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.     

Effect of ageing on transformation kinetics and internal friction of Ni-rich Ni–Ti alloys

In Ni-rich Ni–Ti alloys the ageing treatments create finely dispersed Ti₃Ni₄ precipitates in the B2-based matrix. Formed precipitates are known to influence the phase transformation temperatures and to facilitate the R-phase transformation. In addition to the change in the phase transformation behaviour, the ageing treatments are also reported to affect internal friction and mechanical properties of Ni–Ti alloys.In the present work, by means of systematic DSC and DMA studies the effects of the ageing treatments on the phase transformation as well as on the internal friction and mechanical properties of Ni-rich Ni–Ti alloy were studied. As an extension to earlier studies, the present study concentrates both on the effects of the ageing time and the ageing temperature systematically. By combining the results of the DSC measurements of phase transformation behaviour to the DMA measurements of internal friction and mechanical behaviour of the Ni-rich Ni–Ti alloy, the present study gives yet unpublished information about the comprehensive effects of the ageing treatments on these alloys. These results enable to control the phase transformation temperatures, internal friction and mechanical properties of the Ni-rich Ni–Ti alloy by selecting the suitable ageing treatment.Results showed that the noticed effects of the ageing treatments on mechanical properties could be explained with the changes in the size, distribution, density and coherency of the formed Ti₃Ni₄ precipitates. The high internal friction values can be explained as the contribution of the internal stress fields and increased amount of mobile interfaces as the result of the formed Ti₃Ni₄ precipitates. The amount of mobile interfaces seems to be more dominant factor for the increased internal friction value than the effect of the internal stress fields. Therefore, the optimal internal friction values can be obtained with a proper aging treatment which will yield a high density of small Ti₃Ni₄ precipitates.     

Effect of Ni4Ti3 precipitation on martensitic transformation in Ti–Ni

Precipitation of Ni₄Ti₃ plays a critical role in determining the martensitic transformation path and temperature in Ni–Ti shape memory alloys. In this study, the equilibrium shape of a coherent Ni₄Ti₃ precipitate and the concentration and stress fields around it are determined quantitatively using the phase field method. Most recent experimental data on lattice parameters, elastic constants, precipitate–matrix orientation relationship and thermodynamic database are used as model inputs. The effects of the concentration and stress fields on subsequent martensitic transformations are analyzed through interaction energy between a nucleating martensitic particle and the existing microstructure. Results indicate that R-phase formation prior to B19′ phase could be attributed to both direct elastic interaction and stress-induced spatial variation in concentration near Ni₄Ti₃ precipitates. The preferred nucleation sites for the R-phase are close to the broad side of the lenticular-shaped Ni₄Ti₃ precipitates, where tension normal to the habit plane is highest, and Ni concentration is lowest.     

Phase transformations in a Ti50Ni47.5Fe2.5 shape memory alloy

An optical metallographic study involving surface relief was made of the phase transformations in a Ti₅₀Ni_47.5Fe_2.5 alloy. The formation and reversion of two different kinds of surface relief associated with R-phase and martensitic transformations were observed upon thermal cycling. The nature of the R-phase transition and its microstructural changes during deformation were also observed; these are described.     

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.     

In situ TEM observation of two-step martensitic transformation in aged NiTi shape memory alloy

Martensitic transformation was investigated in an aged NiTi alloy with DSC and a temperature controllable TEM specimen stage to observe the influence of Ti₁₁Ni₁₄ precipitates and R-phase on martensitic transformation in situ. The R-phase, conventional martensitic twins, and a new morphology of interwoven austenite/martensitic structure were observed.     

Microstructures of crystallized Ti51.5Ni48.5−xCux (x = 23.4–37.3) thin films

The microstructures 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_51.2Ni_15.7Cu_33.1 and Ti_51.4Ni_11.3Cu_37.3 films showed very small grain sizes (120 and 50 nm, respectively) compared with the Ti_51.4Ni_25.2Cu_23.4 and Ti_51.3Ni_21.1Cu_27.6 films (1 and 0.3 μm, respectively). They had no precipitates within the B2 grains. On the other hand, the Ti_51.4Ni_25.2Cu_23.4 films annealed at 773 and 873 K showed GP zones and Ti₂Cu precipitates, respectively, and the Ti_51.3Ni_21.1Cu_27.6 film annealed at 773 K showed TiCu precipitates in the grain interiors. The formation of precipitates in the grain interior was discussed in terms of the lattice mismatch between the precipitates and the matrix. The difference in grain size was attributed to different crystallization processes.     

