Ni-rich NiTi Alloys Research Articles

Enhanced mechanical properties and structural stability of Hf-modified NiTi alloy for advanced bearing materials

Ni-rich NiTi alloys suitable for bearing materials exhibit excellent properties such as high hardness, low modulus, non-magnetic, and corrosion resistance. In this study, the addition of Hf elements enhanced the mechanical properties and improved the structural stability of the alloys. The results indicate that the aging-treated Ni55Ti37Hf8 alloy demonstrates outstanding properties, with a Vickers hardness of 738.7 ± 12.5 HV, a nano-hardness of 9.6 ± 0.3 GPa, and an elastic modulus of 106.1 ± 2.2 GPa. The microstructure is primarily composed of the NiTi, Ni4Ti3 phase, and the H-phase. The structural stability of Ni55Ti37Hf8 alloy reached unprecedented levels, with hardness remaining at 673.1 ± 4.6 HV after heat treatment at 600°C for 100 h, meeting the requirements for bearing materials (exceeding 58 HRC). The Ni55Ti37Hf8 alloy exhibits superior thermal stability, making it a revolutionary option for the development of next-generation wear-resistant materials.

The Effect of Ageing on Phase Transformations and Mechanical Behaviour in Ni-Rich NiTi Alloys.

In this article, the results of research on a NiTi alloy with a high nickel content (51.7 at.%), produced using the additive technology SLM method and subjected to isothermal ageing after solution annealing, are presented. The study involved the determination of the sequence of phase transformations occurring using differential scanning calorimetry (DSC) and the determination of the temperature range of these transformations. In parallel, the phase composition was determined using the XRD method; the hardness and the Young's modulus were also determined. The analysis of the DSC results obtained indicates the following characteristic features of the NiTi alloy, which change with ageing time: (1) During cooling (from +150 °C to -50 °C), the type of transformation changes from a one-step transformation after solution annealing to a two-step transformation after the ageing process over 1, 20, and 100 h at 500 °C; (2) during heating (from -50 °C to +150 °C) for all the samples, regardless of the ageing time, only a one-step transformation from martensite M(B19') to austenite A(B2) is observed; (3) the temperature at which the transformation starts increases with the ageing time; (4) the width of the total temperature range of the transformation M(B19') → A(B2) during heating changes from large (ΔT = 49.7 °C), after solution annealing, to narrow (ΔT = 19.3 °C and ΔT = 17.9 °C after 20 h and 100 h of ageing); and, most importantly, (5) a comparison with the literature data shows that, irrespective of the composition of the NiTi alloy and the manufacturing technology of the alloy samples (regardless of whether this was traditional or additive technology), a sufficiently long ageing process period leads to the occurrence of the martensite → austenite transformation in the same temperature range.

Dynamic fracture behavior and its correlation with solidified microstructure of laser powder bed fused Ni-rich NiTi alloys under the impact loading

A Ni-rich NiTi alloy sample was fabricated by laser powder bed fusion and its solidified microstructure included a strong texture orientation of 〈001〉B2//BD and uniformly dispersed Ti4Ni2Ox nanoparticles. The drop weight tearing test was then performed to reveal its impact fracture behavior. The force–displacement curve presented a typical three-stage feature and the steady crack propagation rate reached ∼53.087 m/s. The fracture surface was predominated by the cleavage river pattern with a cleavage direction roughly parallel to the texture orientation. The initiation and propagation of cleavage crack could originate from dislocation pile-ups on {011}B2 plane and micro voids formed at the matrix/nanoparticle interface.

Microstructure-property relationship in Zr-alloyed Ni-rich NiTi alloys: Enhancements in high-temperature stability and superelasticity

