Binary NiTi Alloy Research Articles

A comparative study of the microstructure and shape memory behaviour of NiTi, NiTiHf, and Pt-added NiTiHf alloys

The microstructure, substructure, transformation temperature, and recovery ratio of a novel Ni45Pt 5 Ti30Hf20 alloy are reported in the current work. The properties of binary TiNi and ternary TiNiHf alloys are also characterized to establish a comparative basis for shape memory behaviour. The microstructure of the cast structure of Ni45Pt5Ti30Hf20 alloy showed a dendritic matrix and interdendritic eutectic-like mixture, and that of the homogenized structure consisted of martensitic matrix and secondary phase of dark contrast having the composition (Ti+Hf)2(Ni+Pt). Pt substitution of Ni in the ternary alloy with 20 at.% Hf by 5 at.% leads to a decrease in the Af, As, Ms, and Mf . Ms and Mf represent the temperatures at which austenite to martensite transformation starts and completes, respectively, while As and Af represent the temperatures at which martensite to austenite transformation starts and completes, respectively. The XRD studies showed that the structure of martensite is B19′ and TEM studies showed that the substructure of martensite in Pt modified NiTiHf alloy is similar to that of ternary NiTiHf alloy. The shape memory recovery of the Pt-modified alloy is similar to the ternary alloy at higher strength levels. The recovery ratio determined using Vickers indentation is compared with that determined using compression tests to provide an efficacy of small volume tests for screening of these alloys.

Hot deformation behavior of NiTiHf alloy under compression: Effect of deformation heating on flow softening

High-temperature shape memory alloys are suitable materials for substituting hydraulic elements in the aerospace industry due to light weight and high reactive stresses developing during thermoelastic martensitic transformation and acceptable recoverable strain. These properties may be improved by plastic deformation; however, the plastic deformation of NiTiHf alloys along with structure investigation has yet to be studied systematically. Here, the hot deformation behavior of a NiTiHf alloy with hafnium content >20 at.% has been investigated, and the processing map has been elaborated according to the dynamic material model (DMM) based on the Prasad instability criterion. The deformation heating effect has been shown to affect the flow curves and accounts for profound softening during deformation due to quasi-adiabatic deformation mode at high strain rates. NiTiHf alloys have a much higher activation energy of plastic flow, and the instability region of plastic flow is much larger compared to binary NiTi alloys. Microstructural observations have revealed that in the instability regions, oxide bands and crack nucleation occur, whereas outside this region, such defects are not observed. Electron backscatter diffraction has demonstrated that at high temperatures and slow strain rates, discontinuous dynamic recrystallization occurs resulting in the necklace-type structure, while at high strain rates, this phenomenon has not been observed. Based on the obtained results, the preferable deformation conditions are established to be 850–1000 °C and 0.003–0.05 s−1.

Microstructure and Phase Transformation Behavior of NiTiCu Shape Memory Alloys Produced Using Twin-Wire Arc Additive Manufacturing

NiTiCu thin walls were produced by twin-wire arc additive manufacturing (T-WAAM) using commercial NiTi and Cu wires as the feedstock materials. This approach aims to solve the problems typically associated with large phase transformation hysteresis in NiTi shape memory alloys. The microstructure, mechanical properties, and phase transformation behavior of the as-deposited NiTiCu alloy were comprehensively examined. The results revealed that the as-deposited NiTiCu alloy was well-formed, with its microstructure showed columnar, equiaxed, and needle-like grains, depending on the location within the deposited walls. The microhardness gradually increased from the first to the third layer. The Cu content was 20.80 at%, and Cu-based precipitates were formed in the as-deposited NiTiCu. The volume fractions and lattice parameters of the matrix and precipitates in the as-deposited NiTiCu material were analyzed using high-energy synchrotron X-ray diffraction. The martensitic phase was identified as a B19 crystal structure, and the as-deposited NiTiCu underwent a one-step B2-B19 phase transformation. The tensile strength and fracture strain were approximately 232 MPa and 3.72%, respectively. In particular, the addition of Cu narrowed the phase transformation hysteresis of the as-deposited NiTiCu alloy from 24.4 to 7.1 ℃ compared with conventional binary NiTi alloys. This study expands the potential of T-WAAM in modifying the phase transformation behavior of NiTi-based ternary alloys.

