Internal structures and shape memory properties of sputter-deposited thin films of a Ti–Ni–Cu alloy

Low-temperature heat treatments of the sputter-deposited amorphous films, which were previously proved to be a new method to produce very good shape memory properties for Ti-rich Ti–Ni alloys, have been applied to a ternary Ti–43.0Ni–6.2Cu alloy (at.%). The basically same nanometric structures as in the binary alloy are formed, i.e. the nanometric structures consist of extremely thin plate precipitates of bct structure, which are formed on {100} planes of the parent B2 structure and have the c-axis normal to the habit planes. High-shape recovery stresses of about 500 MPa with recoverable shape strains of 5% are obtained without accompanying any permanent strains. A shape recovery stress of more than 870 MPa is attained if it is allowed to involve about 1% permanent strain. Although these bct precipitates have large tetragonalities, they are perfectly coherent with the parent bcc lattice. The maximum shape recovery stress is nearly twice that of the Ti-rich Ti–Ni binary alloy having a similar nanometric structure. It is suggested that this remarkable increase in recovery stress may be attributed to the change in Burgers vector of dislocations caused by partial disordering in Ti–Ni–Cu alloys. It is emphasized that the shape recovery stress in this ternary alloy is four times that of the Ti₂Ni containing samples and 10 times that of a bulk Ti–45Ni–5Cu alloy.     

Crystallization process and shape memory properties of Ti–Ni–Zr thin films

The crystallization process of as-deposited Ti–Ni–(10.8–29.5)Zr amorphous thin films was investigated. The Ti–Ni–Zr as-deposited films with a low Zr content exhibited a single exothermic peak due to the crystallization of (Ti,Zr)Ni with a B2 structure. In contrast, a two-step crystallization process was observed in the Ti–Ni–Zr thin films with a high Zr content. Shape memory behavior of Ti–Ni–Zr thin films heat treated at 873–1073 K was investigated by thermal cycling tests under various stresses. The martensitic transformation start temperature increased with increasing Zr content until reaching the maximum value, then decreased with further increasing Zr content. The inverse dependence of transformation temperature on Zr content in the thin films with a high Zr content is due to the formation of a NiZr phase during the crystallization heat treatment. The formation of the NiZr phase increased the critical stress for slip but decreased the recovery strain.     

Shape memory behavior of Ti–Ta and its potential as a high-temperature shape memory alloy

In this study, the effect of Ta content on shape memory behavior of Ti–Ta alloys was investigated. The shape memory effect was confirmed in Ti–(30–40)Ta alloys. The martensitic transformation start temperature (M_s) decreased by 30 K per 1 at.% Ta. The amount of ω phase formed during aging decreased with increasing Ta. A stable high-temperature shape memory effect was confirmed for Ti–32Ta (M_s = 440 K) during thermal cycling between 173 and 513 K. On the other hand, the high-temperature shape memory effect of Ti–22Nb, which has a similar M_s to Ti–32Ta, exhibited poor stability due to the large amount of ω phase formed during thermal cycling. It is suggested that Ti–Ta is an attractive candidate for the development of novel high-temperature shape memory alloys.     

Microstructures and transformation characteristics of thin films of TiNiCu shape memory alloy

Both sputtering conditions and crystallizing temperatures have great influence on the microstructures and phase transformation characteristics for TislNi44Cus. By means of the resistance-temperature measurement, X-ray diffraction and atomic fore microscopic study, the results indicate that the transformation temperatures of the thin films increase and the “rock candy“ martensitic relief is more easily obtained with promoting the sputtering Ar pressure, sputtering power, orcrystallizing temperature. However, when sputtering Ar pressure, sputtering power, or crystallizing temperature are lower, a kind of “chrysanthemum“ relief, which is related with Ti-rich GP zones, is much easier to be observed. The reason is that during crystallization process, both of the inherent compressive stresses introduced under the condition of higher sputteringpressure or higher crystallizing temperature are helpful to the transition from GP zones to Ti2(NiCu) precipitates and the increase of the transformation temperatures. The addition of copper to substitute for 5 96 nickel in mole fraction can reduce the transformation hvsteresis width to about 10 - 15 ℃.     

Microstructure and transformation behavior of Ni50Ti45Ta5 shape memory alloy

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Microstructure and shape memory behavior of Ti55.5Ni44.5−xCux (x = 11.8–23.5) thin films

The microstructure and shape memory behavior of Ti_55.5Ni_45.5−xCu_x (x = 11.8–23.5) thin films annealed at 773, 873, and 973 K for 1 h were investigated. None of the films except the Ti_55.4Ni_32.8Cu_11.8 film annealed at 773 K for 1 h had any precipitates in the B2 grain interiors and their grain sizes were small (less than 1 μm). Increasing the annealing temperature caused grain growth and thus a decrease in the critical stress for slip and an increase in the martensitic transformation start temperature (Ms). The grain size was also controlled by the growth of a second phase. In the three-phase equilibrium region of Ti₂Ni, Ti₂Cu and TiNi, Ti₂Cu grains grew faster than Ti₂Ni grains, leading to a decrease in the critical stress for slip and an increase in the Ms temperature with increasing Cu content.     