This study delves into the influence of the Zr element on the thermal stability and superelasticity of Ni-rich NiTi(Zr) alloys. The inclusion of Zr heightened the hardness, reshaped the morphology of the Ni4Ti3 phase, and promoted the formation of nanodomains. The Zr-containing alloys demonstrate remarkable thermal stability, as evidenced by the hardness of 3Zr alloy remaining at 540.6 HV after 100 h of exposure to 600 °C. This marks a substantial 63.9% improvement compared to the 0Zr alloy. After treatments at 400 °C and 500 °C, the 3Zr alloy precipitated the H-phase. Following heat treatment at 600 °C for 100 h, the microstructure of the 0Zr alloy primarily consisted of Ni3Ti2 and Ni3Ti phases, whereas the 3Zr alloy was mainly composed of Ni4Ti3, Ni3Ti2, and Ni3(Ti, Zr)2 phases. The 3Zr alloy showcases exceptional superelastic properties, enduring a cyclic load of 1200 MPa for 5 times while maintaining a recovery rate exceeding 90%. The microstructure of the loaded 3Zr alloy consisted of Ni4Ti3 and nanodomains, exhibiting stable structural characteristics. Relevant mechanisms are discussed, laying the foundation for the application of Ni-rich NiTi alloys.

Transformation- and stress-temperature behavior of hot-drawn Ni-rich NiTi wire after isochronous aging

Aging temperature in the range of 300–500 °C (1 h) strongly affects transformation and mechanical behavior of Ni50.8Ti alloy wire samples obtained as a result of hot drawing at test temperatures of −196 ≤ T ≤ 60 °C. Isochronous aging of hot-deformed Ni-rich NiTi alloy forms a mixed substructure that consists of grains and subgrains, their average size increases from 0.9 to 3.3 μm vs temperature growth. In the aging range of 430–500 °C the diameter of Ti3Ni4 precipitates grows from 10–20 nm to 0.1–0.2 μm along the grain/subgrain boundaries. The widest RsM − Af temperature range (Δ76°C) of the reverse MT is revealed after aging at 300 °C; the minimal RsM − Af temperature range (Δ10°C) – after aging at 500 °C. Aging-induced microstructure strongly affects stress–temperature behavior and Young’s modulus: the obtained regularities are associated with contribution of various competing factors.

Laser-Induced Forward Transfer of Ni-rich NiTi Alloys for Shape Memory Applications

Laser-Induced Forward Transfer of Ni-rich NiTi Alloys for Shape Memory Applications

On Structural Sensitivity of Young’s Modulus of Ni-Rich Ti-Ni Alloy

When developing bone implants, Young’s modulus is one of the primary characteristics of the material that should be considered. This study focuses on regulating the modulus of Ti-50.8 at.% Ni alloy by varying the grain/subgrain size as well as the initial structure using subsequent aging at 430 °C for 10 h. After post-deformation annealing (PDA), the temperature dependence of Young’s modulus exhibits a pronounced V-shaped character with a minimum at the onset temperature of the forward martensitic transformation, Ms, regardless of the structure. The grain/subgrain size of B2-austenite strongly affects the modulus magnitude. This effect is ambiguous for a material with a grain size range of 0.13–3 µm and depends on the test temperature. The effect of aging on the modulus reduction depends on the initial structure; it is most pronounced in an alloy with a relatively coarse grain size of 9 µm and brings a decrease of 3.8 times at a temperature of 37 °C. Aging of the initially recrystallized Ni-rich NiTi alloy makes it possible to obtain a вone-like elastic modulus of E = 12–13 GPa at an operating temperature of 37 °C. An ultrafine-grained substructure exhibits the same Young’s modulus values in the low temperature range from −100 to 25 °C.

Effect of microstructure on the superelasticity of high-relative-density Ni-rich NiTi alloys fabricated by laser powder bed fusion

Effect of microstructure on the superelasticity of high-relative-density Ni-rich NiTi alloys fabricated by laser powder bed fusion

Unconventional precipitation and martensitic transformation behaviour of Ni-rich NiTi alloy fabricated via laser-directed energy deposition

ABSTRACT In this study, we report an unconventional precipitation and martensitic transformation behaviour of directly aged Ni-rich NiTi alloys fabricated via laser-directed energy deposition (LDED). Ni4Ti3 particles precipitate uniformly under all ageing conditions and no traditional multiple-step martensitic transformations are observed. We conclude this unique behaviour to the intrinsic characteristics of the LDED technique, which are metastable microstructures and high residual stresses. On the one hand, these features make grain boundaries no longer a fevered location for precipitation and, on the other hand, significantly suppress the martensitic transformation when ageing at low temperatures (300°C/400°C). As the aging temperature increase (500°C), residual stresses release significantly, accompanied by the growth of Ni4Ti3 precipitates from several nanometres to 452 ± 181 nm with increased interparticle spacing. At the same time, reverse martensitic transformations change from two-step (B19′ → R → B2) to single-step (B19′ → B2).

