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The effect of thermal cycling on the transformation behavior of a Ti–24Nb–1Mo alloy was investigated by means of electrical resistivity measurement, transmission electron microscopy (TEM), X-ray diffraction (XRD), tensile test and Vickers hardness tests. Electrical resistivity changes were not observed in all alloys. It indicates that thermally induced martensitic transformation does not take place in the alloys. After thermal cycling between 298 K and 77 K, clear X-ray diffraction peaks corresponding to ωath phase, which did not exist before thermal cycling, were observed. Volume fraction of ωath phase increased as increasing the number of thermal cycling. ωath phase formed during thermal cycling increased hardness of the alloy. Although thermally induced martensitic transformation did not occur in the alloys, superelastic deformation behavior was observed in the alloys. The superelastic recovery ratio decreased from 81% to 41% by increasing the number of thermal cycling, which came from the increase in the volume fraction of ωath phase.

The effect of annealing temperature and external stress on the thermal expansion of a Ti–23Nb–0.7Ta–2Zr alloy were investigated by means of thermal expansion tests under constant load and X-ray diffraction (XRD). Negative thermal expansion (NTE), which is a shrinkage during heating, was observed in both a cold rolled and annealed specimens. The intensity of (200)β peak decreased while that of (211)β peak increased as the annealing temperature increased. The difference in expansion rate between 50 °C and 250 °C is found to decrease with an increasing annealing temperature from 600 °C to 800 °C, above which it kept almost constant. The expansion rate decreased as the applied stress increased.

Electrochemical anodization of Ti–Ni alloys with different Ni composition was carried out in an ethylene glycol base electrolyte under the various conditions to investigate the effect of crystal structure and chemical composition of the Ti–Ni alloy. X-ray diffraction patterns indicated that Ti-48.0 Ni and Ti-49.0Ni alloys were the martensitic phase at room temperature, while Ti 50.6Ni and 51.0Ni were the austenitic phase. Self-organized nanotubular oxide layers were formed on four Ti–Ni alloys. The thickness of oxide layers increased with increasing anodization time and applied voltages. Energy-dispersive X-ray analysis indicated that the nanotubular oxide layers consist of two kinds of oxides, one of which is Titanium oxide and the other is Nickel oxide. These results indicate that the growth of nanotubular oxide layer formed Ti–Ni alloys are not affected by crystal structure, but by applied voltage and anodization time.

Ti-(50-x)Ni-xSi (at%) (x = 0.5, 1.0, 3.0, 5.0) alloy ribbons were prepared via melt spinning and their crystallization procedure and transformation behavior were investigated using differential scanning calorimtry, X-ray diffraction, and transmission electron microscopy. Ti-Ni-Si alloy ribbons with Si content less than 1.0 at% were crystalline, whereas those with Si content more than 3.0 at% were amorphous. Crystallization occurred in the sequence of amorphous →B2 → B2 → Ti5Si4 + TiNi3 → B2 + Ti5Si4 + TiNi3 + TiSi in the Ti-47.0Ni-3.0Si alloy and amorphous →R → R + Ti5Si4 + TiNi3 → R + Ti5Si4 + TiNi3 + TiSi in the Ti-45.0Ni-5.0Si alloy. The activation energy for crystallization was 189 ±8.6 kJ/mol for the Ti-47Ni-3Si alloy and 212±8.6 kJ/mol for the Ti-45Ni-5Si alloy. One-stage B2-R transformation behavior was observed in Ti-49.5Ni-0.5Si, Ti-49.0Ni-1.0Si, and Ti-47.0Ni- 3.0Si alloy ribbons after heating to various temperatures in the range of 873 K to 1073 K. In the Ti-45.0Ni-5.0Si alloy, one-stage B2-R transformation occurred after heating to 893 K, two-stage B2-R-B19' occurred after heating to 973 K, and two-stage B2-R-B19' occurred on cooling and one-stage B19'-B2 occurred on heating, after heating to 1073 K.

Ti-50.3Ni-0.7Ag (at.%) scaffolds were prepared by sintering of rapidly solidified alloy fibers. The sintered scaffolds possessed high porosity of 80% and large pores of 150–600 μm in size. The yield strength of the scaffold after annealed at 723 K for 3.6 ks was about 2.4 MPa. Ti-Ni-Ag scaffolds had an elastic modulus of 0.8 GPa, similar to that of spongy bone, and exhibited good superelasticity with total recovery strain of 3.4% when 4% pre-strain was loaded at human body temperature. Stable superelastic behavior with complete shape recovery was observed during cyclic deformation in scaffolds.

In the present study, the effects of Sn content on microstructure, texture evolution, transformation behavior and superelastic properties of Ti–20Zr–9Nb‒(2–5)Sn (at.%) shape memory alloys were systematically investigated. After severe cold-rolling at room temperature, the sheet was annealed at 900 °C. A dominant α″ martensite and minor β phase were formed in the 2Sn alloy specimen. (Ti,Zr)5(Nb,Sn)3 s phase started to be detected in the 3Sn alloy specimen, and its area fraction increased from 0.02% to 1.75% with increasing Sn content from 3 at.% and 5 at.%. Increasing Sn content prevented recrystallization, affecting the texture evolution. A dominant ( 112 ) β [ 1 ‾ 3 ‾ 2 ] β and weak γ-fiber deformation textures were observed in the 5Sn alloy specimen and the 001 β 110 > β recrystallization texture was developed in the 3Sn alloy specimen. Also, increasing Sn content was found to effectively stabilize the β phase. 1 at.%‒Sn addition decreased the α″ → β reverse transformation start temperature (As) and finish temperature (Af) by 75 °C and 106 °C, respectively. The superelasticity was distinctly observed in the 900 °C annealed 5Sn alloy specimen at room temperature because its Af was below the room temperature.

