Microstructure and martensitic transformation of Ti49Ni51 − xHfx high temperature shape memory alloys

In present work, the microstructure and martensitic transformation of Ti₄₉Ni_51 − xHf_x (x = 3-15) alloys were studied. The microstructure of Ti₄₉Ni_51 − xHf_x alloys consists of B19′ martensite and (Ti,Hf)₂Ni phase at room temperature. The martensitic transformation behavior is characterized by a single-stage transformation. With increasing Hf content, the transformation temperature increases from 75 to 279 °C resulting from the reduced valence electron concentration, indicating that the replacement of Hf for Ni is effective in increasing the transformation temperatures. The results suggest that the Ti₄₉Ni_51 − xHf_x shape memory alloy is one of potential candidates for high temperature applications.     

Effect of aging on martensitic transformation behavior of Ti48.8Ni50.8V0.4 alloy

In this article, the effect of aging on martensitic transformation of Ti_48.8Ni_50.8V_0.4 alloy was investigated. The results show that the martensitic transformation of the solution-treated shape memory alloy is a typical single-stage transformation process. The transformation temperatures of the samples aged at different temperatures for 0.5 h were lower as compared to that of the solution-treated alloy. With the increase of aging temperature, the transformation temperatures increase. After aging at 500 °C, the samples exhibit a multiple-stage transformation. The samples after aging at 500 °C for more than 5 h resulted in the transformation sequence of A → R→M1 and A → M2 upon cooling and M2 → A and M1 → A upon heating.     

Martensitic transformation behaviors of Ti-rich Ti–Ni alloy fibers fabricated by melt overflow

Ti₅₀Ni₅₀, Ti_50.5Ni_49.5, Ti₅₁Ni₄₉ and Ti_51.5Ni_48.5 fibers were fabricated by melt overflow process. The rapid solidification can increase the solubility above the equilibrium solubility. The effects of the rapid solidification of Ti-rich Ti–Ni alloys on the microstructure, transformation temperatures and shape memory characteristics are investigated. The addition of 0.5 at.% Ti in Ti₅₀Ni₅₀ alloy greatly increases the transformation temperature. However, the transformation temperatures decrease again for Ti content exceeding 51 at.%. Results of thermal cycling tests under various constant stress levels reveal that the recoverable elongation associated with B2–B19 martensitic transformation of Ti_50.5Ni_49.5 fibers is two times larger than that of Ti_51.5Ni_48.5 alloy fiber.     

Microstructure and martensitic transformation and mechanical properties of cast Ni rich NiTiCo shape memory alloys

Abstract

The influence of Co additions on the microstructure, second phase precipitates, phase transformation and mechanical properties of cast Ni_51?xTi₄₉Co_x (x?=?0, 0·5, 1·5 and 4 at-%) shape memory alloys was investigated. At the expense of Ni, Co added to NiTi alloy significantly increases the martensitic transformation temperature. The matrix phase in the microstructure of Ni₅₁Ti₄₉Co₀ alloy is the austenite phase (B2) in addition to martensite phase (B19′) and precipitates of NiTi intermetallic compounds. However, the parent phase in the other three alloys, Ni_50·5Ti₄₉Co_0·5, Ni_49·5Ti₄₉Co_1·5 and Ni₄₇Ti₄₉Co₄, is martensite. Ti₂Ni phase was found in the microstructures of the all investigated alloys; however, Ni₃Ti₂ phase precipitated only in the NiTi alloy with 0 at-%Co. The volume fraction of Ti₂Ni phase decreased by the additions of 0·5 and 1·5 at-%Co, while it is slightly increased with 4 at-%Co. The hardness value of NiTi alloy is affected by Co additions.     

Microstructure, martensitic transformation and superelasticity of Ti49.6Ni45.1Cu5Cr0.3 shape memory alloy

The aim of this study is to investigate the microstructure, martensitic transformation behavior, shape memory effect and superelastic property of Ti_49.6Ni_45.1Cu₅Cr_0.3 alloy, with Cu and Cr substituting for Ni. After annealing, the alloy showed single step A-M/M-A transformations within the whole annealing temperature range of 623 K to 1273 K even in the presence and Ti₂(Ni, Cu) precipitates. With the increase of the annealing temperature, the transformation temperatures exhibited three stages: increasing from 623 K to 873 K, decreasing from 873 K to 1023 K and unchanging from 1023 K to 1273 K. Meanwhile, the critical stress for stress induced martensitic (SIM) transformation decreased to a minimum value and increased after that, exhibiting a V shape curve. The alloy annealed at 623, 773 and 923 K exhibited shape recovery ratio more than 90% when the deformation strain was below 20%.     

