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.     

富镍镨掺杂镍钛形状记忆合金的微结构、相变特性与硬度

采用真空熔炼法向NiTi二元合金中掺杂Pr稀土元素,制备了多组分原子分数的Ni₅₀Ti_50-xPr_x（x=0,0.1,0.3,0.5,0.7,0.9）合金。研究了Pr元素的添加对NiTi合金金相组织、相变温度和硬度的影响。结果表明,Ni₅₀Ti_50-xPr_x合金由NiTi基体与NiPr夹杂相组成,其中Ni₅₀Ti_49.5Pr_0.5合金的马氏体相变温度达73 ℃,合金的热滞窄至37 ℃,维氏硬度约为2850 MPa。Pr元素的添加显著降低了NiTi合金的马氏体相变温度,同时,与其他NiTi基合金相比,NiTiPr合金保持了较窄的热滞和较高的硬度。     

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.     

MAGNETIC--FIELD--CONTROLLED SHAPE MEMORY EFFECT AND SUPERELASTICITY OF Ni53.2Mn22.6Ga24.2 SINGLE CRYSTAL

证实了Ni_53.2Mn_22.6Ga_24.2单晶发生的两步马氏体相变行为是完全热弹性的. 在磁场作用下, 该材料的马氏体相变和中间马氏体相变展现出相同的应变特征, 且具有磁控双向形状记忆效应. 磁场下应力--应变特性的测量结果表明, 磁场不但对压应力诱发马氏体相变过程中变体重取向所需应力的大小有影响, 而且使原来不可逆的形变成为可逆, 这种磁控超弹性特性预示了该合金用作磁控超弹性元器件材料的可能性.     

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.     

The microstructure and properties of rapidly quenched Ti-rich Ti-Ni binary alloys with shape memory effects

The structure, phase composition, and martensitic transformations in binary titanium-rich Ti-Ni alloys with shape memory effects, produced by ultrarapid quenching using melt jet spinning, have been studied using electron microscopy, X-ray diffraction, and measurements of some physicomechanical properties in a wide temperature range. The alloys with a Ti content that exceeded the stoichiometric composition by 5% and more can be produced in an amorphous state. The alloys with a smaller deviation from the stoichiometry, as well as the Ti₅₀Ni₅₀ alloy, are crystallized in a submicrocrystalline state and undergo a B2 → B19’ martensitic transformation at temperatures above room temperature. They have high strength and plastic properties and demonstrate narrow-hysteresis shape-memory effects.     

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.     

Phase transformation and damping behavior of lightweight porous TiNiCu alloys fabricated by powder metallurgy process

Porous TiNiCu ternary shape memory alloys (SMAs) were successfully fabricated by powder metallurgy method. The microstructure, martensitic transformation behavior, damping performance and mechanical properties of the fabricated alloys were intensively studied. It is found that the apparent density of alloys decreases with increasing the Cu content, the porous Ti₅₀Ni₄₀Cu₁₀ alloy exhibits wide endothermic and exothermic peaks arisen from the hysteresis of martensitic transformations, while the porous Ti₅₀Ni₃₀Cu₂₀ alloy shows much stronger and narrower endothermic and exothermic peaks owing to the B2-B19 transformation taking place easily. Moreover, the porous Ti₅₀Ni₄₀Cu₁₀ alloy shows a lower shape recovery rate than the porous Ti₅₀Ni₅₀ alloy, while the porous Ti₅₀Ni₃₀Cu₂₀ alloy behaves reversely. In addition, the damping capacity (or internal friction, IF) of the porous TiNiCu alloys increases with increasing the Cu content. The porous Ti₅₀Ni₃₀Cu₂₀ alloy has very high equivalent internal friction, with the maximum equivalent internal friction value five times higher than that of the porous Ti₅₀Ni₅₀ alloy.     

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.     

