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.     

Martensitic transformation of Ni64Al34Re2 shape memory alloy

As-rolled and annealed Ni₆₄Al₃₄Re₂ shape memory alloy (SMA) exhibits B2 → L1₀ (3R) martensitic transformation with M_s temperature up to about 210 °C. Experimental results indicate that the annealing temperature is the major factor that affects the M_s temperature. It is found that adding 2 at.% Re to replace Al in Ni₆₄Al₃₆ binary SMA can significantly refine the alloy's grain size and enhance the softening behavior during transformation. Meanwhile, Re has the same trend as Ni to affect the M_s temperature, but it has a less effect than Ni. The lattice constants and microstructures of NiAl-B2 phase, NiAl-L1₀ (3R) martensite and Ni₃Al-L1₂ phase are almost similar to those of Ni–Al binary SMAs.     

Multi-stage transformation in annealed Ni-rich Ti49Ni41Cu10 shape memory alloy

Multi-stage transformation (MST) in 500 °C annealed Ni-rich Ti₄₉Ni₄₁Cu₁₀ shape memory alloy (SMA) is investigated by differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as solution-treated alloy undergoes B2 ↔ B19 ↔ B19′ two-stage transformations. Ti(Ni,Cu)₂ precipitates are formed in 500 °C annealed specimens. Alloy annealed at 500 °C for 6–24 h exhibits MST. This MST is confirmed by DMA tests and is composed of B2₁ ↔ B19₁ ↔ B19′₁ and B2₂ ↔ B19₂ ↔ B19′₂ transformations corresponding to the regions near and far from Ti(Ni,Cu)₂ precipitates, respectively. Experimental results show that the more the annealing time, the more the B2₁ ↔ B19₁ ↔ B19′₁ transformations and finally only B2₁ ↔ B19₁ ↔ B19′₁ transformations retain with the transformation temperatures close to those of Ti₅₀Ni₄₀Cu₁₀ SMA.     

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.     

Martensitic transformation and anomalies in resistivity of (Ti–50Ni)1−xCx (x = 0.1, 0.5 at.%) shape memory alloys

Phase transformation of solid solution (Ti–50Ni)_1−xC_x (x = 0.1, 0.5 at.%) alloys have been studied by using differential scanning calorimetry, physical property measurement system and optical microscope. The transformation temperature decreases due to the existence of titanium carbide (TiC) particles compared with that of near-equiatomic Ti–Ni shape memory alloy. The resistivity vs. temperature curves show hysteresis. Thermoelastic martensitic transformation occurred in two alloys despite the difference in TiC content. Nevertheless, the resistivity results show different martensitic transformation routes. A one-step B2 → B19′ transformation occurred in the low TiC content alloy and an R transformation appeared in another alloy, suggesting that the martensitic transformation routes depended on the TiC content. The cumulative effect of the TiC particles causes the local stress field and lattice distortion to restrain the transformation of the B19′. On the other hand, the TiC content has an effect on the temperature coefficient of electrical resistivity (TCR) of alloys. The Ti–Ni–0.5C alloy shows a negative TCR in the range 100–300 K during which transformation occurs. Another alloy shows the opposite result. The cause of the negative TCR is briefly discussed.     

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.     

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.     

Multi-stage martensitic transformations induced by repeated thermal cycling of equiatomic TiNi alloy

Usually a multi-stage martensitic transformation is observed in Ni-rich TiNi alloys after heat treatment at 350–500 °C. It is due to the internal stresses created by the Ni₄Ti₃ participate. In the present work it was found that the multi-stage martensitic transformation appeared in Ti–50.0 at.% Ni alloy after thermal cycles through the temperature range of the phase transitions. Annealed sample undergoing one-stage phase transition was subjected to 32 thermal cycles in the DSC apparatus. The results had shown that three-stage forward martensitic transformation observed after 32 thermal cycle was due to the B2 → R, B2 → B19′ and R → B19′ phase transitions. It was found that the B19′ phase obtained from the B2 phase underwent the reverse transformation at higher temperatures than the B19′ phase obtained from the R phase. After annealing the cycled sample at 400 °C the transformation behavior was similar to the non-thermal cycled alloy. It was concluded that the main reason for the multi-stage phase transition induced by the thermal cycles was the phase hardening.     

