Search Results (99)

The coupled effects of Ni and Ti additions on amorphization, spark plasma sintering (SPS) response, and hardness evolution were investigated in Al-rich Al–Fe–Nb-based metastable alloys. Mechanically alloyed Al₈₂Fe₁₄Nb₂Ni₂, Al₈₂Fe₁₄Nb₂Ti₂, and Al₈₂Fe₁₂Nb₂Ni₂Ti₂ powders showed progressive loss of long-range order, with the quinary alloy exhibiting the strongest amorphization tendency, consistent with its higher configurational entropy (5.420 J·mol⁻¹·K⁻¹) and more negative mixing enthalpy (−9.36 kJ·mol⁻¹). SPS displacement analysis revealed that primary displacement contribution occurs during heating and is progressively limited by crystallization-induced stiffening. Consolidation at 500 °C produced amorphous–nanocrystalline composites containing Al₁₃Fe₄ and Al₃Nb, whereas increasing the temperature to 550 °C promoted further devitrification. The highest hardness, 445.4 HV, was obtained for Al₈₂Fe₁₄Nb₂Ni₂, despite its lower amorphous-forming ability than the quinary alloy. This demonstrates that hardness is controlled not by maximum amorphization, but by the kinetic balance between amorphous retention, fine intermetallic precipitation, and densification efficiency. The results identify SPS as a coupled densification–transformation route for designing high-strength Al-based amorphous–nanocrystalline alloys. Full article

Additive manufacturing of NiTi shape memory alloys is challenging due to their sensitivity to composition and thermal history. The gap between high-resolution powder-based AM and high-productivity wire-based processes for NiTi remains a challenge. This study investigates the technical feasibility of depositing Ni-rich NiTi (56 wt.% Ni) using a micro gas metal arc-based directed energy deposition (µ-GMA-DED) process with a 300 µm wire. The investigation was conducted on a single-bead, single-layer geometry deposited onto a titanium substrate. The deposited layer exhibited a heterogeneous microstructure with dendritic and eutectic-like regions, where phase analysis revealed a mixture of NiTi and Ni₃Ti intermetallics. Differential scanning calorimetry showed suppression of the martensitic transformation in the as-deposited condition, likely due to the high fraction of non-transformable Ni₃Ti, compositional redistribution during rapid solidification, and potential substrate dilution. The nanoindentation results reflected this heterogeneity, with Young’s modulus ranging from 64 to 151 GPa. While post-deposition heat treatment partially restored the martensitic transformation, these results demonstrate the preliminary feasibility of the µ-GMA-DED process, noting that strict control over chemistry and dilution is required before the route can be applied to functional components. Full article

The formation of coarse brittle W₂Zr phases severely limits the dynamic mechanical performance of energetic W-Zr structural materials. In this work, Ti and Ni were introduced into the W-Zr system to modify the phase evolution during sintering, and three alloys with different Zr atomic concentrations, W_xZr_85−xTi_7.5Ni_7.5 (x = 45, 55, and 65), were prepared by vacuum sintering. Microstructural characterization showed that the W₄₅Zr₄₀Ti_7.5Ni_7.5 alloy contained abundant coarse micron-sized W₂Zr particles, whereas both the W₅₅Zr₃₀Ti_7.5Ni_7.5 and W₆₅Zr₂₀Ti_7.5Ni_7.5 alloys exhibited a lower fraction of W₂Zr together with a much finer characteristic size. In particular, decreasing the Zr content reduced the characteristic size of W₂Zr from several micrometers to below 200 nm. Interrupted sintering and thermal analyses suggest that the preferential formation of a Zr(Ti) solid solution and a Zr-Ti-Ni-rich ternary phase at lower temperatures reduces the local availability of free Zr for reaction with W, thereby suppressing the nucleation and growth of W₂Zr. Correspondingly, the dynamic compressive strength increased from 1054 MPa for W₄₅Zr₄₀Ti_7.5Ni_7.5 to 1720 MPa for W₆₅Zr₂₀Ti_7.5Ni_7.5. In addition, the W₆₅Zr₂₀Ti_7.5Ni_7.5 alloy maintained pronounced impact-induced reaction behavior despite its lower Zr content. These results indicate that tailoring the W/Zr ratio in the Ti/Ni-containing W-Zr system provides a feasible route to regulate W₂Zr formation and improve the compressive response under dynamic loading. Full article