Multiple-step martensitic transformations in the Ni51Ti49 single crystal

Multiple-step martensitic transformations of an aged Ni₅₁Ti₄₉ single crystal using calorimetric method were investigated. Results show that for short aging times (10–45 min) multiple-step martensitic transformations on cooling occur in two steps. Applying intermediate aging times (1.25–4 h) results in three steps and long aging times (more than 8 h) lead to two-step martensitic transformations again. This behavior has not been recognized in NiTi single crystals in literatures. It can be related to the heterogeneity of composition and stress fields around Ni₄Ti₃ precipitates.     

Microstructural, mechanical and magnetic properties of shape memory alloy Ni55Mn23Ga22 thin films deposited by radio-frequency magnetron sputtering

The near-stoichiometric Ni₂MnGa ferromagnetic alloys are one of the smart materials, that are of a great interest when they are deposited as a thin film by r.f. sputtering. These thin films of shape memory alloys are prospective materials for micro and nanosystem applications. However, the properties of the shape memory polycrystalline thin films depend strongly on their structure and internal stress, which develop during the sputtering process as well as during the post-deposition annealing treatment. In this study, about 1 μm Ni₅₅Mn₂₃Ga₂₂ thin films were deposited in the range 0,45 to 1,2 Pa of Ar pressure and P = 40 to 120 W. Their composition, crystallographic structure, internal stress and stress gradient, indentation modulus, hardness, deflection induced by magnetic field and magnetic properties were systematically studied as a function of the temperature of the silicon substrate ranging from 298 to 873 K and the vacuum annealing treatment at 873 K for 21,6 ks and 36 ks. A silicon wafer having a native amorphous thin SiO_x buffer layer was used as a substrate. This substrate influences the microstructure of the films and blocks the diffusion process during the heat treatment.The crystal structure of the martensitic phase in each film was changed systematically from bct or 10 M or 14 M. In addition, the evolution of the mechanical properties such as mean stress, stress gradient, roughness, hardness and indentation modulus with the temperature (of substrate or of heat treatment) were measured and correlated to crystal structure and morphology changes.Moreover, it has been shown that it is necessary to associate a high temperature (873 K) annealing during a long time (21 ks and 36 ks) to obtain good ferromagnetic properties. Thus, for the well annealed films (36 ks at 873 K) the magnetostrain is about - 170 ppm for a magnetic field of 1 MA m^- 1 applied along the beams.As a conclusion, the response of free-standing magnetic shape memory films to a magnetic field of 0,2 MA m^- 1 depends strongly on the martensitic structure, internal mechanical stress (mean and gradient) and magnetic properties. The free-standing annealed film at 873 K for 36 ks points out a considerable magnetic actuation associated with bct or 10 M or 14 M martensitic structures.     

Multi-strengthening effects on the martensitic transformation temperatures of TiNi shape memory alloys

The multi-strengthening effects, introduced by combining cold rolling, aging and thermal cycling, on the martensitic transformation temperatures of Ti₅₀Ni₅₀ and Ti₄₉Ni₅₁ shape memory alloys have been studied by using DSC and microhardness measurements. Experimental results show that both pre-cold-rolling of Ti₅₀Ni₅₀ alloy and aging treatments of Ti₄₉N₅₁ alloy can impede the further introduction of dislocations during the thermal cycling and hence effectively reduce the effect of thermal cycling on transformation temperatures. For Ti₄₉Ni₅₁ alloy, whether the cold rolling is conducted before or after the aging treatment, the strengthening of cold rolling can significantly increase the specimen hardness but only slightly affect the following thermal cycling effect. The multi-strengthening effects on the martensitic transformation temperatures are also found to follow the equation of Ms = T₀–K_y.     

Effect of pre-strain on martensitic transformation of Ni43Mn43Co7Sn7 high- temperature shape memory alloy

In the present work, the effect of pre-strain on martensitic transformation of Ni₄₃Mn₄₃Co₇Sn₇ (at.%) alloy was investigated. The results show that Ni₄₃Mn₄₃Co₇Sn₇ alloy undergoes a martensitic transformation at 288 °C upon cooling. The thermal cycling does not affect the transformation behavior of the alloy, indicating the good thermal stability. The reverse transformation of the deformed martensite is pre-strain dependent. When the pre-strain is higher than 7.5%, the reverse transformation occurs in two-stage manner upon first heating due to the nonuniform martensite deformation. In contrast, during the first martensitic and second reverse transformation, the pre-strain shows little effect on the transformation temperatures.     