LiCoO 2 thin films were fabricated using the metal-induced crystallization (MIC) method. The condition to create the MIC effect was investigated. The location and thickness of the Al coating used to manufacture the LiCoO 2 cathode thin film electrodes were varied, and the structural and electrochemical characteristics of the LiCoO 2 thin film electrode were investigated by Raman spectroscopy, field emission scanning electron microscopy and charge-discharge tests. The degree of crystallization of LiCoO 2 increased in the order of uncoated LiCoO 2 (lowest), Al(coating)/LiCoO 2 /Al foil, and LiCoO 2 /Al(coating)/Al foil (highest), for identical heat treatment condition (600 °C, 1 h). The crystallization of LiCoO 2 occurred more quickly as the Al thickness decreased, and it is considered that crystallization of LiCoO 2 is delayed owing to the formation of an alloy between Li-Al when there is more Al.

The structural and electrochemical properties of annealed Si film electrodes with titanium–nickel (TiNi) current collectors have been investigated to identify compounds and microstructures that improve electrochemical performances. The as-deposited Si/TiNi electrode consisted of amorphous Si (α–Si) and crystalline B2–TiNi alloy at room temperature. Only a thin oxide layer was formed at the interface, and any significant microstructural change was not found at 500 °C compared to the as-deposited electrode. The Ni atoms started to migrate from the TiNi alloy to the Si film at 600 °C, which resulted in the formation of a Ti-rich (Ti2Ni) layer at the interface. A large amount of Ni diffused into the Si film at 700 °C led to the formation of nickel silicide (NiSi2) particles being dispersed in the film. The Si/TiNi electrodes that were annealed at a temperature exceeding 600 °C showed improved electrochemical properties such as initial efficiency (~90%) and cycle performance (50% capacity retention after 300 cycles). The excellent electrochemical performance was achieved by enhancing the structural stability in annealed Si/TiNi electrodes.

Highly porous Ti-40Zr-8Nb-2Sn (at.%) shape memory alloy scaffolds were prepared by fiber metallurgy using rapidly solidified alloy fibers for the application of cancellous bone replacement. The phase constituents of alloy fibers and scaffolds were investigated by means of X-ray diffraction (XRD), superelastic behavior of alloy fibers was investigated by means of the dynamic mechanical analyzer (DMA), microstructures and mechanical properties of scaffolds were investigated by means of scanning electron microscopy (SEM) and compressive test, respectively. As-spun alloy fibers consisted of only β phase at room temperature. Clear superelastic behavior was observed in the as-spun alloy fibers at temperatures between 153 K and 298 K. The scaffolds showed three-dimensional networks with fiber-fiber bonds, whose porosity and pore size was about 80% and larger than 100 μm, respectively. The compressive yield stress and elastic modulus of the scaffold annealed at 973 K for 1.8 ks were about 4.0 MPa and 0.3 GPa, respectively, similar to those of cancellous bone. Recoverable strain in the scaffold was improved from 2.8% to 4.0% after annealed at 973 K due to the absence of α phase formed in the as-sintered scaffold. Stable cyclic superelastic behavior was observed in the annealed scaffolds at room temperature.

Ti 50 Ni 49.7 Mo 0.3 , Ti 50 Ni 49.6 Mo 0.4 and Ti 50 Ni 49.5 Mo 0.5 alloy fibers were prepared by a melt overflow process. Two-step B2-R-B19′ transformation was observed in the rapidly solidified Ti-Ni-Mo fibers. Upon increasing the Mo-content from 0.3 to 0.5 at.%, the austenite transformation finish temperature (A f ) of R → B2 decreased from 43 to −42 °C. Porous shape memory alloy pellets with 75% porosity were fabricated by a vacuum sintering technology, using the alloy fibers. Mechanical properties and shape memory effect of the highly porous Ti 50 Ni 49.6 Mo 0.4 alloy, of which A f is lower than room temperature, were investigated using a compressive test. The plateau of a stress-strain curve was observed at about 2 MPa and resulted in 5% elongation associated with stress-induced martensitic transformation. It was also found that a recovered strain was 1.8% on heating after the compressive deformation. Because of the high porosity of this specimen, an elastic modulus of 0.91 GPa could be obtained.

Ti-Ni-Ag scaffolds were prepared by sintering rapidly solidified alloy fibers. Microstructures and transformation behaviors of alloy fibers and scaffolds were investigated by means of electron probe micro-analyzer (EPMA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The B2-R-B19′ transformation occurs in alloy fibers. The alloy fibers have good superelasticity with superelastic recovery ratio of 93% after annealing heat treatment. The as-sintered Ti-Ni-Ag scaffolds possess three-dimensional and interconnected pores and have the porosity level of 80%. The heat treated Ti-Ni-Ag scaffolds not only have an elastic modulus of 0.67 GPa, which match well with that of cancellous bone, but also show excellent superelasticity at human body temperature. In terms of the mechanical properties, the Ti-Ni-Ag scaffolds in this study can meet the main requirements of bone scaffold for the purpose of bone replacement applications.

There has been a tremendous amount of research in the past decade to optimize the mechanical properties and degradation behavior of the biodegradable Mg alloy for orthopedic implant. Despite the feasibility of degrading implant, the lack of fundamental understanding about biocompatibility and underlying bone formation mechanism is currently limiting the use in clinical applications. Herein, we report the result of long-term clinical study and systematic investigation of bone formation mechanism of the biodegradable Mg-5wt%Ca-1wt%Zn alloy implant through simultaneous observation of changes in element composition and crystallinity within degrading interface at hierarchical levels. Controlled degradation of Mg-5wt%Ca-1wt%Zn alloy results in the formation of biomimicking calcification matrix at the degrading interface to initiate the bone formation process. This process facilitates early bone healing and allows the complete replacement of biodegradable Mg implant by the new bone within 1 y of implantation, as demonstrated in 53 cases of successful long-term clinical study.