Ti–Cu–Ni shape memory bulk metallic glass composites

New Ti–Cu–Ni shape memory bulk metallic glass composites were obtained by carefully controlling the cooling rate upon quenching. This allows for the formation of a metastable microstructure consisting mainly of ductile, spherical martensitic Ti(Ni,Cu) precipitates embedded in an amorphous matrix also containing a small volume fraction of TiCu and Ti₂Cu precipitates. These composites exhibit large ductility and high strength combined with a strong work-hardening behaviour. A deformation mechanism is proposed with the help of experimental observations and finite-element simulation. The simulation results demonstrate that stress concentrations occur around the precipitates, which promotes a heterogeneous stress distribution and the formation of multiple shear bands. Additionally, different transformation temperatures were observed for martensitic precipitates depending on whether they are completely or partially embedded in the amorphous matrix.     

Comparison of high temperature shape memory behaviour for ZrCu-based,Ti–Ni–Zr and Ti–Ni–Hf alloys

New ZrCu-based high temperature shape memory alloys with M_s close to 500 K are under development. The shape memory behaviour of this material is compared to those of Ti–Ni–Zr and Ti–Ni–Hf alloys. The optimal compositions show a shape recovery of not less than 3% at temperatures above 470 K.     

Microstructure,mechanical properties and shape memory effect of Ni–Mn–Ga–B high-temperature shape memory alloy

The effects of boron addition on the microstructure, transformation temperature, mechanical properties and shape memory effect of (Ni₅₄Mn₂₅Ga₂₁)_100−xB_x alloys were investigated. The results showed that the martensitic transformation start temperatures M_s decreased monotonically from 465 K for x = 0–278 K for x = 3. Boron addition refined the grain and significantly enhanced the mechanical properties. The compressive fracture strain of 22.3% and reversible strain of 6.8% were obtained in (Ni₅₄Mn₂₅Ga₂₁)_99.5B_0.5 alloy.     

Phase transformation behavior and shape memory characteristics of Ti−Ni−Si alloys

Transformation behavior, microstructures and shape memory characteristics of Ti−(50−X)Ni−XSi (X=2, 4, 6 at.%) and (50−X)Ti−Ni−XSi (X=2, 5, 7, 10 at.%) alloys were investigated by means of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, electrical resistivity measurements and constant load thermal cycling tests. Ti₅Si₃, Ni₁₆Ti₆Si₇ and Ni₄Ti₄Si₇ were formed in Ti−(50−X)Ni−XSi alloys, while Ti₅Si₄, Ni₃Si, Ni₃Ti₂ and Ni₃Ti₂Si were found in (50−X)Ti−Ni−XSi alloys. The total amount of silicides increased with increasing Si content, irrespective of Si content. The B2→B19 transformation occurred in Ti−(50−X)Ni−XSi alloys, and their transformation temperatures appeared to be almost constant. Transformation elongation associated with the B2→B19 transformation decreased with increasing Si content. In contrast to Ti−(50−X)Ni−XSi alloys, a transformation accompanied with structural change did not occur in (50−X)Ti−Ni−XSi alloys.     

Stress–strain behavior of porous Ni–Ti shape memory intermetallics synthesized from powder sintering

The microstructure and stress–strain behavior of porous Ni–Ti intermetallics fabricated by powder sintering of Ni, Ti and/or TiH₂ are investigated. The pores are small and well distributed in the present porous Ni–Ti alloys and the phase constituent and stress–strain behavior of porous Ni–Ti alloys are significantly influenced by the sintering conditions (sintering temperature and sintering time) and TiH₂ additions. With increasing sintering temperature, the pseudoelasticity (PE) increases while the hysteretic width decreases. A second compression also leads to an increase of PE, elastic modulus and related deformation resistance. Further, the stress–strain behavior of porous Ni–Ti alloys is different in some way from that of bulk Ni–Ti alloys and other porous materials.     

Electrochemical behavior of YAG laser-welded NiTi shape memory alloy

Electrochemical behaviors of laser-welded Ti-50.6%Ni（mole fraction） shape memory alloy and the base metal in 0.9% NaCl solution were investigated by electrochemical techniques as corrosion potential measurement, linear and potentiodynamic polarization. The results indicate that the laser-welded NiTi alloy is less susceptible to pitting and crevice corrosion than the base metal, which is demonstrated by the increase in polarization resistance（Rp） and pitting potential（φpit） and decrease in corrosion current density（Jcorr） and mean difference between φpit and φprot values. It is confirmed by scanning electron microscope micrographs that pits could be observed on the surface of base metal but not on the surface of laser-welded alloy after potentiodynamic tests. An improvement of corrosion resistance of laser-welded NiTi alloy could be attributed to almost complete dissolution of inclusions upon laser welding.     