The effect of Fe addition on the transformation temperatures, lattice parameter and magnetization saturation of Ni52.5−XMn23Ga24.5FeX ferromagnetic shape memory alloy

The effect of Fe addition on martensitic transformation temperatures, Curie temperature (T_C), lattice parameters and magnetization saturation was studied in Ni_52.5−XMn₂₃Ga_24.5Fe_X alloys fabricated by arc-melting furnace. The characterizations were performed by DSC, X-ray diffraction and magnetometry. Fe replacing Ni sites leads to an increment on lattice parameter and on the magnetization saturation of the austenitic phase at room temperature. Also, T_C increases from 370 K up to 400 K and remains constant for X ≥ 3.1 at.% Fe. In contrast, martensitic transformation temperatures decrease with Fe substituting Ni.     

Thermal stability of B2 CuZr phase,microstructural evolution and martensitic transformation in Cu–Zr–Ti alloys

The effects of minor Ti addition on the thermal stability of B2 CuZr phase, the microstructure and the martensitic transformation (MT) in Cu₅₀Zr_50−xTi_x (x = 0, 2.5, 7.5 and 10 at%) alloys were investigated. It was found that the crystallization products, i.e. Cu₁₀Zr₇ and Cu(Ti, Zr)₂, of Cu–Zr–Ti amorphous alloys transform to B2 CuZr phase at high temperatures. The corresponding eutectoid transformation temperature gradually increases with increasing Ti content, implying the decrease of the thermodynamic stability of the B2 CuZr phase. The microstructures of Cu–Zr–Ti martensitic alloys were proven to contain B2 CuZr, CuZr martensite, Cu₁₀Zr₇, Cu(Ti, Zr)₂, and ZrTiCu₂ crystals. Dilatometric measurements reveal that the MT temperature reduces with increasing Ti content, which would be of electronic nature. With increasing thermal cycles, the MT temperature gradually decreases while the reverse MT temperature increases, which results from the enhancement of the dislocation density, the partial decomposition of the equiatomic CuZr crystals and the partially reversible MT.     

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.     

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.     

Effects of annealing at high pressure on structure and mechanical properties of Al87Ni7Gd6 metallic glass

The effects of annealing and annealing with a superimposed pressure of 940 MPa on the primary crystallization behaviour of α-Al and the resulting micro-hardness have been studied for as-quenched Al₈₇Ni₇Gd₆ metallic glass. Isothermal annealing experiments were conducted for 30 min at 188 °C, 191 °C, and 205 °C in silicone oil maintained either at atmospheric pressure (i.e. 0.1 MPa) or at 940 MPa. XRD analyses detected the evolution of structure with annealing at 0.1 MPa, while specimens annealed with 940 MPa pressure exhibited sharper diffraction peaks than those annealed at 0.1 MPa. DSC measurements were conducted on the as-received amorphous ribbons as well as ribbons annealed at different temperatures at either 0.1. MPa or with 940 MPa superimposed pressure. Specimens annealed with 940 MPa pressure exhibited higher onset temperatures (i.e. T_x1) and temperatures for the first exothermic peak (i.e. T_p1) for primary crystallization. TEM measurements revealed an increase in the volume fraction of α-Al with increases in annealing temperature, while micro-hardness measurements revealed an increase in hardness with increasing amounts of α-Al. Specimens annealed with 940 MPa pressure exhibited further increases in both the volume fraction of α-Al and resulting micro-hardness.     

Unusual devitrification behaviour in rapidly solidified Ti45Zr38Ni17 alloy

In this study, Ti₄₅Zr₃₈Ni₁₇ ribbons have been elaborated using planar flow casting method (v_quenching = 10⁶ K s⁻¹). The rapidly quenched samples, displaying a dispersion of nanoscaled β phase particles in an amorphous matrix, have been extensively characterised using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Devitrification behaviour, investigated by four-probe resistivity measurements and differential scanning calorimetry as well as high resolution TEM analysis, revealed the formation of nanometric quasicrystals (QC) during the first exothermic phase transformation as well as the precipitation of omega domains inside β particles. On the basis of these data, it has been noticed that nanoscaled β metastable particles had adopted the same temperature dependence as β metastable bulk. Furthermore, particular orientation relationships have been observed for QCs in the vicinity of β particles which have suggested probable influence of crystalline structure on the QCs growth, although the presence of such peculiar materials after annealing treatment could be mainly explained by an icosahedral short-range order prevailing in Ti–Zr–Ni melt and favouring their nucleation.     