Ni-free Ti-based shape memory alloy with superior superelasticity is promising and attractive for biomedical applications. In this study, Ti-18Zr-12.5Nb-2Sn (at.%) alloy fibers were prepared by rapid solidification method and then the effect of various heat treatments on phase constitutions, microstructures, mechanical properties, and superelasticity was investigated by means of X-ray diffraction (XRD), optical microscope (OM), transmission electron microscope (TEM) and tensile test. α phase was observed in the 673 K and 773 K annealed alloy fibers, leading to the degraded ductility. The total recovery strain at room temperature decreased, grain size and transformation temperature increased with increasing annealing temperature from 873 K to 1173 K. Athermal ω was observed in the 1173 K annealed alloy fibers. A good combination of clear superelasticity and high yield stress was achieved by annealing at 873 K followed by aging at 573 K due to the combined effect of the age-hardening caused by isothermal ω phase and small grain size, which indicates these alloy fibers have a good potential in biomedical applications.

Highly porous Ti–18Zr-12.5Nb–2Sn (at.%) scaffolds with porosities of 70%, 75% and 80% were prepared by sintering rapidly solidified alloy fibers. Superelastic behavior of alloy fiber was investigated by the dynamic mechanical analyzer (DMA). Phase identifications of alloy fiber and scaffold were studied by X-ray diffraction (XRD). Precipitates in alloy fiber were investigated by transmission electron microscope (TEM). Porous structure of alloy scaffold was observed by scanning electron microscope (SEM). Mechanical properties and the superelasticity of alloy scaffold were investigated using compressive test. The superelasticity of alloy fibers was improved after aged at 673 K for 3.6 ks due to α precipitates. The sintered scaffolds showed three-dimensional networks with fiber-fiber sintering joints. The compressive yield stress and elastic modulus of the alloy scaffolds aged at 673 K for 3.6 ks were in the range of 5.0–16.7 MPa and 0.33–1.05 GPa, respectively. These values were a close match to those for cancellous bone. The recoverable strain increased from 3.2% to 3.7% with the increase in porosity from 70% to 80% in aged Ti–Zr–Nb–Sn scaffolds when tested at human body temperature (310 K).

The microstructures and transformation behavior of Ti-49Ni-20Hf, Ti-49.5Ni-20Hf and Ti-50.3Ni- 20Hf alloys, when prepared by conventional casting, were investigated and compared with the properties of the alloys prepared by melt spinning. The area fraction of (Ti,Hf)2Ni in Ti-Ni-Hf alloys decreased to 3.9% from 9.4% as Ni content rose to 50.3 at% from 49 at%. Several cracks were observed in the hot-rolled Ti-49Ni-20Hf alloy sheet but none were found in the Ti-50.3Ni-20Hf alloy sheet. The B2-B19' transformation start temperature (Ms) decreased to 476 K from 580 K as Ni content increased to 50.3 at% from 49 at%. All the as-spun ribbons were amorphous, and the activation energy for crystallization ranged from 167.8 kJ/mol to 182.7 kJ/mol based on Ni content. When annealing temperature ranged from 810 K to 873 K, crystalline Ti-Ni-Hf alloys without (Ti,Hf)2Ni particles were obtained. At annealing temperatures higher than 873 K, very fine (Ti,Hf)2Ni particles, less than 20 nm in size, were found embedded in a crystalline matrix.

Microstructures and martensitic transformation behavior of Ti–Ni–Ag alloys prepared by arc melting were investigated by means of scanning electron microscopy (SEM), electron probe micro analysis (EPMA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermal cycling tests under constant load. Ti–Ni–Ag alloys consisted of Ti–Ni–Ag matrices, Ti 2 Ni and TiAg phases. Ti–Ni–Ag matrices contained 0.27–0.52 at.% of solute Ag atoms depending on alloy compositions. The B2–B19′ transformation occurred in Ti–50.1Ni–0.7Ag, Ti–49.2Ni–0.9Ag, Ti–49.2Ni–0.6Ag and Ti–49.0Ni–0.7Ag alloys, while the B2-R-B19′ transformation did in Ti–47.5Ni–1.3Ag and Ti–44.4Ni–1.1Ag alloys. Thermo-mechanical treatment separated the B2-R from the R–B19′ transformation clearly and improved shape recovery by increasing the critical stress for slip deformation in a Ti–50.0Ni–0.7Ag alloy.

β-type Ni-free Ti-based alloys are promising materials for biomedical applications. In this study, highly porous Ti-18Zr-12.5Nb-2Sn (at.%) alloy scaffolds with a porosity of about 80% were fabricated by a process of combining rapid solidification method and fiber metallurgy technique for the replacement of damaged human cancellous bone. Alloy fibers were produced by melt overflow technique. Phase identifications of alloy fiber and scaffold were studied by X-ray diffraction (XRD). Transformation behavior and microstructure of alloy fiber were investigated by differential scanning calorimetry (DSC) and transmission electron microscope (TEM), respectively. Superelastic behavior of alloy fiber was investigated by the dynamic mechanical analyzer (DMA). Porous structure of scaffold was observed by scanning electron microscope (SEM). The mechanical properties and superelasticity of scaffold were investigated by compressive test. The rapidly-solidified fiber consisted of β phase, athermal ω phase and nanodomains. Clear superelastic behavior with the recoverable strain of 4.5% for 5% pre-strain was observed in rapidly-solidified alloy fiber. Three-dimensional networks with fiber-fiber sintering joints were observed in the as-sintered scaffold which consisted of predominant β phase, α phase and isothermal ω phase. The scaffold annealed at 973 K consisted of single β phase. Stress-induced martensitic transformation of β→α" occurred during the compressive test. The compressive plateau stress and elastic modulus of the as-sintered and annealed scaffold were found to be 7.9 MPa and 0.43 GPa, 7.4 MPa and 0.41 GPa, respectively, similar to those of cancellous bone. A recoverable strain of 3.4% was observed in the annealed scaffold at body temperature (310 K).