The influence of the chemical composition of the Ti-Hf-Zr-Ni-Cu-Co shape memory alloys on the structure and the martensitic transformations

Twelve senary alloys were manufactured with different concentrations of doping elements (Hf, Zr, Ni, and Cu) that varied from 1 to 17 at%, and the concentration of the Ni equivalent (Ni, Cu, and Co) atoms changed from 49 to 51 at%. It allowed the production of Ti-Hf-Zr-Ni-Cu-Co alloys with various configuration entropies. It was found that, regardless of the entropy value, all alloys crystallised in the B2 phase, in which the lattice parameter increased upon a rise in the doping elements’ concentration (X parameter) and was hardly affected by the Ni group concentration ([Ni] = Ni + Cu + Co). In the alloys with low and medium entropy, the chemical composition of the B2 phase was homogeneous and close to the composition of the alloy. In the alloys with high entropy, the core and edge of dendrite cells in the matrix were characterised by different concentrations of doping elements, while the concentration of the Ni group was constant. Besides the B2 phase, the secondary phase existed in all alloys, whose type [Ti]2[Ni] or [Ni]4[Ti]3 depended on the [Ni] value, as was observed in the binary NiTi alloys. It was found that the Ti-Hf-Zr-Ni-Cu-Co alloys with low and medium entropy underwent the B2 ↔ B19′ martensitic transformation on cooling and heating. An increase in the [Ni] and X values initiated the formation of the strain nanodomains on cooling before the forward transformation. In the high-entropy alloys with X = 10 at%, the strain nanodomains formed on cooling to − 180 °C without the formation of the B19′ phase. In the alloys with X = 17 at%, strain nanodomains, and martensitic transformation were not found on cooling, but the martensitic transformation was initiated by loading at − 100 °C. It was shown that the variation in the Ni group concentration affected the martensitic transformation parameters only in the low-entropy alloys. In the medium- and high-entropy alloys, the concentration of the doping element had a more significant effect on the transformation than the [Ni] value.

Role of Zr and thermomechanical treatment on phase transformation and functional properties of NiTi-based shape memory alloys

A systematic investigation was conducted to evaluate the role of Zr addition on the martensitic phase transformation behavior and properties of NiTi-based shape memory alloys. Equiatomic binary NiTi alloy and ternary Ni50Ti45Zr5 and Ni50Ti40Zr10 alloys (at.%) were processed by a simple thermomechanical treatment including cold rolling followed by post-deformation annealing at 450 °C for 30 and 60 min. The microstructural investigation and thermal analyses reveal that martensitic phase transformation is suppressed by adding Zr. Solid solution strengthening due to Zr addition, strain hardening due to cold rolling and formation of (Ti,Zr)2Ni precipitates in a dual-phase microstructure during annealing leads to a significant increase in hardness in the thermomechanically treated alloys (e.g., up to 550 Hv for Ni50Ti45Zr5 alloy). The stress-strain curve of martensitic Ni50Ti50 alloy reveals the essential characteristics of shape memory effect, however, Ni50Ti45Zr5 alloy shows typical features of pseudoelasticity with significantly high stress plateau (∼600 MPa) and remarkable recovered strain of 3.5% during unloading.