The recovery of oxygen-affine elements such as tantalum (Ta) using pyrometallurgical routes is difficult because this element cannot easily be enriched in a metal alloy, as is the case with battery recycling for the more noble metals Co, Ni, and Cu. A promising procedure, on the other hand, is to enrich this element in simple oxide compounds formed in a silicate melt. This enrichment in tailored mineral compounds is also known as the “Engineered Artificial Minerals” (EnAM) approach. Currently, the Technological Readiness Level (TRL) of this approach is relatively low and limited to understanding the mechanisms involved in the incorporation of target elements and the search for suitable compounds with a high enrichment factor, favorable morphology, and early crystallization during solidification in order to achieve maximum recovery yield of the selected compound (element). Due to its high ion charge (high field strength) and small ion radius for a heavy element, it is plausible that Ta behaves similarly to the abundant element titanium (Ti), whose chemistry is much better known. Ti minerals such as ulvospinel, perovskite, ilmenite, and pseudobrookite are therefore suitable candidates in the search for a suitable tantalum EnAM. A comparison of the solidification of synthetic silicate melts dominated by iron and calcium with Ti as an additive show that Ta is not incorporated into ulvospinel formed in olivine-containing Fe-rich silicate melts (base composition with 57 wt.% FeO). In contrast, the perovskites formed in silicate melts dominated by calcium-alumosilicate (max. 10 wt.% FeO addition) do incorporate Ta. Crystal size and Ta content increase with increasing iron content (up to a maximum of about 10 wt.%). The results indicate a possible solid solution with the well-known compounds CaTiO₃ and FeTiO₃ and the virtual compounds Ca_0.8TiO₃ and Fe_0.8TiO₃. Full article

Elastocaloric cooling, which leverages stress-induced phase transformation in shape memory materials, represents a sustainable and energy-efficient alternative to conventional vapor-compression cooling systems. Central to optimizing these materials is understanding how thermal processing history dictates phase formation, microstructure, and thermal properties. In this study, we investigated the (Ni₅₀Mn_31.5Ti₁₈)_99.8B_0.2 compound synthesized via vacuum induction melting and arc melting, followed by water quenching. Induction melting results in needle-like, boron-rich precipitates within the martensite lattice. In contrast, vacuum arc melting promoted precipitate growth at the grain boundaries. The vacuum arc melting sample exhibits ~82% martensite phase fraction, a near-ambient transformation temperature of ~277 K, a large transition entropy change of ~75 J·kg⁻¹·K⁻¹, and moderate thermal hysteresis of ~24 K. These results underscore the pivotal role of thermal history in tailoring phase stability and transformation thermodynamics, providing essential design guidelines for subsequent mechanical performance optimization in elastocaloric shape memory alloys for energy-efficient and sustainable thermal management applications. Full article

The influence of Ni and Co additions on microstructure and mechanical properties of (CoCrCuTi)_100−xFe_x high-entropy alloys (HEAs) containing 10 or 15 at. % Fe was investigated. The base HEA consisted of dendritic C14 Laves phases with interdendritic Cu-rich FCC regions. When Ni in the range of 2.5 to 10 at. % was added, a reduction in the Cu-rich phase was observed. Conversely, Co additions in the same range initially increased the Cu-rich phase but eventually led to liquid-phase separation (LPS), forming distinct Cu-lean L1 liquid and Cu-rich L2 globular regions. The average Vickers hardness values of (CoCrCuTi)₉₀Fe₁₀ and (CoCrCuTi)₈₅Fe₁₅ HEAs were measured at 790 ± 33 HV and 760 ± 20 HV, respectively. The additions of Ni and Co decreased overall hardness values. However, while Ni additions caused greater microstructural refinement, Co additions eventually led to heterogeneity due to LPS. For instance, the Vickers hardness of (CoCrCuTi)₉₀Fe₁₀ with 2.5 at. % Ni reached a maximum of 706 ± 95 HV, decreasing in hardness and scatter to 646 ± 19 HV when Ni increased to 10 at. %. In contrast, Co additions led to a marked reduction in hardness, from 574 ± 114 HV at 2.5 at. % Co to 442 ± 246 HV at 10 at. % Co. The fracture toughness (KIC), determined using Vickers indentation testing, indicated that Ni additions reduce fracture toughness, while Co additions increase it. Full article