In this study, Ti-xZr-8Nb-2Sn (x = 40, 45, 50) (at.%) alloys were fabricated by arc melting method and then microstructures, martensitic transformation behavior, superelasticity and mechanical properties were investigated by means of optical m icroscopy, electron microscopy, X-ray diffraction, tensile test and Vickers hardness test. The alloys consisted of β phase only at room temperature and exhibited strong (200)β texture after solution treated at 1173 K for 1.8 ks. Athermal ω phase was not observed from transmission electron microscopy in the alloys. The stress induced β→α″ transformation and the reverse transformation occurred on loading and unloading, respectively. Transformation temperature (Ms(C-C)) decreased from 188 K to 77 K with increasing Zr content from 40 at.% to 50 at.%. The maximum recoverable strains for 40Zr, 45Zr and 50Zr alloys were 7.1%, 7.5% and 7.3%, respectively, which was due to a combined effect of the large lattice deformation and a strong recrystallization texture. All alloys were fractured in ductile manner with fracture strains larger than 25%. Ultimate tensile strength of 835 MPa and superelastic recovery strain of 5.5% were obtained at room temperature in the solution treated Ti-40Zr-8Nb-2Sn alloy.

Stress vs. strain curves of two kinds of Ti–Ni shape memory alloys under dynamic compressive loading condition were obtained by the split Hopkinson pressure bar experiment. One is Ni-rich, 50.75Ni–49.25Ti (at%), alloy and the other is Ti-rich, 49.5Ni–50.5Ti (at%). The former exists as austenite phase to show superelastic behavior at the room temperature, and the latter is martensitic phase at the room temperature. The dynamic stress–strain curves were compared with quasi-static compressive test results. The Ni-rich specimen displayed a much higher stress level at the high strain rate than at the quasi-static test, but the Ti-rich specimen exhibited a stress level alike in the two types of tests. Temperature measurement on the surface of the impact specimens was carried out during the impact test. The temperature rising was about 3–5 °C in the considered test conditions.

This paper presents a three-dimensional calculation model for martensitic phase transformation of shape memory alloy. Constitutive model based on thermodynamic theory was provided. The average behavior was accounted for by considering the volume fraction of each martensitic variant in the material. Evolution of the volume fraction of each variant was determined by a rate-dependent kinetic equation. We assumed that nucleation rate is faster for the self-accommodation than for the stress-induced variants. Three-dimensional finite element analysis was conducted and the results were compared with the experimental data of Ti–44.5Ni–5Cu–0.5 V (at.%) alloy under bias loading.

Graphical abstract: - Highlights: • Amorphous Si-coated TiNi powders were prepared successfully by ball milling. • Ti{sub 4}Ni{sub 4}Si{sub 7} was formed at the interface between Si and TiNi after annealing. • Si-coated Ti–Ni powders displayed the R phase after annealing. - Abstract: Si was coated on the surface of Ti–49Ni (at%) alloy powders by ball milling in order to improve the electrochemical properties of the Si electrodes of secondary Li ion batteries and then the microstructure and martensitic transformation behavior were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ti–Ni powders coated with Si were fabricated successfully by ball milling. As-milled powders consisted of highly deformed Ti–Ni powders with the B2 phase and amorphous Si layers. The thickness of the Si layer coated on the surface of the Ti–Ni powders increased from 3–5 μm to 10–15 μm by extending the milling time from 3 h to 48 h. However, severe contamination from the grinding media, ZrO{sub 2} occurred when the ball milling time was as long as 48 h. By heating as-milled powders to various temperatures in the range of 673–873 K, the highly deformed Ti–Ni powdersmore » were recovered and Ti{sub 4}Ni{sub 4}Si{sub 7} was formed. Two-stage B2–R–B19′ transformation occurred when as-milled Si-coated Ti–49Ni alloy powders were heated to temperatures below 873 K, above this temperature one-stage B2–B19′ transformation occurred.« less

Three batches of Ti–Ni–Cu powders (Ti 50 Ni 45 Cu 5 , Ti 50 Ni 40 Cu 10 and Ti 50 Ni 30 Cu 20 ) were prepared by gas atomization and porous specimens were fabricated by spark plasma sintering (SPS). The microstructure of as-solidified powders exhibited a cellular structure. XRD analysis showed that one-step martensitic transformation of B2–B19 occurred in all alloy powders and SPS specimens. DSC measurements of as-atomized powder, sintered bulk specimens and as-cast ingots were performed in order to study the effect of rapid solidification on martensitic transformation behaviors. The dependence of powder size on martensitic transformation temperature is very small in the rapidly solidified powders. However, the transformation temperatures were strongly dependent on the Cu-content. The martensitic transformation starting temperatures ( M s ) of Ti 50 Ni 45 Cu 5 , Ti 50 Ni 40 Cu 10 and Ti 50 Ni 30 Cu 20 powders ranging between 25 and 150 μm are −4.9, −17.6 and 32.1 °C and the austenite transformation finishing temperatures ( A f ) of Ti 50 Ni 45 Cu 5 , Ti 50 Ni 40 Cu 10 and Ti 50 Ni 30 Cu 20 powders were 15.6, −4.4 and 38.3 °C, respectively. The M s and A f of the as-atomized powders are much smaller than those of SPS specimens and as-cast ingots for all Ti 50 Ni 45 Cu 5 , Ti 50 Ni 40 Cu 10 and Ti 50 Ni 30 Cu 20 alloy systems. The temperature hysteresis ( A f − M s ) of Ti 50 Ni 45 Cu 5 powders was 20 °C and continued to decrease with increasing Cu-content. The temperature hysteresis of Ti 50 Ni 30 Cu 20 powders was only 7 °C.

