**1. Introduction**

Shape memory alloys (SMAs) are small materials with functional and intelligent properties. SMAs exhibit two unique properties: (i) the shape memory effect, in which the material can recover the deformation upon heating or magnetization, and (ii) superelasticity, which is stress-induced during martensitic transformation, and the material can recover the deformation after removing the load. Currently, nickel–titanium (NiTi) SMAs are the most well-developed SMAs and are commonly used in industries including aerospace, automotive, biomedical, robotics, actuators and sensors due to their high recoverable strain in both tension and compression [1–3]. However, the materials used in NiTi SMAs have a high cost and they have difficulty in cold rolling. These factors limit the applications of large quantities of NiTi SMAs due to the cost of materials and processing issues [4]. Compared to NiTi alloys, iron-based SMAs are commercially attractive because they have a low material cost and good machinability and workability. As a result, iron-based SMAs have attracted attention and interest in the industry and academic fields.

The martensitic transformation (MT) of Fe-based SMAs is classified into three groups based on their martensite and austenite structures: (1) face-centered cubic (FCC) (austenite) to body-centered cubic (BCC) or body-centered tetragonal (BCT) (martensite). This transformation system can be observed in an FeNi-based alloy system [5–10]. (2) The martensitic transformation in FeMnSi SMAs are FCC-HCP [11,12]: FCC (austenite) to hexagonal close-packed (HCP) (martensite). (3) The transformation system reported in FeMnAl [13], FeMnGa [14,15] and FeMnAlNi [16] SMAs is BCC (austenite) to FCC (martensite).

**Citation:** Tseng, L.-W.; Chen, C.-H.; Chen, W.-C.; Cheng, Y.; Lu, N.-H. Shape Memory Properties and Microstructure of New Iron-Based FeNiCoAlTiNb Shape Memory Alloys. *Crystals* **2021**, *11*, 1253. https://doi.org/10.3390/ cryst11101253

Academic Editor: Wojciech Polkowski

Received: 14 September 2021 Accepted: 13 October 2021 Published: 15 October 2021

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In general, four distinct morphologies of martensite in iron-based SMAs can be observed: lenticular, lath, butterfly and thin plate [1,17]. Lenticular martensite is formed in FePt SMAs. In FeNi alloys, thin plate martensite forms at the lowest temperature, lenticular martensite forms at intermediate temperatures and lath martensite forms at the highest temperature [17]. The formation of lath martensite typically contains a high density of internal dislocations due to accommodation distortion. Unlike the lath martensite, thin plate types of martensite contain a high density of transformation twins. For butterfly type of martensite, the substructure comprises a mixture of dislocations and twins [17]. Among four types of martensite morphology, only the thin plate martensite shows the shape memory effect [1].

Most Fe-based SMAs, such as FeMnAl, FeNiCoTi, FeMnSi and FeMnGa SMAs, show less than 1% recoverable strain of superelasticity [1,3,5,11,13]. Recently, two new Fe-based SMAs systems were presented, with superelastic strains above 4% at room temperature. The first system is Fe40.95Ni28Co17Al11.5Ta2.5B0.05 (at.%), which shows a 13.5% superelastic strain [9], and in a similar system, FeNiCoAlXB (X: Ta, Nb, Ti) presents a 5% superelastic strain in polycrystalline alloys [9,10,18,19], single crystals [20–24] and wire [25]. The second system is Fe43.5Mn34Al15Ni7.5 (at.%), and this system shows a 5% superelastic strain in polycrystalline alloys [16,26,27], polycrystalline wires [28–30] and single crystals [31,32]. In FeNiCoAlXB (X: Ta, Nb, Ti) alloy systems, three conditions are required to achieve this excellent superelastic property. (1) A strong texture is required in these alloys. If the texture is random, it shows brittle behavior. In FeNiCoAlTaB and FeNiCoAlTiB alloys, the strong texture is in the <100> orientation [8,10]. In FeNiCoAlNbB alloys, the strong texture is in the <110> orientation [9]. (2) A large grain size is required to decrease the grain boundary and triple junction constraints during the superelastic tests. In FeNiCoAlTaB alloys, the average grain size is around 400 μm [8,33]. (3) The alloy is required to undergo aging heat treatment to obtain L12 precipitates. The nano size precipitates not only strengthen the matrix but also change the material from non-thermoelastic to thermoelastic transformation [8]. The precipitate size is around 3 nm for the 600 ◦C–72 h sample [8].