Deposition and dielectric properties of CaCu3Ti4O12 thin films deposited on Pt/Ti/SiO2/Si substrates using radio frequency magnetron sputtering

Polycrystalline CaCu₃Ti₄O₁₂ thin films were deposited on Pt(111)/Ti/SiO₂/Si substrates using radio frequency magnetron sputtering. The phase formation and the physical quality of the films were crucially dependent on the substrate temperature and oxygen partial pressure. Good quality films were obtained at a substrate temperature of 650 °C and 4.86 Pa total pressure with 1% O₂. The dielectric constant (∼ 5000 at 1 kHz and 400 K) of these films was comparable to those obtained by the other techniques, eventhough, it was much lower than that of the parent polycrystalline ceramics. For a given temperature of measurements, dielectric relaxation frequency in thin film was found to be much lower than that observed in the bulk. Also, activation energy associated with the dielectric relaxation for the thin film (0.5 eV) was found to be much higher than that observed in the bulk ceramic (0.1 eV). Maxwell-Wagner relaxation model was used to explain the dielectric phenomena observed in CaCu₃Ti₄O₁₂ thin films and bulk ceramics.     

Pb(Zr0.52Ti0.48)O3/TiNi multilayered heterostructures on Si substrates for smart systems

Ferroelectric/shape memory alloy thin film multilayered heterostructures possess both sensing and actuating functions and are considered to be smart. In this article, Pb(Zr_0.52Ti_0.48)O₃ (PZT) ferroelectric thin films and Ti-riched TiNi shape memory alloy thin films have been deposited on Si and SiO₂/Si substrates in the 400-600 °C temperature range by pulsed laser deposition technique. Deposition processing, microstructure and surface morphology of these films are described. The TiNi films deposited at 500 °C had an austenitic B2 structure with preferred (110) orientation. The surfaces of the films were very smooth with the root-mean-square roughness on a unit cell level. The structure of the TiNi films had a significant influence on that of the subsequently deposited PZT films. The single B2 austenite phase of the TiNi favored the growth of perovskite PZT films. The PZT/TiNi heterostructures with the PZT and TiNi films respectively deposited at 600 and 500 °C exhibited a polarization-electric field hysteresis behavior with a leakage current of about 2 × 10^− 6 A/cm².     

Influence of compression aging on phase transition temperatures in single crystalline Ni‐rich NiTi shape memory alloys

The present paper considers the phase transition behavior of a single crystal Ni‐rich NiTi alloy which was compression aged to produce one single family of Ni₄Ti₃ precipitates. The single crystal material was produced in a two stage process. Polycrystalline material was first melted under an inert atmosphere and remelted when single crystals were produced. Compression aging treatments in <111>‐orientation were carried out in order to suppress all but one family of Ni₄Ti₃‐precipitates which nucleate and grow on {111}‐planes of the B2 matrix. The objective of this study is to investigate the influence of Ni₄Ti₃‐precipitates on the martensitic transformation behavior. It was previously shown that grain boundaries provide heterogeneous nucleation sites for the formation of Ni₄Ti₃; this results in heterogeneous microstructures which undergo multiple step martensitic transformations. Single crystals avoid grain boundaries and the present study aims at clarifying how homogeneously precipitated particles affect martensitic transformations.     

Shape memory characteristics and mechanical properties of high-density powder metal TiNi with post-sintering heat treatment

Through the use of fine titanium and nickel elemental powders and persistent liquid phase sintering, a high density powder metal Ti₅₁Ni₄₉ with a high phase transformation temperature and enthalpy, and narrow transformation hysteresis, which are comparable to those of wrought TiNi alloys, are produced. However, due to the presence of a semi-continuous Ti₂Ni network, the shape recovery and tensile properties in the martensitic state at room temperature, determined using bending tests, are lower than those of cast TiNi counterparts. Through hot isostatic pressing and annealing above the peritectic temperature for sintered specimens, these properties are improved due to the increase in density and spheroidization of the Ti₂Ni compound. The phase transformation temperature and enthalpy are also enhanced due to the continuing carbon absorption by the Ti₄Ni₂X (X = C, O) phase, which decreases the carbon content in the TiNi matrix.     

Microstructure and martensitic transformation behavior of a constant-strain aged Ni–Mn–Ga–Ti magnetic shape memory alloy

The Ni₅₃Mn_23.5Ga_18.5Ti₅ ferromagnetic shape memory alloy has been aged under 2% constant-strain at various temperatures for 3 h, and the microstructure and martensitic transformation behaviors have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). It was found that after constant-strain aging, the amount of the Ni-rich precipitates with lenticular morphology is higher and the size of the second-phase particle is smaller when compared to that of the conventional aged samples. The martensitic transformation temperatures first decrease remarkably with the increase of aging temperature, and then increase when the aging temperature exceeds 973 K, which can be attributed to the change of the Ni-content in the matrix as well as the strengthening effect by fine Ni₃Ti precipitates.     