Nickel-based superalloys are extensively used in fabricating high-temperature gas turbine components, owing to their superior high-temperature strength, excellent structural stability, and remarkable hot corrosion resistance. The influence of impurity sulfur content on their hot corrosion performance is a core scientific issue in hot-end component compositional design and smelting. This study investigated chromium (Cr)-rich nickel-based superalloys with sulfur (S) contents of 3 ppm, 16 ppm, and 42 ppm via XRD, SEM, and an EPMA, focusing on their hot corrosion behavior under a 100% Na₂SO₄ deposit at 900 °C. The results indicated that their hot corrosion products were basically identical, forming a Cr-dominated outer oxide layer rich in Ti, Co, and Ni, an Al₂O₃-based inner corrosion zone, and a CrS_x-dominated sulfide layer. With increasing sulfur content, the outer layer thickness decreased from approximately 30 μm to less than 20 μm, pores in the outer oxide layer increased in quantity and size, and internal sulfides and nitrides accumulated. The average depth of spallation increased from 55 μm for the S3 alloy to 80 μm for the S16 alloy, with the S42 alloy showing even more extensive spallation. The alloy’s hot corrosion performance deteriorated notably with increasing S content. The mechanism of sulfur’s effect on hot corrosion behavior is that sulfur in the alloy segregates at oxide film defects, enhancing defect stability and increasing their quantity and size. These defects serve as rapid diffusion channels for corrosive media, thereby accelerating the alloy’s hot corrosion rate. Full article

A CoCrNi-AlTi medium-entropy alloy was fabricated via laser powder bed fusion (L-PBF), and its microstructural evolution and mechanical response during aging at 500–900 °C for 1 h were systematically investigated. The as-built alloy exhibits a hierarchical microstructure consisting of elongated columnar grains and dislocation-rich cellular substructures, which is associated with an excellent strength–ductility combination (YS: 848 MPa, UTS: 1136 MPa, EF: 32.6%). Upon aging, a pronounced precipitation-hardening response is observed, with a peak hardness of 501 ± 7 HV and an ultimate tensile strength of 1429 MPa achieved at 800 °C. TEM and STEM-EDS analyses indicate that Ti preferentially segregates along dislocation networks and grain boundaries at early aging stages, promoting the heterogeneous nucleation of nanoscale Ni–Al–Ti–rich precipitates that effectively impede dislocation motion. At elevated aging temperatures, additional Cr-enriched regions with diffuse compositional partitioning are observed within the FCC matrix, occurring concurrently with the peak mechanical performance. Further aging at 900 °C leads to strength degradation, which is attributed to precipitate coarsening and recovery-induced dislocation annihilation. These results highlight the critical role of L-PBF-induced defect structures in governing precipitation behavior and the resulting strength–ductility trade-off during post-build heat treatment of CoCrNi-AlTi medium-entropy alloys. Full article