Studies regarding the shape memory behavior of <100>-oriented Fe41Ni28Co17Al11.5Ta2.5 (at.%) single crystals have been carried out. Ma et al. [20] reported a 3.75% recoverable strain in tension at the stress level of 50 MPa. The precipitate size was around 5 nm for a 600 ◦C–90 h sample. Czerny et al. [34,35] found that two stages of aging heat treatment could improve the compressive properties. A FeNiCoAlTa single crystal underwent the first stage of aging at 700 ◦C for 0.5 h and the second stage of aging at 700 ◦C for 3 h, showing a 15% superelastic strain at −196 ◦C [35].

Tseng et al. [23] found that using Ti to replace Ta could reduce the aging heat treatment times. A FeNiCoAlTi single crystal with <100> orientation aged at 600 ◦C for 4 h possessed 6% superelastic strain when the test temperature was at −80 ◦C. Chumlyakov et al. [36–38] showed that adding Nb to this alloy could increase the hardness of the austenite matrix and improve their superelastic properties. FeNiCoAlNb single crystals with <100> orientation in two-step aging (700 ◦C–0.5 h + 700 ◦C–3 h) demonstrated 7% superelastic strain in tension and 13.5% in compression. The precipitates size ranged from 5 to 8 nm [38].

Poklonov et al. [39] reported <100>-oriented FeNiCoAlTiNb single crystals that underwent a thermal cycling tensile test and obtained 2.2% recoverable strain at 50 MPa when they were heat treated at 1277 ◦C for 10 h and subsequently aged at 600 ◦C for 4 h. However, a large irrecoverable strain was observed when the applied stress was 100 MPa. The sample fractured at a stress level of 150 MPa. Tseng et al. [40] investigated the microstructure of FeNiCoAlTiNb alloy and found the homogeneous solution heat treatment condition was 1277 ◦C for 24 h. Second phases were formed in the sample when the heat treatment temperature was above 1300 ◦C.

In this study, Fe41Ni28Co17Al11.5(Ti+Nb)2.5 (at.%) polycrystalline alloys that underwent cold rolling were first investigated for their microstructure, texture, thermo- magnetization and shape memory properties using the three-point bending test. In this paper, we

discuss the aging effect on microstructure, transformation temperatures and shape memory properties in the three-point bending test.

#### **2. Materials and Methods**

Iron, cobalt, nickel, aluminum, niobium and titanium (99.9 wt%) were used as raw materials. Ingots with the nominal composition of Fe41Ni28Co17Al11.5(Ti+Nb)2.5 (at.%) were fabricated by vacuum induction melting (VIM). Wire electrical discharge machining (EDM) was used to cut the bar to the desired block, the dimensions of which were 100 × 25 × 25 mm (length, width and thickness). The block was used for the cold-rolling experiment.

As-received Fe41Ni28Co17Al11.5(Ti+Nb)2.5 (at.%) ingots were prepared using several conditions of thermo-mechanical processing to obtain polycrystalline sheets. The processing conditions are listed below:


The thermo-mechanical processing setup used in this study is summarized in Figure 1.

**Figure 1.** Illustration of the thermo-mechanical processes to obtain Fe41Ni28Co17Al11.5(Ti+Nb)2.5 (at.%) alloy sheets. SHT 1, WQ, CR, SHT 2 and AHT indicate first stage of solution heat treatment, water quenching, cold rolling, second stage of solution heat treatment and aging heat treatment, respectively.

The FeNiCoAlTiNb alloys were ground to 0.05 μm and the surfaces were etched with the solution, which was composed of 7% nitric acid and 93% ethanol. The observation of microstructure was made using the Olympus digital optical microscope. Vickers microhardness measurements were made using a microhardness tester FM-310. The samples for the OM tests were (1) 97%CR + 1277 ◦C–1 h, (2) 97%CR + 1277 ◦C–1 h + 600 ◦C–24 h, (3) 97%CR + 1277 ◦C–1 h + 600 ◦C–48 h and (4) 97%CR + 1277 ◦C–1 h + 600 ◦C–72 h. Hardness values of measurement were made at room temperature.