Microstructural features and martensitic transformation behavior of Ti–Ni–Sn alloys were investigated by means of electron probe microanalysis (EPMA), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). In a Ti–49.5Ni–0.5Sn alloy, Ti 2 Ni phase embedded in Ti–Ni matrix containing 0.7 at%Sn was observed. In a Ti–49.0Ni–1.0Sn alloy, Ti 2 Ni, unknown phase and Ti–Ni matrix containing 1.1 at%Sn were observed, while Ti 2 Ni and two different Ti–Ni matrices with 2.1 at%Sn and without Sn were observed in a Ti–50.0Ni–1.0Sn alloy. In a Ti–48.5Ni–1.5Sn alloy, Ti 2 Ni, Ti 3 Sn and Ti–Ni matrix containing Sn about 1.2 at% were observed. The B2–R–B19′ transformation occurred in Ti–49.5Ni–0.5Sn, Ti–49.0Ni–1.0Sn and Ti–48.5Ni–1.5Sn alloys, while the B2–B19′ transformation occurred in a Ti–50.0Ni–1.0Sn alloy. Substitution of Sn for Ni of an equiatomic TiNi alloy was effective to induce the R phase transformation.

Microstructures and martensitic transformation behavior of 52.7Ti–Ni, 52.0Ti–Ni, 51.1Ti–Ni and 49.7Ti–Ni (at%) alloy ribbons were investigated by means of optical microscopy, transmission electron microscopy, differential scanning calorimetry, X-ray diffraction and thermal cycling tests under constant load. Very fine Ti2Ni particles less than 20 nm which induced the B2-R transformation by suppressing the B2–B19′ transformation were observed in the ribbons. A temperature difference (ΔT) between TR* and Ms* almost kept constant with increasing Ti content from 49.7 at% to 52.0 at% (∼8 K), above which it increases largely (∼17 K), which was ascribed to the fact that volume fraction of Ti2Ni particles almost kept constant with increasing Ti content from 49.7 at% to 52.0 at% (∼8%), above which it increases largely (∼18%).

The effects of Cu addition, replacing Ni in a Ti50Ni25Pd25 alloy, on the microstructure, mechanical properties and shape memory characteristics are investigated. The addition of Cu slightly enhances the strength of B19 martensite through solid solution strengthening mechanism, without deteriorating the strain to fracture. The addition of Cu slightly increases the transformation temperatures but does not affect the thermal hysteresis (13 K). Results of thermal cycling tests under various constant stress levels reveal that the addition of Cu significantly increases the resistance against plastic deformation at high temperatures and at high stress levels (i.e. >450 MPa) indicating a relatively better creep resistance and stable cyclic response. TiNiPdCu alloys show better dimensional stability and demonstrate better recovery ratio upon thermal cycling as compared to that of ternary TiNiPd alloys.

Porous shape memory alloys have attracted great interest as low-weight materials with high energy dissipation properties. In the biomedical field, owing to their biocompatibility and their promise to exhibit high strength and low modulus, porous Ti–Ni alloys have been tested as bone implant materials, successfully exhibiting a significant amount of bone ingrowth. In order to use the excellent superelastic property of shape memory alloys in many medical applications, the austenite transformation finish temperature ( A f ) must be about 37 °C which is human body temperature. In this study, the transformation temperature could be controlled through the substitution of Mo for Ni in a Ti–Ni alloy. For the fabrication of bulk near-net-shape shape memory alloys and porous metallic biomaterials, Ti 50 Ni 49.9 Mo 0.1 and Ti 50 Ni 49.7 Mo 0.3 shape memory alloy powders were prepared by gas atomization and the transformation temperatures and microstructures of those powders were investigated as a function of powder size. The dependence of powder size on the martensitic transformation temperature is also very small in the powders ranging from 25 to 150 μm. The phase transformation temperatures of the porous specimens are almost the same as those of as-atomized powders and the A f of Ti 50 Ni 49.9 Mo 0.1 and Ti 50 Ni 49.7 Mo 0.3 porous specimens is 40.4 and 34.4 °C, respectively. According to compressive tests, the porous samples exhibit shape memory effect and it is found that the recovered stains of the Ti 50 Ni 49.9 Mo 0.1 and Ti 50 Ni 49.7 Mo 0.3 specimens are 1.5 and 2.0%, respectively.

Deformation behavior and shape memory properties of Ti–(20, 23) at.% Nb alloys in a single α″ martensite state were investigated. The Ti–20Nb alloy exhibited a higher stress for the reorientation of martensite variants when compared with the Ti–23Nb alloy. The recovery strain due to the shape memory effect in the Ti–20Nb alloy was smaller than that in the Ti–23Nb alloy. Transmission electron microscope (TEM) observation revealed that the reorientation of martensite variants occurred by the deformation of {1 1 1} type I and 〈2 1 1〉 type II twins. The Nb content dependence of the deformation behavior and shape memory properties was discussed considering the magnitude of twinning shear of the twins.

Amorphous Ti–36Ni–7Sn (at%) alloy ribbons prepared by melt spinning were annealed at various temperatures for crystallization and then microstructures and transformation behavior were investigated by means of electron microscopy, X-ray diffraction and differential scanning calorimetry. Crystallized Ti–36Ni–7Sn alloy ribbons consisted of Ti3Sn, Ti2Ni and TiNi phases. Size of Ti3Sn phase was in the range of 50–850 nm depending on annealing temperature. Average size of TiNi phase increased from 25 nm to 100 nm with raising annealing temperature from 873 K to 1073 K. Volume fraction of Ti2Ni phase decreased by raising annealing temperature through decomposition reaction of 3Ti2Ni + Sn (solved in TiNi phase) → 3TiNi + Ti3Sn and then Ti2Ni almost disappeared by annealing at 1173 K. The TiNi phase in crystallized Ti–36Ni–7Sn alloy ribbons showed the B2–R–B19′ transformation behavior. DSC peak temperature associated with the B2–R transformation (TR*) decreased from 303 K to 292 K with raising annealing temperature from 973 K to 1173 K, while DSC peak temperature associated with the R–B19′ transformation (Ms*) increased from 187 K to 237 K with raising annealing temperature from 1073 K to 1173 K and thus the temperature gap between TR* and Ms* decreases from 112 K to 55 K.