The wear performance of coated and uncoated harrow discs was evaluated under real agricultural field conditions in order to assess the long-term effectiveness of three atmospheric plasma spraying (APS) systems: a Cr₂O₃–SiO₂–TiO₂ ceramic coating, a WC/W₂C–Co carbide coating, and a Co–Cr–Ni–W–C alloy coating. In contrast to most previous studies focused on laboratory testing or short-term trials, the present work provides a comparative long-term field evaluation over 50 ha per disc (1000 ha total) under identical operating conditions in quartz-rich Argic Luvisol soil. Disc wear was quantified through periodic mass-loss and diameter measurements, complemented by microstructural and SEM analyses. The uncoated disc exhibited the most severe degradation, with a total mass loss of approximately 700 g and rapid acceleration of wear after the first 5–10 ha. The ceramic-coated disc showed the highest durability, limiting mass loss to approximately 390 g, corresponding to a reduction of about 44%, and maintaining the largest residual diameter after field operation. The Co-based alloy provided intermediate performance (~16% mass-loss reduction), while the carbide coating showed limited improvement (~7% reduction) due to microcracking and weak carbide–binder interfaces. The results demonstrate that, under real field conditions, coating microstructural integrity is more critical than nominal hardness, and highlight the superior effectiveness of ceramic APS coatings for extending disc service life in abrasive agricultural soils. Full article

This study investigates the control of the phase-structural state in Ni–45Ti–xCu (x = 5, 7 at.%) shape memory alloys fabricated via a shortened powder metallurgy route: mechanical activation → spark plasma sintering (SPS) → heat treatment. Compact samples were produced from mechanically alloyed powders (650–750 rpm, up to 5 h) and sintered at 900 °C. The structure and microstructure were characterized using X-ray diffraction (to identify B2/B19′/Ni₄Ti₃ phases and assess ordering) and SEM–BSE/EDS (to analyze morphology, porosity, and Ni-rich precipitates). Two post-processing treatments were applied: single-stage annealing (500 °C, 2 h) and a three-stage treatment (900 °C/30 min → water quenching → 300 °C/20 min). Mechanical alloying transformed the initial elemental powder mixture (fcc-Ni, hcp-Ti, fcc-Cu) into a supersaturated fcc-(Ni, Cu, Ti) solid solution with emerging NiTi phases, with a minimum particle size achieved after ~300 min at 750 rpm. SPS compaction yielded a high-density matrix consisting predominantly of the B2 phase. Single-stage annealing preserved B19′ martensite and Ni₄Ti₃ precipitates, particularly in the 5 at.% Cu alloy. In contrast, the three-stage treatment dissolved the Ni₄Ti₃ precipitates, suppressed the formation of B19′ and R phases, and stabilized a highly ordered B2 matrix. Increasing the Cu content from 5 to 7 at.% significantly enhanced the B2 phase fraction, reduced secondary nickel-rich phases, and improved structural homogeneity, evidenced by a continuous neck network and closed porosity. The optimized condition—7 at.% Cu combined with the three-stage annealing—produced a microstructure with >95% B2 phase, <1% Ni₄Ti₃, and ~98% relative density. This forms the prerequisite microstructural state for a narrow transformation hysteresis and high functional cyclic stability. Full article

During active brazing of alumina ceramics, active elements react with the ceramic to form a reaction layer, which has significant influence on the mechanical property of the brazed joint. However, the composition and formation mechanism of this layer remain unclear among researchers. To fill this gap, different brazing temperatures (900–1100 °C) and heating rates (2.5 °C/min and 10 °C/min) were used to braze 95% Al₂O₃ ceramics and a Kovar 4J34 alloy using a Ag-Cu-2Ti active brazing filler, and the microstructure and mechanical properties of the joints were investigated. The results show that the joint could be divided into five layers: Al₂O₃, ceramic-side reaction layer, filler layer, Kovar-side reaction layer, and Kovar. The ceramic-side reaction layer could be further divided into a Ti-O-rich layer and an intermetallics (IMC)-rich layer, and the Kovar-side reaction layer consists of TiFe₂ particles, Ag-Cu eutectic, and the remaining Kovar. A belt-like TiFe₂+TiNi₃ IMC could be found in the filler layer. Increasing the brazing temperature enlarged the belt-like TiFe₂+TiNi₃ IMC in the filler layer and increased the thickness of the IMC-rich layer in the ceramic-side reaction layer, but had no significant effect on the thickness of the Ti-O-rich layer in the ceramic-side reaction layer. A lower heating rate (2.5 °C/min) was found to suppress the formation of the IMC-rich layer and shift the fracture location in shear tests from the ceramic-side reaction layer to the filler layer, indicating that the strength of the ceramic-side reaction layer was enhanced by controlling the formation of the IMC-rich layer. A maximum shear strength of 170 ± 61 MPa was obtained at a heating rate of 2.5 °C/min and a brazing temperature of 940 °C. Full article

