Transformation hysteresis and shape memory effect of an ultrafine-grained TiNiNb shape memory alloy

In present work, transformation hysteresis and shape memory effect of an ultrafine-grained Ti₄₄Ni₄₇Nb₉ alloy processed by ECAP were studied. After deformation, the ECAPed sample showed a much wider transformation hysteresis than the initial sample due to the enlarged strength mismatch between matrix and β-Nb phase. The shape memory effect and its cycling stability of the ECAPed sample were obviously improved.     

Powder metallurgical production of TiNiNb and TiNiCu shape memory alloys by combination of pre-alloyed and elemental powders

The common Ti₄₄Ni₄₇Nb₉ and Ti₅₀Ni₄₀Cu₁₀ ternary shape memory alloys were produced by sintering techniques and the microstructure, phase structure and phase transformation behaviour were investigated. A combination of pre-alloyed binary TiNi powder and elemental Nb, Ni and Cu, Ti powders, respectively, were used. In contrast to the use of pre-alloyed ternary powders, which have to be produced in each new composition, a higher flexibility in the alloy composition becomes possible. In case of the Ti₄₄Ni₄₇Nb₉ alloy, liquid phase sintering was done to obtain the eutectic phase structure known from cast material. In case of the Ti₅₀Ni₄₀Cu₁₀ alloy, the pore size and porosity can be improved by choosing a two-step sintering process, as a eutectic melt between Ti and Cu is formed at low temperatures which influences the sintering behaviour. Controlling the impurity contents and the resulting secondary phases is necessary for both alloys in the same way as for binary TiNi alloys.     

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.     

Influence of Nb content on martensitic transformation and mechanical properties of TiNiCuNb shape memory alloys

In present work, microstructure, martensitic transformation behavior and mechanical properties of (Ti₅₀Ni₄₀Cu₁₀)_100−xNb_x (x = 0, 5, 10, 15 at.%) alloys were investigated as a function of Nb content. The addition of Nb to TiNiCu alloy leads to the presence of β-Nb phase. During cooling and heating, the alloys show one-step B2 ↔ B19 transformation. As the Nb content increases, the transformation temperatures almost linearly decrease and the transformation hysteresis monotonously increases due to the decrease of middle eigenvalue of the phase transformation matrix. The addition of Nb is effective in improving the elongation because of the introduction of β-Nb phase. With the increase of Nb content, both the yield strength and the critical stress to induce martensitic transformation increase, resulting in the improved superelastic strain.     

Effect of deformation on the stress-induced martensitic transformation in (Ni47Ti44)100−xNbx shape memory alloys with wide hysteresis

Martensite in TiNi-based alloys is reported to be thermally stabilized after a moderate deformation. Hence, this paper investigates the effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of (Ni₄₇Ti₄₄)_100−xNb_x (x=3, 9, 15, 20, 30 at.%) alloys. The stress-induced martensite appears to be stabilized in relation to the thermal-induced martensite that forms on cooling. This observation is confirmed by an increase in the reverse transformation start temperature, during which time the transformation temperature hysteresis reaches about 200°C. Moreover, the Nb content in Ni−Ti−Nb alloy has a great influence on the transformation temperature hysteresis of stress-induced martensite as well as on the process of stress-induced martensitic transformation. The mechanism of wide transformation temperature hysteresis is explained in terms of the microscopic structure of (Ni₄₇Ti₄₄)_100−xNb_x alloys. Furthermore, the temperature interval of the reverse transformation of stress-induced martensite was found to increase slightly as the strain of the high Nb-content alloy increased, though the value was much smaller than that of the thermally induced martensite. Finally, the paper explains the relation between this unique phenomenon and the elastic strain energy.     