A Two-Way Shape Memory Study on Ni-Rich NiTi Shape Memory Alloy by Combination of the All-Round Treatment and the R-Phase Transformation

Ni₄Ti₃ precipitates are formed in Ni-rich NiTi shape memory alloys (SMAs) after a certain heat treatment. Such a treatment then results in the changed phase transformation behavior of the alloy switching from the one-step reversible phase change B2 ? B19′ to the two-step, B2 ? R ? B19′, phase change. The present study reports a two-way shape memory (TWSM) due to an all-round treatment followed by an R-phase constraint aging at room temperature. The enhanced TWSM behavior was observed upon temperature cycling between 273 K (R-phase) and 77 K (B19′ phase). The effect of various constraint strains in R-phase aging by employing different diameters of the constraint ring was studied. However, the TWSM effect due to constraining the R-phase will be eliminated after a temperature rise of specimen to the fully parent phase (373 K).     

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.     

Influence of aging on microstructure,martensitic transformation and mechanical properties of NiTiRe shape memory alloy

Aging is an effective way to adapt the microstructure, phase transformation and consequently the mechanical properties of NiTi shape memory alloys. In the present study, Ni₅₂Ti_47.7Re_0.3 shape memory alloy was solution treated at 1000 °C for 24 h then aged at various temperatures of 300, 400, 500 and 600 °C for 3 h. The influence of aging treatment on microstructure, martensitic transformation and mechanical properties of Ni₅₂Ti_47.7Re_0.3 was investigated. The microstructure of the solution treated alloy was martensite as a matrix phase and precipitates of Ti₂Ni phase. The aged alloys had a microstructure as same as that of solution treated alloy in addition to the existence of other types of precipitates like Ni₄Ti₃ and Ni₃Ti. The martensitic — austenitic transformation during heating and cooling was going through one stage of transformation. The martensitic phase transformation temperature increased by the increase of aging temperature but still lower than that of solution treated alloy.     

Influence of Post-Weld Annealing on Transformation Behavior and Mechanical Properties of Laser-Welded NiTi Alloy Wires

The influence of annealing on the transformation behavior, mechanical, and functional properties of laser-welded NiTi wires was investigated. The results show that Ti₃Ni₄ precipitates occur after post-weld annealing and coarsen with increasing annealing temperature. The as-welded specimen exhibits one-step B2 → B19′ transformation, while the annealed ones show two-step B2 → R → B19′ transformation. Annealing at 400 °C for 1 h can improve the tensile strength and superelasticity of the welded joints. However, these properties decrease when annealing at 500 °C for 1 h. The change of mechanical and functional properties after annealing is attributed to the different size of Ti₃Ni₄ precipitates. Annealing to produce smaller coherent precipitates (10 nm) improves the mechanical and functional properties of the welded joints. As the Ti₃Ni₄ precipitates coarsen, the mechanical and functional properties decrease.     

Structural and Mössbauer spectroscopic investigation of Fe substituted Ti–Ni shape memory alloys

In the present investigation the effect of Fe substitution in Ti₅₁Ni₄₉ alloy has been studied. The alloys were synthesized through radio frequency induction melting. The alloy was characterized through X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Mössbauer spectroscopy and positron annihilation techniques. It was found that the Fe substitution stabilized the TiNi type cubic (a = 2.998 Å) phase. The microstructure and presence of the oxide phase in Ti₅₁Ni₄₅Fe₄ alloy have been investigated by scanning electron microscopy. The positron annihilation measurements indicated a similar bulk electron density in both the as-cast and annealed (1000 °C for 30 h) alloys, typically like that of bulk Ti. Mössbauer spectroscopy studies of as-cast and annealed iron substituted samples showed regions in the samples where nuclear Zeeman splitting of Fe levels occurred and an oxide phase was found to be present in as cast Ti₅₁Ni₄₅Fe₄ alloy, while annealed sample indicated the presence of bcc iron phase.     