Influence of Current Density upon Hydrogenation on the Shape Memory Effect of Binary TiNi Alloy Single Crystals

Some results concerning the hydrogen effect at electrolytic saturation at a current density of j = 1500 and 3500 A/m2 for 3 h at room temperature on the temperature dependence of the yield stress σ0.1(T) and the shape memory effect (SME) under tension of the [011]-oriented Ti-50.55%Ni (at.%) alloy single crystals are presented. It was shown that hydrogen is in a solid solution and forms particles of titanium hydride TiH2 after hydrogenation at j = 1500 and 3500 A/m2, respectively. Both hydrogen in the solid solution and TiH2 particles led to a decrease in the Ms temperature of the onset of the forward martensitic transformation (MT) upon cooling and the Md temperature (Md is the temperature at which the stresses for the onset of the stress-induced MT are equal to the stresses for the onset of plastic flow of the high-temperature B2 phase), and increased the yield stress σ0.1 of the B2 phase at the Md temperature compared to hydrogen-free crystals. It was found that the SME under stress depends on the tensile stress level and current density. The maximum SME εSME = 10 ± 0.2% at σex = 200 MPa and εSME = 10.5 ± 0.2% at σex = 300 MPa was observed in the hydrogen-free crystals and after hydrogenation at j = 1500 A/m2, respectively, which exceeded the theoretical value of lattice deformation ε0 = 8.95% for the B2-B19′ MT in [011] orientation under tension. At j = 1500 A/m2, the physical reason for the excess of the SME of the theoretical ε0 value was due to the increase in the plasticity of B19′ martensite upon hydrogenation. At j = 3500 A/m2, εSME = 8.0 ± 0.2%, and it was less than ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension. The decrease in SME after hydrogenation at j = 3500 A/m2 was associated with the interaction of two types of B19′-martensite: oriented under stress and non-oriented, formed near TiH2 particles. It was shown that the redistribution of hydrogen in the bulk of the crystals during long-term holding for 168 h at 263 K after hydrogenation at j = 1500 A/m2 increases the SME relative to crystals without long-term holding: 3.5 times at 50 MPa and 1.8 times at 100–150 MPa. After long-term holding, εSME = 9.5 ± 0.2% at 150 MPa, which exceeds the theoretical value ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension.

Effect of Ternary Element Addition on Properties of TiNi-Based Shape Memory Alloys for Engineering and Medical Applications

Shape memory alloys (SMAs) are a type of smart material and have excellent engineering and medical applications. TiNi binary alloys possess remarkable shape recovery, mechanical properties, corrosion resistance, and excellent biocompatibility. By ternary elements addition just like Au, Pt, Pd, Hf, and Zr, increases transformation temperatures, leading to high-temperature shape memory alloys (more than 100°C) but other elements (Fe, Cu, Co, and Mo) form low-temperature shape memory alloys, (lower than 100°C). In the present work, it is reported that the effect of ternary element addition on microstructural properties, shape memory properties, mechanical properties, corrosion resistance, and biocompatibility of ternary shape memory alloys. Ag, Au, and Cu-based TiNi ternary alloys have excellent biocompatibility. The addition of ternary elements such as Ag and Nb increases corrosion resistance, Fe rises the hysteresis loop, Hf enhances thermal stability, and Mo raises workability.

Study of shape memory properties of Ti(50)Ni(50-x) Mx (M= Nb, Mo and Fe) alloys for biomedical applications using first principle approach

The discovery of TiNi binary alloys has revolutionised the scientific and industrial communities more so in biomedical applications due to their excellent shape memory properties and biocompatibility. When TiNi is exposed to an external stimulus such as stress or temperature change, the material is able to recover its original structure after significant or quasi-plastic deformation. The aim of this study was to investigate the shape memory properties of TiNi(50-x)-Mx (M = Nb, Fe, Mo) alloys. In this study, CASTEP was used to perform first-principles calculations and to predict shape memory behaviour and biomechanical properties of these alloys. The results showed a decrease in the formation energy of Ti8Ni7Fe1. The poisons ratio was increasing significantly to the value of 0.39 corresponding to the G/B results. Initial fermi levels increased with the addition of a ternary alloy. Ti8Ni7Fe1 proved to be the best candidate for replacing TiNi alloys.