A high-resolution X-ray diffractometer with Cu-Kα radiation (λ = 0.1542 nm) was used to obtain the crystal structures of 1277 ◦C–24 h, 97%CR and 97%CR after heat treatment at 1277 ◦C for 1 h samples. The as-received, 1277 ◦C–24 h, 97%CR and 97%CR samples after heat treatment at 1277 ◦C for 1 h were cut out for texture measurement using a D5000 X-ray diffraction device (Siemens, Aubrey, TX, USA). The FCC{111}, FCC{200}, and FCC{220} pole figure data were selected to plot the orientation distribution functions (ODFs) [19]. The microstructure and texture of the cold-rolled specimens after being treated with solution at 1277 ◦C for 1h were determined using electron backscatter diffraction (EBSD). The etching solution used in preparing the sample surface was 10% HClO4 + 90% C2H5OH. The SEM images of fractured surfaces for the 600 ◦C–48 h sample after three-point bending test were obtained using a JSM-7800F device (JEOL, Musashino, Akishima, Tokyo).

In order to find the martensitic transformation temperatures and the thermo- magnetization properties, the cold-rolled samples that were aged at 600 ◦C for 24, 48 and 72 h were measured by using a superconducting quantum interference device (SQUID) under fields of 0.05 and 7 Tesla. The cooling and heating rate was determined to be 5 K/min. The magnetic results were used to determine the martensitic transformation temperatures used to calculate the temperature hysteresis. The sample was first heated up to 110 ◦C under a zero magnetic field, and then cooled down to −260 ◦C and heated up around to 110 ◦C under a constant magnetic field of 0.05 Tesla. After the 0.05 Tesla test, the magnetic field was increased to 7 Tesla. The sample was cooled from 120 ◦C to −260 ◦C and heated up to 120 ◦C again to complete one cycle. In thermo-magnetization curve, magnetization was used to describe how a material responds to an applied magnetic field and one magnetization (emu/g) corresponded to 1 Am2/kg.

The shape memory effects of FeNiCoAlTiNb samples were determined using the three-point bending method with an ElectroForce 3230 (TA Instruments, New Castle, DE, USA) equipped with a hot/cold chamber. The support span was 20 mm. Three-point bending shape memory samples were cut from the cold-rolled sheet. The samples were solution- treated at 1277 ◦C for 1 h and underwent aging heat treatment at 600 ◦C for 24, 48 and 72 h. The samples were straight before the bending test. Figure 2 presents the setup of the bending test. For this test, the sample was first loaded with a stress level of 50 MPa at room temperature and was then cooled and heated between −150 ◦C and room temperature to complete one thermal cycle. This process was repeated for increasing stress levels. A heating/cooling rate was selected as 5 K/min. When the sample completed one thermal cycle, the applied stress level increased to 100 MPa and the next heating/cooling cycle was carried out. The same procedure was applied using the stress levels of 150 and 200 MPa. The 600 ◦C–48 h sample failed when the applied stress level was 250 MPa. The 600 ◦C−72 h sample fractured during the 150 MPa applied stress level.

**Figure 2.** Setup for the bending test.

#### **3. Results**

*3.1. Microstructure and Microhardness Results*

Figure 3a–d show the microstructure of the FeNiCoAlTiNb polycrystalline in different thermo-mechanical processing conditions. Figure 3a shows the 97%CR sample after being heat treated at 1277 ◦C for 1 h. The grain boundaries were clearly seen in this condition. Figure 3b–d display the 97%CR sample after being treated with solution at 1277 ◦C for 1 h and aging heat treatment at 600 ◦C for 24, 48 and 72 h. The microstructure results show that leaf-like β phases accumulate in the grain boundary, especially in triple junctions. Increasing the aging times from 24 to 72 h, large number of β phases form in the aging sample. The generation of β phases can also be observed in FeNiCoAlXB (X: Ta, Ti, Nb) systems [8–10,12].

**Figure 3.** Microstructure of FeNiCoAlTiNb alloy in different thermo-mechanical processing conditions: (**a**) 97%CR+1277 ◦C–1 h, (**b**) 97%CR+1277 ◦C–1 h + 600 ◦C–24 h, (**c**) 97%CR + 1277 ◦C–1 h + 600 ◦C–48 h and (**d**) 97%CR + 1277 ◦C–1 h + 600 ◦C–72 h.