The mechanism of multistage martensitic transformations in aged Ni-rich NiTi shape memory alloys

Usually aged Ni-rich NiTi alloys undergo martensitic transformations on cooling from high temperatures in two steps: B2 to R and then R to B19′ (normal behaviour). But under certain ageing conditions, the transformation can also occur in three or more steps (unusual multiple step behaviour). In the present study we use differential scanning calorimetry (DSC) for a systematic investigation of the evolution of transformation behaviour with ageing temperature and time. We demonstrate that during ageing of Ni-rich NiTi alloys, DSC curves exhibit two transformation peaks on cooling after short ageing times, three after intermediate ageing times and finally again two peaks after long ageing times (2–3–2 transformation behaviour). In the present study we propose a new explanation for the 2–3–2 transformation behaviour which consists of two basic elements: (1) The composition inhomogeneity which evolves during ageing as Ni₄Ti₃ precipitates grow. (2) The difference between nucleation barriers for R-phase (small) and B19′ (large). These two elements explain all features of the evolution of DSC charts during ageing including the number of distinct DSC peaks and their positions.     

Stability,phase transformation and deformation behavior of Ti-base metallic glass and composites

Melt-spun ribbons and copper-mold cast cylinders of (Ti_0.5Cu_0.23Ni_0.2Sn_0.07)_100−xMo_x bulk glass-forming alloys are prepared. Both Ti₅₀Cu₂₃Ni₂₀Sn₇ and (Ti_0.5Cu_0.23Ni_0.2Sn_0.07)₉₅Mo₅ melt-spun glassy ribbons exhibit large supercooled liquid regions, high reduced glass transition temperatures, and good thermal stabilities. During continuous heating of the melt-spun ribbons, both alloys present a two-stage crystallization behavior. Mo slightly lowers the glass-forming ability but significantly decreases the temperature of the second stage crystallization. For both alloys, the stable phases after heating are Ti₂Ni, TiCu, Ti₃Sn and β-(Cu,Sn). As-cast Ti₅₀Cu₂₃Ni₂₀Sn₇ cylinders contain dendritic hcp-Ti solid solution precipitates, as well as interdendritic glassy and Sn-rich crystalline phases. The ultimate compression stress reaches 2114 MPa with 5.5% plastic strain for 2-mm diameter cylinders. Yielding occurs at 1300 MPa, and Young’s modulus is 85.3 GPa. Mo improves and stabilizes the precipitation of a β-Ti solid solution but prevents glass formation in as-cast (Ti_0.5Cu_0.23Ni_0.2Sn_0.07)₉₅Mo₅ bulk alloys. The bulk samples contain dendritic β-Ti solid solution precipitates, Ti₂Ni particles and Sn-rich phases. The ultimate compression stress is 2246 MPa with about 1% plastic strain for a 3-mm diameter cylinder. σ_0.2 is about 1920 MPa and Young’s modulus is 104 GPa. The high strength is attributed to both Mo solution strengthening and Ti₂Ni particle strengthening. The limited ductility is induced by the precipitation of brittle Ti₂Ni particles.     

Superelastic properties of nanocrystalline NiTi shape memory alloy produced by thermomechanical processing

Effects of thermomechanical treatment of cold rolling followed by annealing on microstructure and superelastic behavior of the Ni₅₀Ti₅₀ shape memory alloy were studied. Several specimens were produced by copper boat vacuum induction melting. The homogenized specimens were hot rolled and annealed at 900 °C. Thereafter, annealed specimens were subjected to cold rolling with different thickness reductions up to 70%. Transmission electron microscopy revealed that the severe cold rolling led to the formation of a mixed microstructure consisting of nanocrystalline and amorphous phases in Ni₅₀Ti₅₀ alloy. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold-rolled specimens was crystallized and a nanocrystalline structure formed. Results showed that with increasing thickness reduction during cold rolling, the recoverable strain of Ni₅₀Ti₅₀ alloy was increased during superelastic experiments such that the 70% cold rolled–annealed specimen exhibited about 12% of recoverable strain. Moreover, with increasing thickness reduction, the critical stress for stress-induced martensitic transformation was increased. It is noteworthy that in the 70% cold rolled–annealed specimen, the damping capacity was measured to be 28 J/cm³ that is significantly higher than that of commercial NiTi alloys.     