The combined effects of work hardening and precipitation strengthening were employed to improve the cyclic stability of TiNiPdCu-based high-temperature shape memory alloys. Annealing after cold deformation resulted in the formation of nano-scale TiPdCu and Ti2Pd precipitates, stable at high temperatures in Ti50Ni25−xPd25Cux alloys. The nano-scale precipitates were also observed to retard recovery/recrystallization processes at higher temperatures. It was found that the combined effects of work hardening and precipitation strengthening remarkably enhanced the high-temperature stability of the Ti50Ni20Pd25Cu5 alloy and increased its maximum working temperature range while keeping the transformation temperatures and recovery strains at sufficiently high levels. Precipitation strengthening helped to greatly improve the high-temperature cyclic stability of the alloy. Creep tests at 673 K under 500 MPa confirmed that the better high-temperature cyclic stability of the precipitate-containing alloy was mainly due to its higher creep resistance.

Crystallization behavior of rapidly solidified Ti–Ni–Sn alloys amorphous ribbons and martensitic transformation behavior after crystallization were examined by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Glass forming ability of Ti–Ni–Sn alloys increased with increasing Sn content and activation energy for crystallization increased from 151.2 ± 7.0 kJ/mol to 165.1 ± 9.0 kJ/mol with increasing Sn content from 5 at% to 7 at%, above which it almost kept constant. Crystallization occurs in the sequence of amorphous → (Ti,Sn) 2 Ni and B2 → B2 and Ti 3 Sn when Sn content is ≤5 at%, amorphous → (Ti,Sn) 2 Ni → B2 → B2 and Ti 3 Sn when Sn content is in the range of 5 at% and 10 at%, amorphous → (Ti,Sn) 2 Ni → Ti 3 Sn → B2 and Ti 3 Sn when Sn content is ≥10 at%. The crystallized Ti–Ni–Sn alloys showed the B2-R-B19′ martensitic transformation behavior. Rapid solidification was effective to separate the B2-R transformation from the R-B19′ transformation in Ti–Ni–Sn alloys.

Transformation behavior, shape memory characteristics and superelasticity of thermo-mechanically treated Ti–(45− x )Ni–5Cu– x V (at%) ( x = 0.5–2.0) alloys were investigated by means of differential scanning calorimetry, transmission electron microscopy, X-ray diffractions, thermal cycling tests under constant load and tensile tests. The B2–B19′ transformation occurred when V content was 0.5 at%, above which the B2–B19–B19′ transformation occurred. The B2–B19 transformation was not separated clearly from the B19–B19′ transformation. Thermo-mechanically treated Ti–(45− x )Ni–5Cu– x V alloys showed perfect shape memory effect and transformation hysteresis(Δ T ) of Ti–43.5Ni–5.0Cu–1.5V and Ti–43.0Ni–5.0Cu–2.0V alloys was about 9 K which was much smaller than that of a Ti–44.5Ni–5.0Cu–0.5V alloy(23.3 K). More than 90% of superelastic recovery ratio was observed in all specimens and transformation hysteresis (Δ σ ) of a Ti–44.5Ni–5.0Cu–0.5V alloy was about 70 MPa, which was much larger than that of a Ti–43.0Ni–5.0Cu–2.0V alloy (35 MPa).

The effects of thermomechanical treatment on the microstructure and high-temperature shape-memory characteristics of a TiNiPdCu alloy were investigated. An unexpected precipitation behavior was identified in a Ti50Ni15Pd25Cu10 alloy. Very high densities of nanoscale precipitates of TiPdCu and Ti2Pd types were found to be formed in the thermomechanically treated Ti50Ni15Pd25Cu10 alloy. A spinodal type of decomposition process was expected to be the cause of the observed precipitation behavior. It was noticed that the preferential diffusion of Cu atoms towards the heterogeneous nucleation sites promoted the precipitation of TiPdCu-type precipitates, which in turn promoted the precipitation of fine Ti2Pd-type precipitates. These precipitates greatly increased the resistance against the transformation-induced plasticity and creep deformation, especially at high stresses and high temperatures, mainly because of the high-temperature stability of these precipitates. High densities of these nanoscaled precipitates caused an anomalous increase in hardness and retarded the martensitic transformation. It was expected that the current research results could be highly beneficial for the development of high-temperature shape-memory alloys stable at temperatures >773 K, while keeping the benefits of ease of fabrication.

This paper reports the creation of functionally graded NiTi thin plates with varying composition through thickness. The composition gradient is created by surface diffusing Ni into thin equiatomic NiTi plates. The compositional gradient results in a gradient of the critical temperatures for martensitic transformation in the alloy. The functionally graded NiTi plates exhibit unique reversible one-way shape recovery behavior in a “fishtail-like” motion. Such mechanical behavior offers new options for innovative designs of NiTi, in particular actuators and bionics devices.

