Phase transformation and microstructure of quaternary TiNiHfCu high temperature shape memory alloys

The effect of Cu addition on the phase transformation and microstructure of TiNiHf high temperature shape memory alloy has been studied. The experimental results show that the TiNiHfCu alloy undergoes a B2↔B19′ transformation with a concentration of 3 at.% Cu. And a two-step phase transformation occurs upon heating when the Cu content is 5 at.%. The constitutional phases of TiNiHfCu quaternary alloys are the matrix and (Ti,Hf,Cu)₂Ni particles. The substructure of martensite is mainly (001) compound twin in TiNiHfCu alloys. The martensite variants are (011) type I twin related. The phase transformation temperatures decrease rapidly during the initial several thermal cycles and then keep constant with further increasing of the thermal cycles. It should be noticed that the R-phase transition is separated from the martensitic transformation during the cooling process in the TiNiHfCu alloys. The underlying reasons have been discussed.     

Effect of precipitation on the shape memory effect of Ti50Ni25Cu25 melt-spun ribbon

The present research aims to provide accurate understanding of the relation between precipitation (volume fraction, morphology, type) and shape memory effect of Ti₅₀Ni₂₅Cu₂₅ melt-spun ribbon. Rapid thermal annealing was used to control the microstructural development while the shape memory effect of the ribbon was determined under constraint thermal cycling. The results show that the precipitation process takes the following sequence: B11 TiCu → B11 TiCu + Ti₂(Ni, Cu) → Ti₂(Ni, Cu) with increasing annealing temperature or duration. The shape memory effect is found to depend on both the volume fraction and the distribution of the precipitates. The former affects the shape recovery strain through reduction of the transformation volume participating the shape recovery. The latter affects the shape recovery strain through strengthening the matrix thus reducing the martensite strain which is more predominant under low constraint stresses. Precipitation strengthening, on the other hand, reduces the tendency of dislocation generation/movement, thus reducing the irreversible strain and improving shape recovery strain. This understanding provides guidelines on the optimization of the shape memory properties of the Ti₅₀Ni₂₅Cu₂₅ melt-spun ribbon via post-processing annealing.     

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.     

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.     

Shape memory behavior and internal structure of Ti–Ni–Cu shape memory alloy thin films and their application for microactuators

Internal structure and shape memory behavior of Ti–38.3Ni–9.3Cu (at.%) thin films heat-treated at 873 K, 923 K, 973 K and 1023 K were investigated by TEM observation and thermal cycling tests under various constant stresses. The thin film heat-treated at 873 K contained two types of precipitates, i.e., fine platelets and Ti₂Ni particles. The density of the platelets decreased with increasing heat-treatment temperature and annihilated completely when the heat-treatment temperature reached 973 K. The Ti₂Ni precipitates increased in volume fraction with increasing heat-treatment temperature from 873 K to 923 K, then their volume fraction was almost kept constant above 923 K. The recoverable strain decreased and the M_s increased with increasing heat-treatment temperature from 873 K to 923 K. Both the recoverable strain and the M_s became almost constant when the heat-treatment temperature was above 923 K. A diaphragm-type microactuator utilizing a Ti–38.0Ni–10.0Cu (at.%) thin film was fabricated. The diaphragm was square with the width of 500 μm. The actuation properties were investigated under conditions of both quasi-static and dynamic actuation. The M_s and the transformation temperature hysteresis of the microactuator were determined to be 352 K and 6 K, respectively. The microactuator operated at a working frequency of 100 Hz.     

New modelling of the B2 phase and its associated martensitic transformation in the Ti–Ni system

A symmetric two-sublattice model (Ni, Ti, Va)_0.5(Ni, Ti, Va)_0.5 is applied to describe the intermediate B2 compound in order to cope with the order–disorder transition in the Ti–Ni system. Using this model, the ordered B2 and the disordered Ti-rich b.c.c. are described by a single Gibbs free energy function. The B2 phase is the parent phase of the martensitic transformation in the TiNi shape memory alloys (SMAs), and its thermodynamic properties are then reassessed with emphasis on its composition range that is critical for SMAs. The low temperature B19′ phase is also evaluated on the basis of the selected experimental data from the martensitic transformation. Properties related to the transformation are studied in comparison with experimental data. The magnetic contribution is examined for the martensitic transformation. All calculations are in satisfactory agreement with experimental phenomena.     