Influence of Ti additions on martensitic transformation and magnetic properties of cast Ni51Fe22−xGa27Tix shape memory alloys

The effect of Ti addition on the microstructure, martensitic transformation, magnetic and mechanical properties of polycrystalline Ni₅₁Fe_22?x Ga₂₇Ti _x (x=0, 2 and 4) ferromagnetic shape memory alloy was investigated by scanning electron microscope, differential scanning calorimetry and X-ray diffraction. The results showed that the martensitic transformation temperature increases monotonously with the increase of fraction of Ti substitution for Fe. The increase in the martensite transformation temperatures should be related to the change of the electron concentration after the addition of Ti to Ni₅₁Fe_22?x Ga₂₇Ti _x alloys. According to the results of X-ray diffraction and magnetic properties, Ti has significant effect the structure of Ni₅₁Fe_22-x Ga₂₇Ti _x . Adding of 4 at% Ti altered the structure of the matrix from five-layered tetragonal martensite of Ni₅₁Fe₂₂Ga₂₇ and Ni₅₁Fe₂₀Ga₂₇Ti₂ alloys to non-modulated tetragonal martensite. Magnetic properties proved that the alloy transits from ferromagnetic, five-layered tetragonal martensite, to paramagnetic, non-modulated martensite structure, with increasing Ti content to 4 at.%. Saturation magnetization, remnant magnetization and coercivity of the alloy were significantly influenced by Ti additions. Hardness values of Ni₅₁Fe₂₂Ga₂₇ increased by the addition of Ti.     

The effect of aging on the B2-R transformation behaviors in Ti-51at%Ni alloy

Aging is an effective way to control the transformation behaviors of Ti–Ni shape memory alloys. In the present study, the transformation behaviors of Ti-51at%Ni polycrystals after aging at various temperatures (250 °C–500 °C) for different time were investigated by DSC measurement. It is found that B2-R transformation temperature (R_S) of the aged samples is uniquely determined by the aging temperature for aging above 400 °C, being almost independent on aging time. Higher aging temperature causes a lowering of R_S temperature. At lower aging temperatures (e.g., 250 °C, 300 °C), R_S gradually increases with aging time and finally saturates. The saturated R_S value is also a unique function of the aging temperature. These facts can be explained by the phase equilibrium between Ti–Ni matrix (B2) and Ti₃Ni₄ precipitates. The Ti₃Ni₄ precipitates formed at different aging temperatures alter the Ni concentration in B2 matrix and consequently cause an aging temperature dependence of the R-phase transformation of B2 matrix. By considering the kinetics of precipitation reaction, the “time-independence” of R_S for high temperature aging and the “time-dependence” of R_S for low temperature aging can also be reasonably understood. Besides, the R–B19′ transformation temperature (M_S) gradually increases with aging time as well, which can also be understood by considering the growth of precipitates during aging.     

Phase transformation behavior of Ti49Ni49.5Fe1V0.5 and Ti48Ni48.5Fe1V2.5 alloys after different heat treatments简

The effects of heat treatments on the phase transformation behavior of Ti49 Ni49.5 Fe1 V0.5 and Ti48 Ni48.5 Fe1 V2.5 alloys were investigated. The results indicate that the alloys subjected to different heat treatments have B2 structure at room temperature. All the specimens exhibit a twostage B2→R→B190martensitic transformation on cooling, but a B190→B2 one-stage reverse martensitic transformation on heating except aged A1 alloy, which undergoes an abnormal two-stage transformation upon heating. The phase transformation temperatures are affected by heat treatments and V content, which can be attributed to the variation of the second-phase particles content in the matrix.     

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。     

Effects of equal channel angular extrusion and aging treatment on R phase transformation behaviors and Ti3Ni4 precipitates of Ni-rich TiNi alloys

Ni-rich TiNi alloys were subjected to the effect of multiple equal channel angular extrusion (ECAE) treatments by B_C path at 500 °C. The characteristics of R phase transformation in aging treatment were dissimilar in the appearance and the temperature range to those counterparts induced by ECAE treatments. The fine lens-like shape Ti₃Ni₄ particles precipitated mainly in the regions of near grain boundaries and on the tangled grain boundaries after ECAE treatments. The effects and mechanisms of aging treatments and ECAE treatments on R phase transformation behaviors and Ti₃Ni₄ precipitates were investigated and discussed.