Effect of Vanadium on the Microstructure, Transformation Temperatures, and Corrosion Behavior of NiTi Shape Memory Alloys

Abstract Ni50Ti50-xVx (x = 0, 1, 2, 3 at%) shape memory alloys were prepared by vacuum induction melting. They were homogenized and then hot rolled. Carbon hydrogen nitrogen oxygen sulfur (CHNOS) and X-ray diffraction (XRD) analyses were carried out on the alloys to find out the oxygen and carbon contents and the phases present in the alloys. Transformation temperatures determined by differential scanning calorimetry indicate that the addition of vanadium reduces the transformation temperatures. Corrosion studies were carried out in Hanks’ solution, while potentiodynamic polarization tests were done to calculate the rate of corrosion of the alloys. Two significant parameters were analyzed from the Tafel graph, namely, corrosion rate and corrosion potential. A comparison of these properties of the alloys was also made with commercially pure titanium and binary NiTi alloys. Among the NiTiV alloys, Ni50Ti47V3 (at%) alloy was found to undergo the least rate of corrosion. With the increasing vanadium content, the rate of corrosion was found to decrease. Scanning electron microscopy (SEM) analysis of the corroded surface shows that pitting was the main mechanism of corrosion in these alloys. Results show that the addition of V to NiTi has a positive effect on the corrosion properties of the alloys. Elaborate results are discussed in detail in this article.

Deformation induced martensite stabilization of NiTi in constrained composite systems

This study investigated the effect of tensile deformation on the transformation behaviour of NiTi within the constrained environment of NiTi–Nb composites. Particular attention is given to the deformation-induced stabilisation of the stress-induced martensite manifested as the increase of the reverse transformation temperature. Two different forms of the NiTi–Nb composites were used, including nanowires and microparticles, in comparison with the single-component binary NiTi alloy. This stabilisation effect has been reported for NiTi in the literature. The effect is attributed to the changes in the internal elastic and plastic states of the martensite variant structure caused by the deformation. The NiTi–Nb nanocomposites present different internal mechanics environments. It was found that the presence of a second phase within the NiTi matrix hinders the lattice distortion martensitic transformation, enlarging its hysteresis in the order of the binary NiTi, the NiTi–Nb microparticle composite and the NiTi–Nb nanowire composite. After deformation, the NiTi–Nb microparticle composite exhibited the highest stabilisation effect, followed by the NiTi–Nb nanowire composite and then the binary NiTi alloy. These contrasting observations are attributed to dual effects of the Nb inclusions within the NiTi matrix, to obstruct the lattice distortions of the martensitic transformation, and to retain elastic strains and thus stresses that may assist and resist a transformation depending on their relative directions.

Effect of welding current on the microstructure, residual stresses, mechanical properties and nano-mechanical behaviour of P-TIG welded TiNi binary shape-memory alloy

In the present work, an autogenous TIG welding technique was used to join 2 mm thick sheets of a TiNi binary shape-memory alloy at three different values of current (80 A, 90 A and 100 A). The effects of the welding current on microstructures, residual stresses, mechanical properties and nano-mechanical behaviour were investigated. Microstructure analysis revealed that with an increase in current, the columnar grain size decreased, which had a significant effect on mechanical properties. The tensile strength of the weldment at 100 A was ∼92% of that base alloy’s (BA) and elongation of approx. 14%. The reduction in elongation was due to the formation of Ti2Ni and Ti3Ni4 type precipitates. The microhardness profile in all the weldments showed an increase of approx. 30% between the base alloy and the fusion zone due to the formation of Ti2Ni and Ti3Ni4 type precipitates during welding. Residual stress analysis suggested the tensile residual stresses in the longitudinal direction are minimum in the weldment performed at 100 A. The nanoindentation results revealed that the weldment obtained at 100 A had the least plastic deformation (∼45.5% less than the 80 A weldment) owing to a decrease in inter-dendritic spacing and high proportion of IMCs: Ti2Ni and metastable Ni4Ti3.