Thermal and transport properties of as-grown Ni-rich TiNi shape memory alloys

Electrical resistivity, Seebeck coefficient, specific heat and thermal conductivity measurements on the Ti_50−xNi_50+x (x = 0.0–1.6 at.%) shape memory alloys are performed to investigate their thermal and transport properties. In this study, anomalous features are observed in both cooling and heating cycles in all measured physical properties of the slightly Ni-rich TiNi alloys (x ≤ 1.0), corresponds to the transformation between the B19′ martensite and B2 austenite phases. Besides, the transition temperature is found to decrease gradually with increasing Ni content, and the driving force for the transition is also found to diminish slowly with the addition of excess Ni, as revealed by specific heat measurements. While the signature of martensitic transformation vanishes for the Ni-rich TiNi alloys with x ≥ 1.3, the characteristics of strain glass transition start to appear. The Seebeck coefficients of these TiNi alloys were found to be positive, suggesting the hole-type carriers dominate the thermoelectric transport. From the high-temperature Seebeck coefficients, the estimated value of Fermi energy ranges from ∼1.5 eV (Ti_48.4Ni_51.6) to ∼2.1 eV (Ti₅₀Ni₅₀), indicating the metallic nature of these alloys. In addition, the thermal conductivity of the slightly Ni-rich TiNi alloys with x ≤ 1.0 shows a distinct anomalous feature at the B19′ → B2 transition, likely due to the variation in lattice thermal conductivity.     

Stress-induced martensitic transformation in （Ni47Ti44）100-xNbx shape memory alloys with wide hysteresis

The effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of the （Ni47Ti44）100-xNbx （x=3, 9, 15, 20, 30, mole fraction, %） shape memory alloys was investigated in detail by differential scanning calorimetry （DSC） after performing cryogenic tensile tests at a temperature of Ms＋30 ℃. The results show that Nb-content has obvious effect on the process of stress-induced martensitic transformation. It is also observed that the stress-induced martensite is stabilized relative to the thermally-induced martensite （TIM） formed on cooling, and Nb-content in Ni-Ti-Nb alloy has great influence on the reverse transformation start temperature and transformation temperature hysteresis of stress-induced martensite（SIM）. The mechanism of wide transformation temperature hysteresis was fully explained based on the microscopic structure and the distribution of the elastic strain energy of （Ni47Ti44）100-xNbx alloys.     

Influence of Sc addition on microstructure and transformation behaviour of Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy

Vacuum-arc melted Ni_24.7Ti_50.3Pd_25.0 and Ni_24.7Ti_49.3Pd_25.0Sc_1.0 (at.%) alloys were investigated to study effect of Sc micro-addition on microstructure and transformation behaviour of NiTiPd alloy. Study showed that microstructure of homogenized NiTiPd alloy consisted of NiTiPd matrix interspersed with Ti₂(Ni,Pd) precipitates. In contrast, NiTiPdSc alloy showed a single phase NiTiPdSc matrix with a few scandium oxide particles at isolated places. TEM and X-ray diffraction studies confirmed matrix phase of the alloys to be of orthorhombic B19 structure. TEM observations showed that NiTiPdSc alloy had relatively larger martensite plates with a smaller twin ratio compared to that of NiTiPd alloy. Also, APB (anti-phase boundary) like regions with twinless martensites was observed in both the alloys, area fraction of APB-like regions being more in NiTiPdSc alloy. Thermal analysis showed that transformation temperatures (TTs) of NiTiPd alloy decreased significantly with addition of Sc. The martensite finish temperature (M_f) of 181 °C for NiTiPd alloy lowered to 139 °C upon 1.0 at.% Sc addition. The transformation hysteresis of Ni_24.7Ti_49.3Pd_25.0Sc_1.0 (at.%) alloy was measured to be 7 °C, significantly lower than that of 15 °C for Ni_24.5Ti_50.0Pd_25.0Sc_0.5 alloy, reported in literature. Alloy purity, lower volume fraction of second phase and presence of twinless/small twin ratio martensite in microstructure is believed to be the reasons for such low transformation hysteresis. The transformation behaviour of the alloys upon stress-free thermal cycling was found stable, variation in TTs being within 1–2 °C.     

Effect of post-deformation annealing on the R-phase transformation temperatures in NiTi shape memory alloys

In NiTi shape memory alloys, both the annihilation of dislocations and the formation of Ni₄Ti₃ precipitates may occur during post-deformation annealing. Different responses of the R-phase transformation temperatures to the annealing conditions have been reported. In order to find out the main factor(s) affecting the R-phase transformation temperatures during post-deformation annealing, a Ti-49.8 at% Ni and a Ti-50.8 at% Ni alloy were subjected to various post-deformation annealing and thermal cycling treatments. The results show that the R-phase transformation temperatures are very stable in the Ti-49.8 at% Ni alloy, while a significant variation is observed in the Ti-50.8 at% Ni alloy with respect to the annealing and thermal cycling conditions. These findings suggest that the R-phase transformation temperatures are not susceptible to the change of dislocation density and depends mainly on the Ni concentration of the matrix, which can be modified by the formation of Ni₄Ti₃ precipitates.     