Even though Ti-Ni-Cu alloys have attracted a lot of attention because of their high performance in shape memory effect and decrease in thermal and stress hysteresis compared with Ti-Ni binary alloys, their poor workability restrains the practical applications of Ti-Ni-Cu shape memory alloys. Consolidation of Ti-Ni-Cu alloy powders is useful for the fabrication of bulk near-net-shape shape memory alloy. Ti50Ni30Cu20 shape memory alloy powders were prepared by gas atomization, and the sieved powders with the specific size range of 25 to 150 μm were chosen for this study. The evaluation of powder microstructures was based on a scanning electron microscope (SEM) examination of the surface and the polished and etched powder cross sections. The typical images showed cellular/dendrite morphology and high population of small shrinkage cavities at intercellular regions. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis showed that a B2-B19 one-step martensitic transformation occurred in the as-atomized powders. The martensitic transformation start temperature (Ms) of powders ranging between 25 and 50 μm was 304.5 K (31.5 °C). The Ms increased with increasing powder size. However, the difference of Ms in the as-atomized powders ranging between 25 and 150 μm was only 274 K (1 °C). A dense cylindrical specimen of 10 mm diameter and 15 mm length were fabricated by spark plasma sintering (SPS) at 1073 K (800 °C) and 10 MPa for 20 minutes. Then, this bulk specimen was heat treated for 60 minutes at 1123 K (850 °C) and quenched in ice water. The Ms of the SPS specimen was 310.5 K (37.5 °C) whereas the Ms of conventionally cast ingot is found to be as high as 352.7 K (79.7 °C). It is considered that the depression of the Ms in rapidly solidified powders is ascribed to the density of dislocations and the stored energy produced by rapid solidification.

Rapidly solidified Ti–36Ni–7Sn (at.%) alloy amorphous ribbon was crystallized in the sequence amorphous–(Ti,Sn) 2 Ni–B2–Ti 3 Sn. The crystallized ribbon consisted of the B2, (Ti,Sn) 2 Ni and Ti 3 Sn phases with grain sizes of R ∗ and M S ∗ in the ribbon was larger than that of the ingot prepared by conventional casting; this difference was ascribed to precipitation hardening due to nanostructuring of the (Ti,Sn) 2 Ni and Ti 3 Sn particles.

The effects of Ta and Sn contents on the martensitic transformation temperature, crystal structure and thermal stability of Ti–Ta–Sn alloys are investigated in order to develop novel high temperature shape memory alloys. The martensitic transformation temperature significantly decreases by aging or thermal cycling due to the formation of ω phase in the Ti–Ta binary alloys. The addition of Sn is effective for suppressing the formation of ω phase and improves stability of shape memory effect during thermal cycling. The amount of Sn content necessary for suppressing aging effect increases with decreasing Ta content. High martensitic transformation temperature with good thermal stability can be achieved by adjustment of the Ta and Sn contents. Furthermore, the addition of Sn as a substitute of Ta with keeping the transformation temperature same increases the transformation strain in the Ti–Ta–Sn alloys. A Ti–20Ta–3.5Sn alloy reveals stable shape memory effect with a martensitic transformation start temperature about 440 K and a larger recovery strain when compared with a Ti–Ta binary alloy showing similar martensitic transformation temperature.

This study investigated a fundamental aspect of thermoelastic martensitic transformations in different shape memory alloys by means of interrupted thermal analysis technique using differential scanning calorimetry (DSC). The objective of this study was to determine the true transformation temperature interval. It also provides the opportunity to further the discussion of time dependence of the transformations. The study applied a technique of thermal arrest amidst phase transformations. The transformation temperature intervals were found to be 8.4 and 12.9 K for the forward and reverse B2↔B19′ martensitic transformation in a near-equiatomic Ti-50.2 at.% Ni alloy and 14.7 and 12.8 K in a Ni-rich Ti-50.8 at.% Ni alloy and 7.3 and 9.1 K for the L21↔orthorhombic transformation in a Ni43Co7Mn39In11 alloy. These values were significantly smaller than those commonly reported in the literature. The experimental evidences also demonstrated that the apparent time dependences of the martensitic transformations manifested in DSC analysis were artifacts caused by instrumental thermal inertia.

For the fabrication of bulk near-net-shape Ti-Ni-Cu shape memory alloys, consolidation of Ti-Ni-Cu alloy powders are useful because of their brittle property. In the present study, shape memory alloy powders were prepared by gas atomization and martensitic transformation temperatures and microstructures of those powders were investigated as a function of powder size. The size distribution of the powders was measured by conventional sieving, and sieved powders with the specific size range of 25 to were chosen for this examination. XRD analysis showed that the B2-B19 martensitic transformation occurred in the powders. In DSC curves of the as-atomized powders as a function of powder size, only one clear peak was found on each cooling and heating curve. The martensitic transformation start temperature() of the powders was . The increased with increasing powder size and the difference of between powders and powders is only . The typical microstructure of the rapidly solidified powders showed cellular morphology and very small pores were observed in intercellular regions.

Effect of substitution of Mn for Ni on transformation behavior, shape memory characteristics and superelasticity of Ti45Ni-5Cu alloy has been investigated by means of electrical resistivity measurements, X-ray diffraction, thermal cycling tests under constant load and tensile tests. The one-stage B2-B19' transformation occurred when Mn content was 0.5 at%, above which the two-stage B2-B19-B19' transformation occurred. A temperature range where the B19 martensite exists was expanded with increasing Mn content because decreasing rate of Ms (60 K / % Mn) was larger than that of Ms' (40 K / % Mn). Ti-(45-x)Ni-5Cu-xMn alloys were deformed in plastic manner with a fracture strain of 60 % ~ 32 % depending on Mn content. Clear superelasticity was found in fully annealed Ti-(45-x)Ni-5Cu-xMn alloys with Mn content more than 1.0 at%, which was ascribe to a solid solution hardening by substitution of Mn for Ni.

Ti–30Ni–20Cu (at.%) alloy ribbons with an average grain size of 0.25–0.28 μm were obtained by applying two-step annealing, comprising a first annealing step at 773–823 K and a second annealing step at 748 K, to amorphous melt-spun ribbons, which was very small compared with ribbons obtained by one-step annealing at 748–823 K (0.64–1.20 μm). Hysteresis and elongation associated with the B2–B19 transformation were decreased by two-step annealing, while the fracture stress and strain increased.