Constant-strain tensile cyclic behavior and microstructure of Ni-rich NiTi(-Sc) alloys

The superelastic behavior of Ni52Ti48, Ni55Ti45, and Ni55Ti44.9Sc0.1 alloys after solution treatment was investigated by cyclic tensile tests, electron probe microanalyzer (EPMA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The Ni-rich NiTi(-Sc) alloys exhibited excellent superelastic properties and underwent stress-induced martensite transformation during tensile loading. The residual strain accumulated in the Ni52Ti48 alloy was approximately 0.05% after loading/unloading tests. Coarse martensite laths were observed, indicating incomplete recovery after unloading. In contrast, no residual martensite was found in Ni55Ti45 alloy and Ni55Ti44.9Sc0.1 alloy after unloading. This was due to the presence of a large amount of Ni4Ti3 phases, which cut off the continuity of the matrix and confined the martensite transformation in a narrow matrix, thus inhibiting the formation of large-size martensite. The recovery ratio of Ni55Ti44.9Sc0.1 alloy was maintained at 96.7% after 5 cycles at 2.5% tensile strain. Moreover, the tensile strength of Ni55Ti45 and Ni55Ti44.9Sc0.1 were 1567.8 and 1667.9 MPa, respectively, which represented an apparent improvement compared to Ni52Ti48 (1223.5 MPa). This was attributed to the increased Ni content, resulting in an increase in the volume fraction of the strengthening phases including the Ni4Ti3 phase and the Ni3Ti2 phase.

Simulation of recoverable strain variation during isothermal holding of the Ni51Ti49 alloy under various regimes

The aim of the present paper is to simulate the strain variation on isothermal holding of Ni-rich NiTi alloy under various regimes. The modified Likhachev-Volkov microstructural model and a new Nelder-Mead algorithm for the determination of the model parameters were used. To determine the model parameters, the experimental data obtained during holding of the Ni51Ti49 alloy under a stress of 200 MPa were chosen. Using these parameters, the strain variation on holding of the NiTi alloy under a stress in two regimes was simulated. It was shown that the modified Likhachev-Volkov microstructural model allowed one to calculate the isothermal strain variation on holding after cooling under a stress (Regime 1) and a good correlation between the theoretical and experimental results were found. At the same time, the simulation of the strain variation on holding after active deformation (Regime 2) did not fit to experimental data because the model did not consider the difference in the stored elastic energy in two regimes. It was shown that a decrease in the elastic energy stored during the transformation increased the strain during holding under a stress after active deformation and made the simulated curves to be close to the experimental.

Evidence for strain glass transition in a highly nickel-rich ferroelastic alloy

Highly Ni-rich NiTi alloys have attracted interest due to their unique mechanical and functional properties. Most research on these alloys has focused on mechanical properties under heat treatment. Here, we investigate the dynamic mechanical characteristics and microstructural evolution of the B2 matrix in Ni54Ti46 alloy. The findings demonstrate that softening in storage modulus is accompanied by the successive formation and constrained growth of nanodomains over a wide temperature range, indicating a strain glass transition occurs in the B2 matrix of Ni54Ti46 alloy.

Microstructure evolution of Ni-rich NiTi(-Hf) alloy during thermal oxidation at different temperatures

The oxidation behavior of Ni-rich NiTi(-Hf) alloy at the temperature range from 400 to 950 °C is investigated in this paper. For Ni-rich NiTi alloy, the results show that the oxidation kinetics follow the logarithmic law at 400 °C and obey the parabolic law from 500 to 950 °C. After high-temperature oxidation, the observed multi-layered scale structure consists of TiO2+Ti2O3+NiTiO3, Ni(Ti)+Ti2O3+rich Ti, and Ni3Ti from outside to the inside. The addition of the Hf element improves the high-temperature oxidation resistance. A schematic oxidation mechanism of Ni-rich NiTi(-Hf) alloy is proposed to explain the observed results.

Effect of Severe Plastic Deformation and Post-Deformation Heat Treatment on the Microstructure and Superelastic Properties of Ti-50.8 at.% Ni Alloy.