Continuous Heating Dissolution and Continuous Cooling Precipitation Diagrams of a Nickel-Titanium Shape Memory Alloy

Binary NiTi alloys are the most common shape memory alloys in medical applications, combining good mechanical properties and high biocompatibility. In NiTi alloys, the shape memory effect is caused by the transformation of an austenite phase to a martensite phase and the reverse process. Transformation temperatures are strongly influenced by the exact chemical composition of the NiTi phase and the presence of precipitates in the microstructure induced by thermo-mechanical treatment, especially solution annealing and ageing. Isothermal time–temperature precipitation diagrams can be found in the literature. Cooling is frequently not considered, as water quenching is typically assumed to be sufficient. To the best of our knowledge, continuous heating dissolution (CHD) and continuous cooling precipitation (CCP) diagrams do not exist. Differential scanning calorimetry (DSC) is a common method to analyse the austenite/martensite transformation in shape memory alloys, but it has not yet been used to analyse precipitation processes during continuous temperature changes. We have enabled DSC to analyse dissolution and precipitation processes in situ during heating as well as during cooling from the solution annealing temperature. Results are presented as CHD and CCP diagrams, including information from microstructure analysis and the associated changes in the austenite/martensite transformation temperatures.

Dry Sliding Wear and High-Velocity Impact Behaviour of Spark Plasma Sintered Ti-Ni Binary Alloys

This work investigated the dry sliding wear behaviour of spark plasma sintered (SPSed) Ti-Ni binary alloys produced at varying nickel content with alloy steel ball as the counterface material, at room temperature under varied applied normal loads. Finite element modeling was used to investigate the high-velocity impact response of the sintered alloys due to the dimensional constraint associated with SPSed samples. Microstructural analysis results revealed the presence of intermetallic phases of Ti-Ni with increasing nickel content. The best wear resistance ranging from 0.25 x 10-3 mm3/Nm to 0.22 x 10-3 mm3/Nm across all applied loads was obtained in Ti-6Ni alloy. This was attributed to the compaction of the protective triboxide and carbide layers on the surface of the sample. Oxidative and wear by adhesion were observed at low applied normal load while at high loads the prevalent wear mechanism was abrasive with reduced influence of oxidative and adhesive wear. Finite element analysis results also showed that Ti-6Ni alloy possessed the optimum combination of absorbed energy and ductility to reduce the possibility of brittle failure under impact loading. Keywords: Ti-Ni binary alloys; Spark plasma sintering; Dry sliding wear; High-velocity impact; Finite element analysis.

Fabricating TiNiCu Ternary Shape Memory Alloy by Directed Energy Deposition via Elemental Metal Powders

In this paper, a TiNiCu shape memory alloy single-wall structure was fabricated by the directed energy deposition technique with a mixture of elemental Ti, Ni, and Cu powders following the atomic percentage of Ti50Ni45Cu5 to fully utilize the material flexibility of the additive manufacturing process to develop ternary shape memory alloys. The chemical composition, phase, and material properties at multiple locations along the build direction were studied, using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Vickers hardness testing, tensile testing, and differential scanning calorimetry. The location-dependent compositions of martensitic TiNi and austenitic TiNi phases, mechanical properties, and functional properties were investigated in detail. Variations were found in atomic compositions of Ti, Ni, and Cu elements along the build direction due to the complex interaction between elemental powders and laser processing. Good correlations were present among the chemical composition, phase constituent, hardness, and feature of phase transformation temperatures at various locations. The ultimate tensile strength of the as-deposited TiNiCu alloy is comparable with the previously reported additively manufactured TiNi binary alloys. By adding Cu, a much lower thermal hysteresis was achieved, which shows good feasibility of fabricating ternary TiNiCu shape memory alloys, using elemental powders in the directed energy deposition to adjust the thermal hysteresis.