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.     

The four-step multiple stage transformation in deformed and annealed Ti49Ni51 shape memory alloy

A four-step multiple stage transformation is observed in 20% deformed and 500 °C annealed Ti₄₉Ni₅₁ shape memory alloy. Two extra B2 → B19^′ transformation peaks appear before the previously described B2  → R and R → B19^′ peaks while cooling, and these correspond to one new peak, which appears after the original B19^′ → B2 peak during heating. These two extra peaks are caused by the combined effect of severe cold-working and long-time annealing on Ti₄₉Ni₅₁ alloy, and they come separately from the B2 → B19^′ transformation occurring in regions with low and high dislocation densities, which are originally suppressed by cold-working.     

Enhancing magnetocrystalline anisotropy of the Fe70Pd30 magnetic shape memory alloy by adding Cu

Fe–Pd–Cu thin films are of great interest for applications in magnetic shape memory microsystems due to their increased martensitic transformation temperature. Here we analyse the consequences of Cu addition to Fe–Pd on the binding energy and magnetic properties by a combination of thin film experiments and first-principles calculations. Strained epitaxial growth of Fe₇₀Pd_30-xCu_x with x = 0, 3, 7 is used to freeze intermediate stages during the martensitic transformation. This makes a large range of tetragonal distortion susceptible for analysis, ranging from body-centred cubic to beyond face-centred cubic (1.07 < c/a_bct < 1.57). We find that Cu enhances the quality of epitaxial growth, while spontaneous polarization and Curie temperature are reduced only moderately, in agreement with our calculations. Beyond c/a_bct > 1.41 the samples undergo structural relaxations through adaptive nanotwinning. Cu enhances the magnetocrystalline anisotropy constant K₁ at room temperature, which reaches a maximum of ?2.4 × 10⁵ J m^?3 around c/a_bct = 1.33. This value exceeds those of binary Fe₇₀Pd₃₀ and the prototype Ni–Mn–Ga magnetic shape memory system. Since K₁ represents the maximum driving energy for variant reorientation in magnetic shape memory systems, we conclude that Fe–Pd–Cu alloys offer a promising route towards microactuator applications with significantly improved work output.     

Effect of heat treatment temperature on the microstructure and actuation behavior of a Ti–Ni–Cu thin film microactuator

Ti–Ni–Cu/SiO₂ two layer diaphragm-type microactuators were fabricated by sputter deposition and micromachining. The influence of heat treatment temperature on the actuation behavior was investigated under quasi-static conditions. The interfacial structure of Ti–Ni–Cu/SiO₂ and internal structure of the Ti–Ni–Cu layer were also investigated using transmission electron microscopy. The reaction layer formed between the Ti–Ni–Cu and SiO₂ layers, and preferentially grew into the SiO₂ side. The reaction layer formed at 1023 K mainly consisted of Ti₄(Ni,Cu)₂O. The maximum height of the diaphragm decreased with increasing heat treatment temperature. The growth of the reaction layer also affected the microstructure of the Ti–Ni–Cu layer. The density of fine platelets and Ti₂Ni precipitates decreased with increasing heat treatment temperature from 873 to 923 K, and they disappeared at 973 K due to the fact that the reaction layer mainly consisted of a Ti-rich phase. The microactuator heat treated at 973 K showed the highest transformation temperature with the lowest transformation temperature hysteresis, which is attractive for high speed actuation.     

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.     