V含量对等原子比NiTi形状记忆合金微观组织、相变行为和显微硬度的影响

采用光学显微镜、扫描电子显微镜、X射线衍射仪、差示扫描量热仪和显微硬度计等测试手段,研究了V含量对等原子比NiTi形状记忆合金微观组织、相变行为和显微硬度的影响规律。结果表明：当V含量为0.5at%时,具有等轴晶组织的NiTiV形状记忆合金包含B19'' 和 Ti₂Ni相;当V 含量大于0.5at%时,NiTiV形状记忆合金形成B19''相、Ti₂Ni相和V的富集相,并且随着V含量增加,V的富集相越来越多聚集于晶界。进一步分析表明,Ni_49.75Ti_49.75V_0.5和Ni_49.25Ti_49.25V_1.5 形状记忆合金发生了B2?B19''的一级相变,而Ni_48.75Ti_48.75V_2.5和Ni_48.25Ti_48.25V_3.5形状记忆合金发生了B2?R?B19''的二级相变,尽管降温过程中同时发生了部分的R相变与B19''马氏体相变。随着V含量增加,相变温度随着V含量增加逐渐降低,其主要原因是Ni/Ti比例的增加。此外,随着V含量增加,合金的显微硬度值呈现先降低然后几乎保持不变的变化规律。     

Microstructure,phase transformation and mechanical property of Nb-doped Ni–Mn–Ga alloys

This study investigated the microstructure, phase transformation and mechanical property of (Ni_49.8Mn_28.5Ga_21.7)_100-xNb_x (x = 1, 3, 6, 9) alloys. The Nb1 alloy exhibited a single austenite phase at room temperature. With increasing Nb content for Nb3, Nb6 and Nb9, the alloy changed to a dual phase consisting of austenitic matrix and Nb-rich second phase with a hexagonal structure, and the amount of the second phase increased with the increase of Nb content. The martensitic transformation temperature and Curie temperature were changed and the transformation enthalpy was gradually reduced with increasing Nb content. The change of martensitic transformation temperature and Curie temperature was related to the introduction of Nb in the Ni–Mn–Ga structure that decreased valence electron concentration (e/a), increased unit cell volume and reduced magnetic exchange of the alloys. The decrease of transformation enthalpy was mainly attributed to the formation and increase of the Nb-rich second phase that reduced volume fraction of the matrix taking part in phase transformation. All the alloys presented a similar compression behavior with progressively fracturing characters (occurrence of several stress drops before complete fracturing). The fracture strength was slightly enhanced with increasing Nb content from Nb0 to Nb9, but the ductility has no apparent improvement.     

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.     

Microstructure and martensitic transformation of cast TiNiSi shape memory alloys

The martensitic transformation behavior, second phases and hardness of Ti₅₁Ni_49−xSi_x shape memory alloys (SMAs) with x = 0, 1 and 2 at.% are investigated. The transformation temperature of one stage martensitic reaction B2 ↔ B19′ is associated with the forward (M_s) and reverse (A_s) martensitic transformations, respectively. All experimental DSC results such as martensitic transformation peaks (M*) and reverse martensitic transformation peaks (A*) are increased and became sharper with increasing Si-content. The microstructure investigation of the studied SMAs (Ti₅₁Ni_49−xSi_x) showed that there are two types of precipitated second phase particles. The first one is Ti₂Ni which mainly located at grain boundaries and intermetallic compound of Ti₂(Ni + Si) phase distributed inside the matrix. The volume fraction of these two phases is increased with Si content. Additionally, a small amount of Si remained in solid solution of the matrix of Ti₅₁Ni_49−xSi_x SMAs. Moreover, hardness of Ti₅₁Ni_49−xSi_x SMAs is increased as the Si-content increases.     