Table 1 shows the Vickers microhardness of the FeNiCoAlTiNb alloy in different conditions of thermomechanical processing. The hardness value of the 97%CR sample is around 430HV. After the solution heat treatment, the hardness is 260HV. For the aging heat treatment condition, the microhardness values are 462 HV, 510 HV and 552 HV for 600 ◦C–24 h, 600 ◦C–48 h and 600 ◦C–72 h, respectively. The microhardness results of three aging conditions are summarized in Figure 4. From the results, as the aging times increase, the hardness values increase their values due to the formation of precipitation hardening. In FeNiCoAlTa single crystal studies, the hardness values increased when the aging times increased from 24 to 90 h at an aging temperature of 600 ◦C, which suggests precipitates strengthen the austenite matrix [21].

**Figure 4.** Hardness results of FeNiCoAlTiNb samples under different aging conditions.


**Table 1.** Microhardness results of FeNiCoAlTiNb alloy in different thermo-mechanical processing conditions.

#### *3.2. XRD Results*

Figure 5 presents the XRD results for samples' conditions: 1277 ◦C–24 h, 1277 ◦C–24 h + 97%CR and 1277 ◦C–24 h + 97%CR + 1277 ◦C–1 h. The XRD profile results show that face-centered cubic (FCC) phases are found in three conditions. In the cold-rolled samples after 1277 ◦C–1 h treatment, a strong peak intensity is observed in the (111) plane.

**Figure 5.** XRD results of FeNiCoAlTiNb samples under different processing conditions: 1277 ◦C–24 h, 1277 ◦C–24 h + 97%CR and 1277 ◦C–24 h + 97%CR + 1277 ◦C–1 h.

#### *3.3. Magnetization Results*

The transformation temperatures of the cold-rolled samples after being aged at 600 ◦C for 24, 48 and 72 h were determined using the magnetic field test. After solution heat treatment, the 97% cold-rolled sample does not show here, since martensitic transformation is not observed in this condition. Figure 6a–f display the thermo-magnetization results of FeNiCoAlTiNb that underwent aging heat treatment at 600 ◦C for 24, 48 and 72 h under the magnetic fields of 0.05 and 7 Tesla. The martensitic transformation temperatures of the sample aged at 600 ◦C for 24, 48 and 72 h were determined using the magnetic field results of 0.05 Tesla, as shown in Figure 6a,c,e. The tangent line method was used to determine the transformation temperatures. From the result, the martensitic transformation temperatures of the 600 ◦C–24 h aging sample were: austenite finish temperature (Af) = −103 ◦C, austenite start temperature (As) = −147 ◦C, martensite start temperature (Ms) = −135 ◦C and martensite finish temperature (Mf) = −189 ◦C. The temperature hysteresis was defined as ΔT = |Af − Ms|, and was calculated to be 36 ◦C. For the 600 ◦C–48 h aging sample, Af = −68 ◦C, As = −105 ◦C, Ms = −106 ◦C and martensite finish temperature (Mf) = −154 ◦C. The temperature hysteresis was 30 ◦C. For the 600 ◦C–72 h aging sample, Af = −53 ◦C, As = −100 ◦C, Ms = −90 ◦C, and martensite finish temperature (Mf) = −134 ◦C. The temperature hysteresis was 37 ◦C. The transformation temperatures

and temperature hysteresis of the FeNiCoAlXB (X: Ta, Nb, Ti) alloys and the FeNiCoAlTiNb alloy reported by the present authors [8–10] are summarized in Table 2.

**Figure 6.** Thermo-magnetization curves of FeNiCoAlTiNb aging sample. (**a**,**b**) 0.05 and 7 Tesla for 600 ◦C−24 h, (**c**,**d**) 7 Tesla for 600 ◦C−48 h, (**e**,**f**) 0.05 and 7 Tesla for 600 ◦C−72 h.

Figure 6b,d,f present the magnetic results of the 7 Tesla experiments; the magnetization of both aging conditions is 140 emu/g. The Ms can also be obtained using tangent line methods, and Ms was −108 ◦C, −81 ◦C and −78 ◦C in 600 ◦C–24 h, 600 ◦C–48 h and 600 ◦C–72 h conditions, respectively. The results show the transformation temperatures move to higher temperatures as the aging times increase. A similar trend was reported in FeNiCoAlTa single crystals [21].

**Table 2.** Transformation temperatures and temperature hysteresis of the present FeNiCoAlTiNb alloy in comparison with data for FeNiCoAlXB (X: Ta, Nb, Ti) alloys [8–10].