Martensite structure in Ti–Ni–Hf–Cu quaternary alloy ribbons containing (Ti,Hf)2Ni precipitates

The martensite structure in a Ti₃₆Ni₄₄Hf₁₅Cu₅ ribbon annealed at different temperatures is investigated. When the annealing temperature is <873 K, spherical (Ti,Hf)₂Ni particles 20–40 nm in diameter precipitate in the grain interior. Transmission electron microscopy analysis shows that (0 0 1) compound twins are dominant in the ribbon containing homogeneously distributed (Ti,Hf)₂Ni precipitates. When the annealing temperature is 773 K, the boundaries between the martensite domains with the (0 0 1) twins are blurry and vague. When the annealing temperature is 873 K, four types of boundaries among the martensite domains are found: {1 1 1}, (0 0 1)//{1 1 1}, {1 1 3} and (1 1 0)//{1 1 3} types. When the annealing temperature is 973 K, the (0 1 1) twins become dominant, and the martensite variants show mainly spear-like and mosaic-like morphologies. However, martensite domains with (0 0 1) twins also exist around the coarse (Ti,Hf)₂Ni precipitates. Fine (Ti,Hf)₂Ni precipitates should be responsible for the improvement in shape memory effect and the superelasticity of Ti–Ni–Hf–Cu ribbons.     

Variation of atomic spacing and thermomechanical properties in Ni–Mn–Ga/alumina film composites

Ni_51.4Mn_28.3Ga_20.3 thin films deposited on alumina ceramics have been studied by X-ray powder diffraction and dynamic mechanical analysis. Substantial temperature vs. film thickness dependencies of interatomic spacing measured in the direction of the film normal are observed in the range of 25–200 °C and 0.1–5 μm, respectively. The coefficient of thermal expansion (CTE) of the film in the paramagnetic cubic phase has been determined to be equal to (15 ± 1) × 10⁻⁶ K⁻¹ for all the films, in agreement with the CTE of bulk material. The thickness dependent shrinkage of the pseudo-cubic lattice along the film normal direction is attributed to the thermally induced tensile stress in the film plane. The thickness dependence of the elastic modulus of submicron films is obtained. It is shown that the internal stresses result in both the thickness dependence of martensitic transformation temperature and the reversible, thermally induced change in shape of the Ni–Mn–Ga/alumina cantilever actuator.     

Mechanical properties of highly porous Ti49.5Ni50.5 biomaterials

Ti_49.5Ni_50.5 shape memory alloy fibers were prepared by a melt overflow process. The martensitic transformation starting temperature of B2 → B19′ in the rapidly solidified fibers was 19 °C. Cylindrical billets of Ni-rich Ti–Ni alloy with 75% porosity were produced by a vacuum sintering technology using as-cast alloy fibers. The mechanical properties and shape memory properties of the highly porous Ti–Ni alloy is investigated using a compressive test. The plateau of the stress–strain curve was observed at about 7 MPa and resulted in 8% elongation associated with stress-induced B2 → B19′ transformation. Because of the high porosity of this specimen, the elastic modulus of about 0.95 GPa could be obtained. It was also found that a recovered strain was 5.9% on heating after the compressive deformation. This recovery of the length is ascribed to the shape memory effect which occurs during the martensitic transformation.     

Deformation-induced γ ↔ α2 phase transformation in TiAl alloy compressed at room temperature

Deformation-induced γ ↔ α₂ phase transformation in a Ti–47Al–2Cr–2Nb–0.2Y (at.%) alloy compressed at room temperature was investigated by high-resolution transmission electron microscopy (HREM) and energy dispersive spectrum (EDS). The deformation-induced (DI) γ → α₂ phase transformation occurred in a twin intersection region. On the other hand, the deformation-induced α₂ → γ transformation nucleated at the stacking faults of α₂ phase. The composition analysis of the γ and α₂ laths by EDS suggested that composition of some laths deviated much from their equilibrium values. These composition deviations promoted the deformation-induced γ ↔ α₂ phase transformation to occur; EDS results also suggested that there was no composition difference between the DI-γ plate and the primary γ phase. Based on the HREM and EDS experimental results, the mechanism of the deformation-induced γ ↔ α₂ phase transformation has been discussed.     