Microstructure and martensitic transformation of an ultrafine-grained TiNiNb shape memory alloy processed by equal channel angular pressing

Microstructure, martensitic transformation and mechanical properties of an ultrafine-grained Ti₄₄Ni₄₇Nb₉ shape memory alloy processed by equal channel angular pressing were investigated. The as-ECAP processed sample is characterized by an inhomogeneous and refined microstructure. In β-Nb phase-rich region, the grains of matrix are elongated with high density dislocations. In β-Nb phase-free region, the microstructure is partial recovery and characterized by near-equiaxed grains. The heterogeneous microstructure is attributed to presence of β-Nb phase. Martensitic transformation behavior of the as-ECAP processed sample is characterized by a single-stage transformation. The thermal cycling stability of transformation and the mechanical properties are considerably improved due to a strengthening effect resulting from refined grain size and high dislocation density.     

Effect of minor Cu content on microstructure and mechanical property of NiTiCu bulk alloys fabricated by crystallization of metallic glass powder

Ultrafine-grained Ni_50.2−xTi_49.8Cu_x (x = 0, 2.5, 5, and 7.5) bulk shape memory alloys were fabricated by sintering of metallic glass (MG) powder and crystallization of amorphous phase. Non-isothermal crystallization kinetic analysis reveals that the crystallization mechanism of the synthesized x = 5 MG powder is typical interface-controlled two dimensional growth of nuclei followed by volume diffusion-controlled three dimensional growth of nuclei. In contrast, the crystallization mechanism of the synthesized x = 7.5 MG powder is typical volume diffusion-controlled three dimensional growth of nuclei in whole crystallization process. Correspondingly to different crystallization mechanisms, the two sintered and crystallized (SCed) bulk alloys have the same crystallized phases of bcc B2, fcc NiTi₂ phases, and monoclinic B19′, but these phases display different morphologies and distributions. The SCed x = 5 bulk alloy has a microstructure of bcc B2 matrix surrounding fcc NiTi₂ phase region, while the SCed x = 7.5 bulk alloy possesses discontinuous bcc B2 phase region. Consequently, the different crystallization mechanisms and microstructures causes extreme high yield strength and large plasticity for the SCed x = 5 bulk alloy and low strength and no plasticity for the SCed x = 7.5 bulk alloy. Especially, the yield strength of the SCed x = 5 bulk alloy is at least two times of that of the counterpart alloy prepared by melt solidification. The results provide a method fabricating high performance bulk alloys by tailoring crystallization mechanism using powder metallurgy.     

Influence of Fe addition on phase transformation behavior of NiTi shape memory alloy

Three different NiTi-based alloys, whose nominal compositions were Ni₅₀Ti₅₀, Ni₄₉Ti₄₉Fe₂, Ni₄₅Ti_51.8Fe_3.2 (mole fraction, %), respectively, were used in the current research to understand the influence of Fe addition on phase transformation behavior in NiTi shape memory alloy (SMA). The microstructure and phase transformation behavior of the alloys were investigated by optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis. The results show that the matrix of the Ni₅₀Ti₅₀ alloy consists of both B19′ (martensite) phase and B2 (austenite) phase. Moreover, the substructures of twins could be observed in the B19′ phase. However, the ternary alloys of NiTiFe exhibit B2 phase in the microstructures. Such microstructures were also characterized by large presence of Ti₂Ni precipitates dispersed homogenously in the matrix of the two kinds of alloys. The addition of Fe to the NiTi SMA results in the decrease in phase transformation temperatures in the ternary alloys. Based on mechanism analysis, it can be concluded that this phenomenon is primarily attributed to atom relaxation of the distorted lattice induced by Ni-antisite defects and Fe substitutions during phase transformation, which enables stabilization of B2 phase during phase transformation.     

Effects of Hydrogen on Ni47Ti44Nb9 Shape Memory Alloy

The effects of hydrogen on the transformation characteristics of Ni₄₇Ti₄₄Nb₉ shape memory alloy were investigated. Cathodic hydrogen charging was performed at a current density of 20 mA/cm² in 0.5 mol/L H₂SO₄ solution at room temperature. The transformations of the hydrogenated specimens were characterized by differential scanning calorimetry, x-ray diffraction, and electrical resistivity measurement in details. For the hydrogen-charged NiTiNb alloy, the original reversible transformation between B2 and B19′ phase disappeared. Meanwhile, new transformation around 120 °C was present. This high temperature reversible transformation was confirmed to be the transformation between β-NbH phase and α-Nb(H) solid solution phase.     