Novel eutectoid Ti-5Ni alloy fabricated via direct energy deposition

Direct energy deposited (DED) Ti-5Ni (wt.%) alloys promote significant grain refinement with fine equiaxed grains (~30–50 μm); they have discontinuous grain boundary α, fine eutectoid α laths (~1 μm width), and α + Ti2Ni phases. Post-heat treatment below beta transus for 24 h stimulates the formation of near equiaxed α, and the temperature significantly influenced the microstructural constituents, which included pre-eutectoid α + retained β + Ti2Ni (760°C), pre-eutectoid α + α’ + Ti2Ni (800°C), and fully α’ + Ti2Ni (850°C) phases. The tensile properties of the as-fabricated and the heat-treated alloy remarkably improved compared with the other conventionally processed eutectoid Ti-Ni binary alloys. Thus, the proposed approach opens up new opportunities and possibilities for fabricating other eutectoid-forming alloy systems with improved manufacturing efficiency, which can overcome limitations in the other manufacturing processes.

Development of Ni44Ti35Zr15Cu6 Quaternary Shape Memory Alloy: Experimental and Density Functional Theory Studies

The effect of Zr and Cu additions on the phase stability, microstructure and martensitic transformation temperature of binary NiTi alloys has been investigated with a combined approach of experimental measurements and first principle density functional theory calculations. In this work, multilayer Ni/Ti/Zr/Cu films have been deposited using the magnetron sputtering on Si substrates at room temperature and subsequently annealed at 350 °C for complete inter-diffusion of multilayers to create amorphous phase followed by high temperature annealing at 600 °C for 5 min to achieve Ni44Ti35Zr15Cu6 shape memory alloys. The high temperature annealing is required for the formation of crystalline phases. The x-ray diffraction pattern of the film annealed at 600 °C indicated the presence of martensite phase. The surface and interface studies were performed focusing on morphology and interlayer diffusion over the nano-level structure at 600 °C. The elemental mapping showed near-uniform distribution of constituent elements after annealing. Atomic force microscopy results showed the increase in surface roughness at annealing temperature of 600 °C due to grain growth. The formation energy results indicate that the Zr and Cu additions to NiTi favor the formation of monoclinic B19’ phase. The differential scanning calorimetry results show that the martensitic transformation temperature increases with Zr and Cu additions. Nanoindentation studies for Ni44Ti35Zr15Cu6 films showed an increase in depth recovery for the crystalline film. A theoretical model has been used, to determine the hysteresis of shape memory alloys as predicted by the geometric nonlinear theory of martensite.

Shape Memory Behavior of Rapidly Quenched High-copper TiNiCu Alloys

Rapidly quenched quasibinary TiNi–TiCu system alloys with high copper contents (above 20 at.%) exhibit excellent shape memory effect and have considerably narrower hysteresis as compared with the TiNi binary alloy, this advantage being of special importance for cyclic load applications, e.g. for microelectromechanics (MEMS). The aim of this work is to study the effect of annealing parameters and copper content on the shape memory effect in TiNiCu alloys. Thin amorphous ribbons of TiNi-TiCu alloys with copper contents of 25 to 40 at.% were produced by planar flow casting at a melt cooling rate of about 106 K/s. The alloys were crystallized by isothermal annealing with variable duration and by exposing specimens to a short (10 ms) electric pulse. Increasing the copper content to above 30 at.% considerably reduces the plasticity and shape memory effect of the alloys. However, significant reduction of annealing duration greatly improves the shape memory performance due to prevention of the formation of brittle Ti-Cu phases in the alloys structure.

Heat treatment – microstructure – hardness relationships of new nickel-rich nickel-titanium-hafnium alloys developed for tribological applications