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.     

Evolution and stability of phases in a high temperature shape memory alloy Ni49.4Ti38.6Hf12

Ni_49.4Ti_38.6Hf₁₂ shape memory alloy has been characterized for structure, microstructure and transformation temperatures. The microstructure of the as-cast sample consists of B19′ and R-phases, and (Ti,Hf)₂Ni precipitate phase along the grain boundaries in the form of dendrites. The microstructure of the solution treated sample contains only B19′ martensite phase, whereas a second heat treatment after solutionizing results in reappearance of the R-phase and the (Ti,Hf)₂Ni grain boundary precipitate phase in the microstructure. A detailed microstructural examination shows the presence of precipitates having both coherent and incoherent interface with the matrix, the type of interface being dictated by the crystallographic orientation of the matrix phase. The present study shows that the (Ti,Hf)₂Ni precipitates having coherent interface with the matrix, drive the formation of the R-phase in the microstructure.     

Vacuum brazing of TiAl-based intermetallics with Ti–Zr–Cu–Ni–Co amorphous alloy as filler metal

An amorphous Ti41.7–Zr26.7–Cu14.7–Ni13.8–Co3.1 (wt%) ribbon fabricated by melt spinning was used as filler to vacuum braze Ti–48Al–2Nb–2Cr (at%) intermetallics. The influences of brazing temperature and time on the microstructure and strength of the joints were investigated. It is found that intermetallic phases of Ti₃Al and γ-Ti₂Cu/Ti₂Ni form in the brazed joints. The tensile strength of the joint first increases and then decreases with the increase of the brazing temperature in the range of 900–1050 °C and the brazing time varying from 3 to 15 min. The maximum tensile strength at room temperature is 316 MPa when the joint is brazed at 950 °C for 5 min. Cleavage facets are widely observed on all of the fracture surfaces of the brazed joints. The fracture path varies with the brazing condition and cracks prefer to initiate at locations with relatively high content of γ-Ti₂Cu/Ti₂Ni phases and propagate through them.     

Martensite in a TiAl alloy quenched from beta phase field

Hexagonal martensite with well-developed midribs was obtained by iced-brine quenching of Ti44Al4Nb4Hf0.1Si from the beta phase field. Crystallographic characterisation shows great resemblances of the martensite in this alloy to the lenticular martensite in Fe–Ni alloys in the morphology and substructures of martensite plates. It also bears similarities with the martensite in titanium and its alloys in habit planes and orientation relationship with parent phase. Martensitic transformation under the conditions employed in this work was incomplete and the remaining beta phase after martensitic transformation, thus, underwent diffusional beta-to-alpha transformation or massive transformation, depending on the local circumstances.     

Microstructure and martensitic transformation of TiNiNbB shape memory alloys

In present work, microstructure, martensitic transformation and mechanical properties of Ti₄₄Ni_47−xNb₉B_x (x = 0, 0.5, 1, 5 at.%) alloys were investigated as a function of B content. The results show that the addition of B significantly influences the microstructure of the alloys. The microstructure of Ti₄₄Ni₄₇Nb₉ alloy consists of B2 parent phase matrix and β-Nb phase. When the B content is 0.5 at.%, Nb₃B₂ phase presents. With further increasing B content to above 1 at.%, TiB and NbB phases present instead of Nb₃B₂ phase. With increasing B content, the transformation temperatures increase due to the reduced Ni/Ti ratio and Nb content in the matrix. The mechanical properties can be optimized by the addition of 1 at.% B.     

An investigation on the crystal structures of Ti50Ni50−xCux shape memory alloys based on density functional theory calculations

The present research has investigated the martensite crystal structures and electronic structures of Ti₅₀Ni_50−xCu_x (x = 0, 5, 12.5, 15, 18.75, 20, 25) shape memory alloys using density functional theory (DFT). The computational results are compared with the reported data and it is found that the equilibrium lattice constants are in good agreement with reported values. It is also found that with Cu addition to NiTi, the lattice parameters (a and c) and the monoclinic angle decrease, whereas the lattice parameter b increases. With increasing Cu content, fewer electrons were transferred from Ti to Ni in comparison with that in binary NiTi alloys, and the NiTi monoclinic structure becomes unstable. When Cu content is increased to around 20 at%, an orthorhombic crystal structure is formed which agrees well with reported experimental observations.