Comparative study of R-phase martensitic transformations in TiNi-based shape memory alloys induced by point defects and precipitates

We studied the influence of point defects (Fe) and precipitates (Ti₃Ni₄) on the characteristics of R-phase martensitic transformation by comparing the transport and thermal properties of as-quenched Ti₅₀Ni₄₆Fe₄ and annealed Ti_48.7Ni_51.3 shape memory alloys. Both alloys undergo a weak first-order R-phase transformation with a small thermal hysteresis (less than 7 K) and non-zero transformation strain, suggesting the introduction of point defects and precipitates lead to a stable R-phase in these alloys due to the defects induced local lattice deformations. Furthermore, our study revealed that the transition temperature, transformation width, and transformation strain of the investigated R-phase TiNi-based alloys are strongly affected by the induced defects. As a result, the annealed Ti_48.7Ni_51.3 has a higher transition temperature than that of Ti₅₀Ni₄₆Fe₄, as expected.     

Effect of aging and ball milling on the phase transformation of Ni50Mn25Ga17Cu8−xZrx alloys

This study investigated phase transformation of Ni₅₀Mn₂₅Ga₁₇Cu_8-xZr_x (x = 0, 4, 8) alloys after aging and ball milling. For the Cu₄Zr₄ and Zr₈ dual phase alloys, aging has enhanced magnetic susceptibility and magnetic exchange of the matrix, resulting in an increase of the Curie temperature of austenitic matrix. The decrease of martensitic transformation temperatures for the Cu₄Zr₄ alloy and the increase of martensitic transformation temperatures for the Zr₈ alloy after aging should be related to the dissolution and precipitation of the second phase in the matrix, respectively. Ball milling is effective to smash the Cu₄Zr₄ and Zr₈ alloys to fine particles, but cannot fracture the Cu₈ alloy to particles, indicating an inherent high ductility and strength of the Cu₈ alloy. Therefore, the macroscopic brittleness of the polycrystalline Cu₈ alloy was mainly caused by the weak grain boundaries. For the Cu₄Zr₄ and Zr₈ particles, the martensitic transformation and Curie transition of austenitic matrix disappeared and the Curie transition of second phase remained after ball milling. After post-annealing at 800 °C, the Curie transition of austenite was recovered due to the restoration of atomic order, but the martensitic transformation cannot be retrieved which might be caused by the grain refinement of the austenitic matrix after ball milling.     

Cu content and annealing temperature dependence of martensitic transformation of Ti36Ni49−xHf15Cux melt spun ribbons

The martensitic transformation in Ti₃₆Ni_49−xHf₁₅Cu_x (x = 1, 3, 5, 8) ribbons has been investigated. Only B2 to B19′ transformation was detected in all the present ribbons. The martensitic transformation temperatures do not change obviously with increase in the Cu content except that they decrease when the Cu content is 3 at.%. The lattice parameters of B19′ martensite, a and c increase, b almost remains constant, while the monoclinic angle β decreases with increase in the Cu content. For the ribbons with Cu content of 1 and 3 at.%, the martensitic transformation temperatures change slightly when the annealing temperature increases. For the ribbons with Cu content of 5 and 8 at.%, with increase in the annealing temperature, the martensitic transformation temperatures almost do not change and then decrease rapidly when the annealing temperature is higher than 873 K. TEM observation shows that the microstructure of the ribbons with Cu content of 1 and 3 at.% contains the martensite matrix and the (Ti,Hf)₂Ni particles with the size of about 150 nm, which does not change obviously when the annealing temperature increases. This results in that the martensitic transformation temperatures are not sensitive to the annealing temperature in the ribbons with 1 and 3 at.% Cu content. However, nano-scale (Ti,Hf)₂Ni particles precipitate in the ribbons with Cu content of 5 and 8 at.% when the annealing temperature is 773 and 873 K, and then the (Ti,Hf)₂Ni particles grow and coarsen rapidly with further increase in the annealing temperature. The coarsening of the (Ti,Hf)₂Ni particles should be responsible for the dramatic decrease of the martensitic transformation when the annealing temperature is higher than 873 K. For all the present ribbons, the substructure of B19′ martensite is (001) compound twins, and the inter-variant relationship is mainly (011) type I twinning.