7.5Ni150–40046.3–291.7600–1200302023[69]
LPBFFe-17Mn-5Si-10Cr-4Ni38095800502021[70]
400100
420105
LPBFFe-17Mn-5Si-10Cr-4Ni115–17563.89–194.44300–600302021[71]
LPBFFe-34Mn-14Al-7.5 Ni200-680302021[89]
LPBFFe-30Mn-6Si100-400–60052023[90]
125
150
175
LPBFFe-17Mn-5Si-4Ni-10Cr-(V, C)420105800502024[91]
LPBFFe-34Mn-8Al-7Ni200--402021[74]
LPBFFe-17Mn-5Si-10Cr-4Ni-1(V, C)175178–389225302022[92]
LPBFFe-20Mn-6Si-9Cr-5Ni17056850-2023[100]
63750
73650
18560850
69750
79650
20065850
74750
85650
LPBFFe-7Si-3B-2Cr-1C90-700–130030–702023[73]
100
120
150
LPBFFe-17Mn-5Si-10Cr-4Ni420105800502022[67]
LPBFFe-17Mn-5Si-10Cr-4Ni-1(V,C)175-225302023[62]
Fe-17Mn-5Si-10Cr-4Ni300
LPBFFe-17Mn-5Si-10Cr-4Ni175194.44300302023[101]
Laser-DED (LDED)Fe-20Mn-6Si-9Cr-5Ni1000-13.33002024[93]
DEDFe-17Mn-5Si-10Cr-4Ni-1(V, C)--8-2023[75]
SLMFe-34Mn-14Al-7.5Ni-65 J788302016[94]
Table 2. Representative Fe-Mn-Si SMA systems developed in recent years.
Table 2. Representative Fe-Mn-Si SMA systems developed in recent years.
Added ElementShape Recovery Stress (MPa)Shape Recovery Strain (%)Used for AMExisting and Potential ApplicationsRefs
Cr130–3502–5-Civil engineering structures requiring higher corrosion resistance; Connectors for special environments[1,2,24,104]
Cr, Ni300–4605–6LPBF; DEDAdvanced mechanical components[104]
Co250–3503–4.5-Connectors for special environments[105]
Al200–3003–4-General mechanical components[106]
Table 4. Shape recovery properties of AMed Fe-SMAs.
Table 4. Shape recovery properties of AMed Fe-SMAs.
Chemical CompositionPre-Strain (%)Loading ModeRecovery Strain (%)Recovery Stress (MPa)Processing MethodRef.
Fe-17Mn-5Si-10Cr-4Ni-1(V,C)2Compression0.39-LPBF[64]
Fe-21Mn-5Si-9Cr-5Ni11Bending6-LPBF[65]
Fe-17Mn-5Si-10Cr-4Ni-(V, C)4Tension0.43-LPBF[66]
0.79428
0.89314
Fe-17Mn-5Si-10Cr-4Ni4Tension1.64-LPBF[71]
Fe-34Mn-14Al-7.5 Ni4Tension3.4-LPBF[89]
Fe-17Mn-5Si-4Ni-10Cr-(V, C)4.62Bending3.73-LPBF[91]
Fe-34Mn-8Al-7Ni3.5Compression1.5-LPBF[74]
Fe-17Mn-5Si-10Cr-4Ni-1(V, C)4Tension1.41-LPBF[92]
Fe-20Mn-6Si-9Cr-5Ni11Tension3.49-LDED[93]
Fe-17Mn-5Si-10Cr-4Ni-1(V, C)3Tension2.4-WAAM[75]
Fe-34Mn-14Al-7.5Ni12.5Compression7.5-SLM[94]
Fe-17Mn-5Si-10Cr-4Ni-1(V, C)4Tension1.11-LPBF[107]
Fe-17Mn-5Si-10Cr-4Ni5.26Bending4.04-LPBF[67]
Fe-17Mn-5Si-10Cr-4Ni4Tension1.47-LPBF[101]
Table 5. Comparison of physical and mechanical characteristics of AMed Fe-SMAs and AMed Ni-Ti SMAs.
Table 5. Comparison of physical and mechanical characteristics of AMed Fe-SMAs and AMed Ni-Ti SMAs.
PropertiesFe-SMAsNi-Ti SMAs
Density (g/cm3)7.4–7.491 [65,75]4.1485 [114]
Poisson’s ratio−0.1, −0.67 [51]−1.65 [126]
Young’s modulus (GPa)141–200 [66,92]40–60 [127,128,129]
Ultimate tensile strength (MPa)658.6–1068.6 [65]564–1025 [129]
Yield stress (MPa)230–750 [51,68] 1050 [128]
Elongation (%)0.6–54 [51,65]6.7–15.6 [129]
Corrosion resistancePoor [130]Superior [131]
Cost of powder (¥/kg)4003000
SME (Recovery strain, %)0.43–3.49 [66,93]~5.9 [132]
WorkabilityGood [107]Poor [107]
ProcessingEasy [107]Hard [107]
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Sun, Q.; Tan, X.; Ding, M.; Cao, B.; Iwamoto, T. A Review of Additively Manufactured Iron-Based Shape Memory Alloys. Crystals 2024, 14, 773. https://doi.org/10.3390/cryst14090773

AMA Style

Sun Q, Tan X, Ding M, Cao B, Iwamoto T. A Review of Additively Manufactured Iron-Based Shape Memory Alloys. Crystals. 2024; 14(9):773. https://doi.org/10.3390/cryst14090773

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Sun, Qian, Xiaojun Tan, Mingjun Ding, Bo Cao, and Takeshi Iwamoto. 2024. "A Review of Additively Manufactured Iron-Based Shape Memory Alloys" Crystals 14, no. 9: 773. https://doi.org/10.3390/cryst14090773

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