The effects of various heat treatments on the microstructure and hardness of new Ni56Ti41Hf3 and Ni56Ti36Hf8 (atomic %) alloys were studied to evaluate the suitability of these materials for tribological applications. A solid-solution strengthening effect due to Hf atoms was observed for the solution annealed (SA) Ni56Ti36Hf8 alloy (716 HV), resulting in a comparable hardness to the Ni56Ti41Hf3 alloy containing 54 vol.% of Ni4Ti3 precipitates (707 HV). In the Ni56Ti41Hf3 alloy, the maximum hardness (752 HV), achieved after aging at 300°C for 12 h, was attributed to dense, semi-coherent precipitation of the Ni4Ti3 phase. Unlike the lenticular morphology usually observed within binary NiTi alloys, a “blocky” Ni4Ti3 morphology formed within Ni56Ti36Hf3 due to a smaller lattice mismatch in the direction normal to the habit plane at the precipitate/matrix interface. The maximum hardness for Ni56Ti36Hf8 (769 HV) was obtained after applying an intermediate aging step (300 °C for 12 h) followed by normal aging (550 °C for 4 h). This two-step aging treatment induces dense nanoscale precipitation of two interspersed precipitate phases, namely H-phase and a new cubic Ni-rich precipitate phase, resulting in the highest hardness exhibited yet by this family of alloys. The composition and structure of this new precipitate phase was characterized using atom probe tomography and transmission electron microscopy techniques to be cubic with a lattice parameter of a = 8.87 Å and symmetry belonging to one of the primitive subgroups of m3¯m with approximately Ni61.5Ti31Hf7.5 composition .

Thermal, microstructural and elastic modulus behavior of Ti50Ni50−xNbx (x = 0–25%at) shape memory alloys obtained by plasma arc melting

Ternary alloys based on the NiTi system with the addition of Nb have attracted attention for several areas of engineering, due to improvements in the shape memory properties. In this work, six TiNiNb compositions were obtained by plasma arc melting under controlled atmosphere, with Nb contents of 5%, 10%, 15%, 20% and 25% Nb in the Ti50Ni50−xNbx system (% at). This work aims to evaluate the influence of Nb content on the microstructure, phase transformation and elastic modulus behavior of the ternary alloy compared to the equiatomic binary NiTi alloy. Differential scanning calorimetry (DSC) show that after a heat treatment of 900 °C for 1 h, all studied compositions present a single-step phase transformation during cooling and heating. Nb addition has a limited effect over phase transformation temperatures, with the onset of martensite transformation remaining practically constant. The microstructure of the alloys is constituted by B2 NiTi austenite matrix in the samples with 0% and 5% Nb, and by B19′ NiTi martensite in the samples with 10–25% Nb, all surrounded by an eutectic phase rich in Nb (β-Nb). Scanning electron microscopy (SEM) revealed microstructures with the presence of dendrites with a large amount of β-Nb that evolved in size with increasing Nb content. The Nb addition and consequent microstructure changes contributed to the decrease in the average hardness, going from 527 HV in NiTi to values between 259 and 396 HV in TiNiNb alloys. The Nb content, however, had a weaker effect over the highest modulus of elasticity, which was highest in the Ti50Ni30Nb20 alloy (66 GPa) and lowest in the Ti50Ni45Nb5 alloy (52 GPa), representing a less than 10% variation.

Role of Nickel Content in One-Way and Two-Way Shape Recovery in Binary Ti-Ni Alloys

The shape recovery characteristics of titanium nickelide with an Ni content of 50.0 at % and 50.7 at % were studied in a wide range of structures obtained as a result of cold drawing with an accumulated true strain of e = 0.52 and subsequent annealing in the 250 to 700 °C temperature range. Shape memory effect (SME) inducing was carried out by bending using a non-isothermal loading mode, which made it possible to reveal implementing elastic strain in the equiatomic alloy up to 12% and thereby increase the total shape recovery by a factor of 1.5. The obtained results prove that the Ni content strongly affects the value and specific features of changes of the shape recovery characteristics with loading strain as well as grain/subgrain size. In equiatomic alloy, the total recovery strain manifests its maximum of 13.5–15% and the recovery strain of 9% at a loading strain range of 12 to 14%. In Ni-rich alloy, the total recovery strain manifests its maximum of 20% and the recovery strain of 14% at a loading strain range of 15 to 21%. The maximum two-way SME value correlates with the residual strain in both alloys and reaches its maximum of 3.0% in a material with a recrystallized structure. Varying the loading strain value under bending in the 11 to 21% range allows regulation of the temperature of shape recovery in Ni-rich alloy in the 45 to 80 °C range.