Shape memory behavior of Ti–20Zr–10Nb–5Al alloy subjected to annealing treatment

The effects of annealing temperature on microstructures, phase transformation, mechanical properties, and shape memory effect of Ti–20Zr–10Nb–5Al alloy were investigated. X-ray diffraction(XRD) patterns show that the alloy is composed of single hexagonal ɑ'-martensite phase for both as-rolled sample and sample annealed at773 K for 30 min, while single orthorhombic ɑ' phase exists in the samples annealed at 873 and 973 K for30 min. The optical observations indicate that the alloy is recrystallized when annealed at 873 K, and the grain size of the sample annealed at 973 K is about five times larger than that annealed at 873 K. Both of the samples annealed at 873 and 973 K show almost the same reverse martensite transformation start temperature of 483 K as demonstrated by thermal dilatation tests. The critical stress values for martensite reorientation(σ_M) are 392 and 438 MPa for the alloys annealed at 873 and 973 K, respectively. The maximum shape memory strain is 2.8 %, which is obtained in the alloy annealed at 873 K due to the lower σ_M. Moreover,the sample annealed at 873 K exhibits larger tensile stress and tensile strain due to the smaller grain size.     

Structure of martensite in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures

After annealing at different temperatures, there are different types of precipitates in Ti-rich Ti–Ni–Cu thin films: plate-like Guinier–Preston (GP) zones, Ti₂Cu precipitates and spherical Ti₂Ni precipitates. The (0 1 1) compound twins and (1 1 1) type I twins are dominant in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures, which suggests that the precipitates do not change the twinning modes of the B19 martensite. However, the amount of the (0 1 1) compound twin increases with increasing annealing temperature due to its small twinning shear. In thin films with GP zones or Ti₂Ni precipitates, the amount of martensite with a single-pair morphology is less than that in thin films without precipitates. And in thin film with Ti₂Cu + Ti₂Ni precipitates, hardly any martensite with a single-pair morphology is observed. For the different types of precipitates at the different annealing temperatures, the obstacle of the precipitates to the growth of the B19 martensite plate also varies. The GP zones slightly hinder the growth in the width of martensite, resulting in wavy twin boundaries at the martensite variant tip. The Ti₂Cu precipitates can change both the width and the direction of the martensite plates. Ti₂Ni precipitates also significantly disturb or impede the growth of the martensite variants. These effects lead to a decrease in the maximum shape recoverable strain with increasing annealing temperature.     

Differences of transformation behavior between Ni-rich TiNi shape memory films and alloys

The differences of transformation behavior between Ni-rich TiNi shape memory film (SMF) and shape memory alloy (SMA) age-treated at 773K after solution-treatment at 973K have been investigated, using Ti-51.5Ni thin film and Ti-51.5Ni bulk alloy as examples, by differential scanning calorimetry (DSC), SEM and EDX. It is found that the age-treated Ni-rich TiNi SMF and SMA are of the same types of transformation, i.e., A→R→M (during cooling), and M→A (during heating) (A: parent phase; R: R-phase; M: martensite); the transformation temperature of the TiNi SMF is lower than that of the SMA, but the SMFs hysteresis is larger. The transformation heat of the TiNi SMF and SMA is nearly the same. The reason that TiNi SMFs strain is sensitive to temperature is not hysteresis, but its thickness is thinner, and the temperature is easy to distribute homogeneously.     

Structure of martensite in sputter-deposited (Ni,Cu)-rich Ti–Ni–Cu thin films containing Ti(Ni,Cu)2 precipitates

The martensite structure in sputter-deposited thin films of Ti_48.6Ni_35.9Cu_15.5 was studied. The Ti(Ni,Cu)₂ phase precipitates during the annealing process. Fine Ti(Ni,Cu)₂ precipitates can be deformed by the shear deformation of martensitic transformation, but they obstruct the movement of the twin boundaries to some extent. Coarse Ti(Ni,Cu)₂ precipitates seriously impede the growth of martensite plates and lead to a rectangular-cell-like structure of martensite in the film annealed at 873 K. The resistance of Ti(Ni,Cu)₂ precipitates to the growth of the martensite plates enhances with the coarsening of Ti(Ni,Cu)₂ precipitates, which is one of the reasons for the decrease in the maximum recoverable strain with increasing annealing temperature. B19′ martensite with (0 0 1) compound twinning is frequently observed near coarse Ti(Ni,Cu)₂ precipitates and grain boundaries in films annealed at 873 and 973 K. The local stress concentration should be responsible for the presence of B19′ martensite.