Phase transformation,thermoelastic and magnetic properties of Ni50Mn30Ga20−xCux ferromagnetic shape memory alloys

Effect of addition of Cu on phase transformation temperatures, enthalpy and entropy changes, Curie temperature, magnetization saturation were investigated in Ni–Mn–Ga ferromagnetic shape memory alloy. The results show that the Ni₅₀Mn₃₀Ga_20−xCu_x alloys exhibit thermoelastic martensitic transformation. The martensitic and reverse martensitic transformation temperatures, enthalpy and entropy changes and thermal hysteresis increase with increase of Cu content. Martensite structure changes from 7 M with 0–0.5 at.% Cu to non-modulated T martensite when the content of Cu is more than 0.5 at.%. In addition, the Curie temperature almost remains unchanged at low-Cu content and subsequently decreases obviously. Magnetization saturation of alloys decrease with increasing Cu content since it is sensitive to ordered atomic arrangement.     

Precipitation of W2B5 and β-WB from Ti0.3W0.4Cr0.3B2 solid solutions

We investigated the formation of W₂B₅ and β-WB precipitates in supersaturated Ti_0.3W_0.4Cr_0.3B₂ solid solutions during isothermal annealing at temperatures between 1400 and 1600 °C. X-ray diffractometry measurements showed that the precipitation process is very complex including metastable phase formation and mutual transformation between the two precipitation phases. The rate constants of precipitation are determined as a function of reciprocal temperature and discussed in framework to literature data.     

Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy with narrow thermal hysteresis and high thermal stability

High temperature shape memory alloys with operating temperatures above 100 °C are in demand for use as solid-state thermal actuators in aerospace, automobile and other engineering applications. The present study deals with transformation behaviour and thermal stability of Ni_24.7Ti_50.3Pd_25.0 (at.%) high temperature shape memory alloy, in cast and homogenized condition. The martensite finish temperature and transformation hysteresis of the alloy were determined to be 181.0 °C and ∼8.5 °C respectively. The alloy showed high stability upon stress-free thermal cycling, variation in transformation temperatures being ±1 °C. The narrow thermal hysteresis and high thermal stability of the alloy upon transformation cycling has been discussed and correlated with its microstructural features, activation energy and elastic strain energy of thermoelastic martensitic transformation. The alloy exhibited modulus of ∼82 GPa and hardness of ∼4.7 GPa in martensite phase.     

Effects of plate thickness on reverse martensitic transformation of prestrained NiTi/NiTi alloy

In this paper, differential scanning calorimeter (DSC) was used to study the effects of predeformation and plate thickness on the reverse martensitic transformation of explosively welded NiTi/NiTi alloy. Results showed that there was a constraint between Ni_50.4Ti (NiTi-1) and Ni_49.8Ti (NiTi-2), which led to that the thickness of NiTi-1 or NiTi-2 strongly affected the reverse martensitic transformation behavior because residual stress variations in thickness wound enable bias force to be built inside the composite. The DSC measurements showed that after deformation, the reverse martensitic transformation temperature of the composite was increased with the increasing thickness of NiTi-2. Also, the XRD results revealed that the microstructure of NiTi/NiTi alloy changed from B2 phase to B19’ phase along the thickness direction.     

Martensitic transformation of an aged/thermal-cycled Ti30.5Ni49.5Zr10Hf10 shape memory alloy

Ti_30.5Ni_49.5Zr₁₀Hf₁₀ alloy undergoes a B2 B19 one stage martensitic transformation. Martensitic transformation temperatures decrease, but the hardness increases with increasing aging time, resulting from the effect of point defects/atoms rearrangement in B2 phase induced by aging at 450°C. Thermal cycling can also decrease the transformation temperatures while increasing the hardness of the alloy. The strengthening effects of aging and thermal cycling on the M _s (M*) temperature of this alloy follows the expression M _s = T ₀ – KK y, in which K values are affected by different strengthening processes. Experimental results indicate that martensitic transformation temperatures of Ti_30.5Ni_49.5Zr₁₀Hf₁₀ alloy can be more effectively depressed by aging than by thermal cycling. The K value of this aged alloy is smaller than that of aged Ti_40.5Ni_49.5Zr₁₀ alloy due to the former having the higher aged temperature and the less lattice distortion.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.