Grain refinement in γ-TiAl-based alloys by solid state phase transformations

The colony size of a fully lamellar Ti–46Al–2Cr–2Mo–0.25Si–0.3B ingot was refined from 120 to 30–65 μm by well defined heat-treatments which exploit the suppression of the α → α + γ transformation or involve cyclic annealing around the α-transus temperature. In addition, an extremely fine lamellar spacing in the range of 30–40 nm was obtained. For coarse-grained fully lamellar Ti–46Al–9Nb a massive phase transformation was used for microstructural refinement. The thermal stability of the massively transformed material was tested by annealing treatments and characterized by hardness measurements and the variation of the c/a-ratio of the tetragonal γ-TiAl cell as obtained from X-ray diffraction. After annealing at 1200 °C α₂-Ti₃Al lamellae appear within the former massively transformed γ-TiAl grains parallel to all four (111)_γ-planes causing an increase in hardness.     

Ordered ω phases in a 4Zr–4Nb-containing TiAl-based alloy

A considerable amount of B2 phase with a cellular morphology is retained in a 4Zr–4Nb-containing TiAl-based alloy. Heterogeneous precipitation of ordered ω from B2 is found to occur readily after HIPping: B2 → ω with the B8₂-structure in cell regions and B2 → ω with the D8₈-structure in cell-wall regions. Congregated D8₈-ω domains and particles form as a network surrounding the well-developed B8₂-ω cells. The heterogeneous formation of different ω variants is caused by a heterogeneous distribution of Zr + Nb elements across B2, which plays an important role in stabilizing vacancies and promotes the formation of D8₈-ω. Fine D8₈-ω particles are also observed to precipitate from the B8₂-ω cell matrix after ageing at 700 °C for 1000 h, showing a transformation path of β → B2 → B8₂-ω → D8₈-ω for the aged cells. The heterogeneous formation of a D8₈-ω network and B8₂-ω cells is found to be detrimental to ductility and fatigue strength. A very brittle fine-grained TiAl alloy is produced as a result.     

Shape memory behaviour of Ti51.5Ni(48.5−x)Cux (x = 23.4–37.3) thin films with submicron grain sizes

The shape memory behaviours of Ti_51.4Ni_25.2Cu_23.4, Ti_51.3Ni_21.1Cu_27.6, Ti_51.2Ni_15.7Cu_33.1 and Ti_51.4Ni_11.3Cu_37.3 thin films annealed at 773, 873 and 973 K for 1 h were investigated. The Ti_51.3Ni_21.1Cu_27.6 film annealed at 773 K, the Ti_51.2Ni_15.7Cu_33.1 film annealed at 873 K, and the Ti_51.4Ni_11.3Cu_37.3 films annealed at 873 and 973 K showed a perfect shape memory effect at a stress as high as 1 GPa. This improvement in shape memory behaviour was attributed to their fine grain sizes less than 500 nm. Whereas the Ti_51.2Ni_15.7Cu_33.1 and Ti_51.4Ni_11.3Cu_37.3 films annealed at 873 K or higher showed a martensitic transformation start temperature above room temperature, these films annealed at 773 K were in the parent phase at room temperature owing to their very fine grain sizes. The effects of Cu content and annealing temperature on the shape memory behaviour of the Ti_51.5Ni_48.5?xCu_x (x > 27) films with submicron grain sizes were discussed in comparison with those of the Ti_51.5Ni_48.5?xCu_x (x < 24) films on the basis of their microstructures.     

Martensitic phase transformations in nanocrystalline NiTi studied by TEM

By high pressure torsion (HPT) deformation almost complete amorphization is obtained in bulk Ni–50.3at.%Ti containing B19^′ martensite. During low temperature annealing tiny crystallites retained after the HPT deformation are acting as nuclei and trigger the nanocrystallization of B2 austenite. It is shown that the density of the nuclei is a function of the HPT strain and determines together with the annealing temperature the grain size of the nanocrystals ranging from 5 to 350 nm. Upon cooling the nanostructures transform to B19^′ partially since the grain boundaries hinder the autocatalytic formation of martensite. The large transformation strains of B19^′ are reduced by very fine (0 0 1) compound twins. With decreasing grain size an increasing energy barrier arises and the martensitic transformation is completely suppressed in grains smaller than 60 nm. The R-phase transformation causing only small transformation strains is observed in grains between 15 and 60 nm. Whereas in grains below 15 nm B2 remains indicating that no transformation occurs at all.     

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.