Severe plastic deformation via high-ratio differential speed rolling (HRDSR) was applied to the Ni-rich Ti-50.8Ni alloy. Application of HRDSR and a short annealing time of 5 min at 873 K leads to the production of a partially recrystallized microstructure with a small grain size of 5.1 μm. During the aging process for the annealed HRDSR sample at 523 K for 16 h, a high density of Ni3Ti4 particles was uniformly precipitated over the matrix, resulting in the formation of an R phase as the major phase at room temperature. The aged HRDSR sample exhibits excellent superelasticity and superelastic cyclability. This achievement can be attributed to an increase in strength through effective grain refinement and particle strengthening by Ni3Ti4 and a decrease in the critical stress for stress-induced martensite (B19') due to the presence of the R-phase instead of B2 as a major phase at room temperature. The currently proposed method for using HRDSR and post-deformation heat treatment allows for the production of Ni-rich NiTi alloys with excellent superelasticity in sheet form.

Effect of crystallographic anisotropy on phase transformation and tribological properties of Ni-rich NiTi shape memory alloy fabricated by LPBF

This article investigated the effects of crystallographic anisotropy on the microstructure, phase transformation and tribological properties of NiTi Shape memory alloy (SMA) fabricated by laser powder bed fusion (LPBF). The maximum tensile recovery strain of 4.13 ± 0.16% was obtained at room temperature by adjusting the hatch spacing. The contributions of metallurgical factors under different crystal orientations, such as active twinned correspondence variant pairs (CVP), Schmid factor, critical stress for the stress-induced martensitic transformation (SIMT), Yong's modulus and nanohardness to superelastic response and tribological properties was clarified through various building orientations (0°, 45°, 90°). The results indicated that the superelasticity of LPBF-fabricated NiTi alloy was crystallographic orientation-dependent. Sample 0° with a strong (001) texture possessed the highest recoverable superelastic strain of 7.91 ± 0.14%. We describe, in detail, how the sliding friction mechanisms such as delamination, adhesive and formation of iron base oxide layers were attributed to superelasticity and the friction pairs under different loading conditions and building orientations. This new understanding reveals how different LPBF-induced microstructures affect mechanical properties and wear, providing a powerful guide for the tribological design or application and tailoring the functional behavior in LPBF-fabricated Ni-rich NiTi alloy.

Effect of energy density on the superelastic property of Ni-rich NiTi alloy fabricated by laser powder bed fusion

Laser powder bed fusion (LPBF) has proved to be a promising process for tuning the microstructure and mechanical response in NiTi-shaped memory alloys using a varied energy density input by modifying the process parameters such as laser power or scanning speed. The microstructure, precipitation, chemical composition, and their relationship with austenite–martensite transformation temperatures (TTs) and superelastic properties have been extensively investigated in Ni-rich NiTi alloys processed at varying volume energy densities (52–110 J/mm3) through LPBF. A specific amount of Ni3Ti precipitation was observed and primarily located along the boundary and at the center of the melt pool, and nanoscale spherical NiTi2/Ni2Ti4OX precipitation was homogeneously dispersed in the B2 matrix and B19’ martensite, but no Ni4Ti3 was observed as frequently reported in the literature. Decreasing the volume energy density (VED) led to lower TTs and a higher recovery strain ratio. However, there is an exception for the sample built with the highest laser power of 250 W that possesses the largest Ni/Ti atomic ratio and therefore the highest superelastic property despite the mediated VED (∼87 J/mm3) employed. We concluded that the variation in TTs and superelastic properties among the samples is caused by the varying amounts of Ni, which causes competition among alloying elements in evaporation, condensation, and precipitation during LPBF.

Fabrication of Ni-Rich 58NiTi and 60NiTi from Elementally Blended Ni and Ti Powders by a Laser Powder Bed Fusion Technique: Their Printing, Homogenization and Densification.