In this study, the effect of annealing temperature on microstructure and mechanical properties of a Ti–18Zr–12.5Nb–2Sn (at.%) alloy was investigated by using optical microscopy (OM), X-ray diffraction (XRD) measurement and tensile test. The cold-rolled plate was annealed at temperatures between 773[Formula: see text]K and 1173[Formula: see text]K. Recrystallization occurred in the specimen annealed at 873[Formula: see text]K. Grain size increased from 8[Formula: see text][Formula: see text]m to 80[Formula: see text][Formula: see text]m with increasing temperature from 873[Formula: see text]K to 1173[Formula: see text]K. The ultimate tensile strength decreased from 1590[Formula: see text]MPa to 806[Formula: see text]MPa with increasing annealing temperature from 773[Formula: see text]K to 973[Formula: see text]K, and then showed similar value in the specimens annealed at temperatures from 973[Formula: see text]K to 1173[Formula: see text]K. The fracture strain increased from 3.8% to 41.0% with increasing annealing temperature from 773[Formula: see text]K to 1173[Formula: see text]K due to the recovery and recrystallization. The recovery strain increased with increasing of annealing temperature attributed to the evolution of recrystallization texture.

The shape memory alloy ribbons of Ti50Ni15Cu35 had been fabricated by two rapid solidification processes of melt spinner and melt overflow to achieve different rapidly solidified structures. The ribbon fabricated by the melt spinner was an amorphous and any martensitic transformations occurred in the crystallized ribbon due to the formation of an equilibrium intermetallic phase (Cu4Ti3). However, the microstructure of the ribbon prepared by the melt overflow exhibited columnar grains normal to the ribbon surface. X-ray diffraction analysis showed that one-step martensitic transformation of B2–B19 occurred in the ribbon. According to the DSC analysis, it was known that the martensitic transformation temperature of B2 → B19 was 71.2 °C. During thermal cyclic deformation with the applied stress of 120 MPa, transformation hysteresis and elongation associated with the B2–B19 transformation were observed to be 4.6 °C and 1.68%, respectively, in the Ti50Ni15Cu35 alloy. It is considered that these shape memory characteristics were ascribed to solid–solution strengthening by the supersaturation of excess Cu content.

The composition dependence of the mechanical properties and martensitic transformation behavior of Ti–Nb–4Zr–2Ta–N alloys is investigated. The effect of annealing temperature on the microstructural evolution and superelastic properties in the N-added and N-free alloys is compared. The addition of N decreases M s of Ti–Nb–4Zr–2Ta alloys by about 200 K per 1 at.%N and improves the superelastic properties of Ti–Nb–4Zr–2Ta alloys. The dissolution of α phase increases the martensitic transformation start temperature and decreases the superelastic recovery strain for the N-free alloy, whereas it causes opposite effects for the N-added alloy. The different annealing temperature dependences of superelastic properties are discussed on the basis of microstructure observation.

Ternary Ti-Nb-Zr alloys were prepared by a magnetron sputtering method with porous structures observed in some of them. In bulk, in order to control the porous structure, a space holder (NH4HCO3) is used in the sintering method. However, in the present work, we show that the porous structure is also dependent on alloy composition. The results from Young&rsquo, s modulus tests confirm that these alloys obey d-electrons alloy theory. However, the Young&rsquo, s modulus of ternary thin films (&asymp, 80&ndash, 95 GPa) is lower than that for binary alloys (&asymp, 108&ndash, 123 GPa). The depth recovery ratio of ternary Ti-Nb-Zr thin films is also higher than that for binary &beta, Ti-(25.9&ndash, 34.2)Nb thin film alloys.

Mo is added to Ti–Nb alloys in order to enhance their superelasticity. The shape memory properties of Ti–(12–28)Nb–(0–4)Mo alloys are investigated in this paper. The Ti–27Nb, Ti–24Nb–1Mo, Ti–21Nb–2Mo and Ti–18Nb–3Mo alloys exhibit the most stable superelasticity with a narrow stress hysteresis among Ti–Nb–Mo alloys with Mo contents of 0, 1, 2 and 3 at.%, respectively. The ternary alloys reveal better superelasticity due to a higher critical stress for slip deformation and a larger transformation strain. A Ti–15Nb–4Mo alloy heat-treated at 973 K undergoes (2 1 1)〈1 1 1〉-type twinning during tensile testing. Twinning is suppressed in the alloy heat-treated at 923 K due to the precipitation of the α phase, allowing the alloy to deform via a martensitic transformation process. The Ti–15Nb–4Mo alloy exhibits stable superelasticity with a critical stress for slip deformation of 582 MPa and a total recovery strain of 3.5%.

Silicon thin film annealed at 973 K for 7.2 ks after being deposited on a Ti–50.3Ni(at%) substrate was not detached from the substrate after 2.2% tensile deformation, which was ascribed to a diffusion bonding between the Si film and substrate. The B2–B19′ transformation start temperature ( M s ) of the Ti–Ni substrate with Si thin film increased by annealing, which was ascribed to a tensile stress developed by the difference in thermal expansion coefficient between the Si film and substrate.

Spatially varying temperature profile generated by the Joule heating of a Ti–45Ni–5Cu (at%) alloy wire was investigated by experiment and analysis. The Joule heating is a simple and effective method to fabricate shape memory alloy (SMA) actuators for proportional control. By applying a voltage drop of 1.338 V, a temperature gradient between 504 K and 413 K was obtained experimentally over a length of 30 mm SMA wire. A closed-form expression derived from a simple analytic model was suggested to predict the temperature profile, and the result of the finite element analysis was also provided. Finally, it is demonstrated that the Joule heated specimen shows a low shape recovery rate of 0.02%/K and a unique Luders-type deformation with a progressively increasing stress level from 340 MPa to 380 MPa.