Laser powder bed fusion (LPBF) technology offers an effective approach for fabricating high-performance superelastic NiTi alloys. This study achieved Ni_51.9Ti_48.1 alloys with outstanding superelastic properties through a triple optimization design of the initial powder composition, printing process parameters, and post-processing. The phase transformation behavior and microstructure of the alloys were systematically investigated. The results indicate that as energy density increases, the size and quantity of pore defects in LPBF-fabricated Ni_51.9Ti_48.1 alloys increase, phase transformation temperatures rise, and hardness conversely decreases. Ni_51.9Ti_48.1 alloys produced at lower energy densities exhibit fewer dislocations. After annealing at 600 °C, Ni₄Ti₃ and R phases form internally, resulting in a maximum superelasticity of 6.64%. Conversely, Ni_51.9Ti_48.1 alloys produced at higher energy densities exhibited a large number of dislocations and formed subgrains after annealing at 600 °C. Additionally, due to the high void volume fraction, they demonstrated deteriorated superelasticity. Full article

NiTi alloys are widely used in biomedical applications due to their shape memory and superelastic properties. However, their surface reactivity requires protective, biofunctional coatings. To enhance NiTi performance, its surface was modified with an Ag-SiO₂-TiO₂ nanocoating containing small amounts of silica and silver. The coating’s primary phase was rutile with structural defects and a silver solid solution. It showed good adhesion, high scratch resistance, and improved corrosion behavior in Ringer’s solution, as demonstrated by EIS and cyclic polarization. EIS revealed high low-frequency impedance and two time constants, suggesting both barrier protection and slower electrochemical processes. Despite low breakdown and repassivation potentials, the coating effectively limited uniform corrosion. SEM/EDS confirmed localized degradation and partial substrate exposure, while elemental mapping showed well-dispersed silica and silver in a TiO₂-rich matrix. The proposed pitting mechanism involves chloride-induced depassivation and galvanic effects. Surface potential mapping indicated electrostatic heterogeneity, mitigated by silica. The coating offers a balanced combination of corrosion protection and biofunctionality, supporting its potential for implant use. Full article

This study investigates the effect of Al_x and Ti_x content (x = 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) on the microstructural evolution and mechanical properties of Co-free high-entropy Al_xTi_xCrFe₂Ni alloys in both as-cast and homogenized conditions. The research focused on the characterization of structural features, melting behavior, and mechanical performance. Microstructural characterization was carried out using optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and differential thermal analysis (DTA). Mechanical properties were evaluated through Vickers hardness testing and uniaxial compression tests. Increasing the Al and Ti content induced a transformation from a single-phase FCC structure to a dual-phase BCC structure, with the primary BCC phase strengthened by spherical precipitates rich in Al, Ti, and Ni. Homogenization annealing at 1100 °C led to an overall improvement in the mechanical properties. The Al_0.3Ti_0.3CrFe₂Ni alloy exhibited the most balanced combination of strength and ductility after annealing, achieving a compressive yield strength of 1510 MPa, a compressive strength of 3316 MPa, and a compressive plastic strain of 45%. Full article

This research investigates the effects of the addition of the rare-earth element La on the microstructure, phase transformation, and mechanical properties of Ni₅₀Ti₃₀Hf_20−xLa_x (x = 0, 0.5, 1, 2) alloys. The results show that a primary matrix composed of Ni-Ti-Hf and featuring La-rich second phases formed. The temperature at which the martensitic phase transformation starts decreases with an increase in La content. As the amount of La increases, hardness decreases slightly, while the elastic modulus increases. Full article