Compared to the equiatomic or near-equiatomic NiTinol alloys, Ni-rich NiTi alloys are suitable to be employed in structural applications as they exhibit higher hardness and are dimensionally stable. This research aimed to process two different grades of Ni-rich NiTi alloys, 58NiTi and 60NiTi, from Ni–Ti powder mixtures having about 58 wt.% and 60 wt.% Ni, respectively. This was performed by a laser powder bed fusion technique. At the first stage of this research, the printability of the used powder mixtures was investigated by applying different sets of printing parameters. Two appropriate sets were then selected to print the samples. Microstructural study of the printed parts revealed the existence of inhomogeneity in the microstructures. In addition, depending on the applied set of parameters, some amounts of cracks and pores were also present in the microstructure of these parts. Postprinting hot isostatic pressing procedures, performed at different temperatures, were developed to cause the reaction of phases, homogenize the parts, and possibly eliminate the existing flaws from the samples. Effects of these applied treatments on the microstructure, phase composition, density, dimensional integrity, and hardness of parts were sequentially studied. In essence, 58NiTi and 60NiTi parts having phase compositions complying with those of the equilibrium phase diagram were obtained in this research. However, the mentioned cracks and pores, formed in the microstructure of as-printed parts, could not be fully removed by postprocessing treatments.

High temperature wear behavior of Ni-rich NiTi-based alloys

Ni-rich NiTi-based alloys have been regarded as promising candidates for tribological applications because of their high hardness and excellent corrosion resistance. In this work, the effects of aging and Hf addition on microstructure and unlubricated wear properties of Ni-rich NiTi alloys, Ni55Ti45 and Ni55Ti40Hf5 alloys, were investigated. Both the aging and addition of Hf increase the microhardness of Ni55Ti45 alloy. The aged Ni55Ti40Hf5 alloy shows the largest value of microhardness resulting from the combination of the precipitation strengthening of Ti3Ni4 and H phases, and the solid-solution strengthening of Hf atom. The wear resistance of the studied alloys can be improved by the aging and Hf addition. The coefficient of friction values obtained at 400 °C are lower than that obtained at room temperature. Among the studied alloys, the aged Ni55Ti40Hf5 alloy shows the lowest coefficient of friction. The studied alloys show the same wear mechanisms, i.e. abrasive and oxidative wear at 400 °C.

Influence of Annealing Treatment on Microstructure and Properties of Ni-Rich NiTi Alloy Coating Prepared by Laser Cladding.

NiTi alloys are widely known for their shape memory effect and super-elasticity. In this study, the laser cladding method was applied to prepare Ni-rich NiTi alloy coatings on 316L stainless steel substrate. The microstructure, phase composition, element distribution and phase transformation behavior of the coatings were investigated in as-fabricated and annealing-treated states. The results indicated that the recrystallized microstructure obtained and the content of Ni3Ti and Ti2Ni phases increased significantly with a rising annealing temperature. Annealing treatment also induced a decrease in the phase-transition enthalpy and a rise in the transformation temperature, even though no obvious martensite transformation was observed. This was suppressed due to the Fe element diffused from the substrate and was probably retarded by the mounting metallic compounds formed during annealing as well. The mechanical properties have also improved obviously; coatings annealed under 850 °C exhibited the highest microhardness of 839 HV, and the wear resistance of the coatings after annealing was enhanced with an 11% average wear mass loss reduction.

The role of Sc on the microstructures and mechanical properties of the Ni-rich NiTi alloy

The microstructure and mechanical properties of Ni-rich alloys with the addition of Sc range from 0.15 at% to 0.5 at% were investigated by X-ray diffractometer(XRD), electro-probe microanalyzer(EPMA), transmission electron microscopy(TEM), microhardness, room-temperature tensile properties, and room-temperature fracture toughness in this paper. The as-cast binary alloy and Sc modified ternary alloy are mainly composed of NiTi matrix phase, Ni4Ti3, and Ni3Ti2 phases. After solution treatment, the Ni3Ti2 phase is dissolved back into the matrix, and the alloy mainly consists of NiTi matrix phase and nanoscale Ni4Ti3 phases. The addition of Sc inhibits the precipitation of brittle Ti2Ni phases because O is prone to combine with Sc to form Sc2O3. Trace Sc addition has a weak effect on the microhardness, and it can improve the room-temperature tensile properties and fracture toughness of the alloy. Trace Sc addition moderately reduce the size of the Ni4Ti3 phases and change the aspect ratio of the Ni4Ti3 phases. The reduced precipitate size is responsible for the improvement of alloy mechanical properties.