Phase transformation behaviors and mechanical properties of Ti50Ni49Fe1 alloy with severe plastic deformation

In this work, transformation behaviors and mechanical properties of cold-rolled shape memory alloy TisoNia9Fel by severe plastic deformation （SPD） were intensively investigated. The phase transformation behaviors, phase analysis, and microstructures were characterized by differential scanning calorimetry （DSC）, X-ray diffraction （XRD）, and transmission electron microscopy （TEM）, respectively. Tensile testing was performed to analyze the effect of SPD on the mechanical properties and shape memory of TisoNi49Fel alloy. When the thickness reduction is beyond 30 %, the martensitic transformation is suppressed. After cold-rolling, the alloy is mainly com- posed of B2 parent phases with some stress-induced martensitic B 19t phases, and high density of dislocations are generated and the grains are obviously refined. The yield stress ab significantly raises from 618 MPa of 0 % cold rolling to 1,338 MPa of 50 % SPD. Shape-memory effect increases from 6.5 % without cold rolling to 8.5 % after 30 % SPD, ascribed to the induced defects in cold rolling. Those results indicate that TisoNi49Fel alloy has improved mechanical properties and potential commercial applications after SPD.     

Effect of Aging Treatment on Superelasticity of a Ti48.8Ni50.8V0.4 Alloy

In this study, effect of aging treatment on microstructure, deformation behavior, and superelasticity of Ti_48.8Ni_50.8V_0.4 alloy was investigated. After aging at 400?°C for 30?min, Ti₃Ni₄ precipitates formed. With increasing aging temperature from 300 to 450?°C, the yield strength of reoriented martensite increased due to the strengthening effect of Ti₃Ni₄ phase, thus improved the shape recovery ratio and reduced the stress hysteresis. Further increasing the aging temperature, the size of Ti₃Ni₄ precipitates increased and the coherency between precipitate and matrix gradually lost, leading to the decreasing yield strength of reoriented martensite and shape recovery ratio. Simultaneously, the stress hysteresis increased resulting from the hinder of plastic deformation to the interfacial movement during phase transformation. The critical stress to induce martensitic transformation continuously decreased with increasing aging temperature.     

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.     

Phase transformation behavior and mechanical properties of Ti50Ni49−x Fe1Co x shape memory alloys

In this article, the influence of Co addition on phase transformation behavior and mechanical properties of TiNiFe shape memory alloy was investigated extensively. Differential scanning calorimetry (DSC) measurements shows that martensitic start transformation temperatures (Ms ) decrease drastically with increasing Co content, while the R phase transformation start temperatures (Rs ) vary slightly. Nevertheless, the substitution of Ni with Co does not exert substantial influence on the two-stage transformation behavior of the TiNiFe alloy. The results from stress-strain curves indicate that higher critical stress for stress-induced martensitic transformation (rSIM ) has been obtained because of Co addition. In such cases, the Ti50Ni48Fe1 Co1.0 alloy maintains a good shape memory effect, and a maximum recoverable strain of 7.5 % can be obtained.     

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.     

Microstructure and martensitic transformation of Ni–Fe–Ga–Si ferromagnetic shape memory alloys

The effects of the fourth element Si on the martensitic transformation and magnetic properties of Ni–Fe–Ga magnetic shape memory alloys were investigated. A complete thermoelastic martensitic transformation in Ni–Fe–Ga–Si alloys was observed in the temperature range of 218–285 K. The martensitic transformation temperatures of Ni–Fe–Ga alloys are obviously decreased by the substitution of Si for Ga element, that is, the substitution of 1 at.% Si for Ga leads to a decrease of martensitic transformation temperature of about 39.6 K. Moreover, the substitution of Si for Ga leads to a decrease of the saturation magnetic field and the magnetic anisotropy constant K₁ obviously.     

Martensitic transformation and mechanical properties of Ti-rich Ti-Ni-Cu melt-spun ribbon

Martensitic transformation, mechanical and thermomechanical properties of a Ti-rich Ti₅₂Ni₂₃Cu₂₅ melt spun ribbon annealed at a temperature below the crystallization temperature were studied by XRD, DSC and DMA. After annealing the initially amorphous ribbon at 400 °C for 10 h, the ribbon is fully crystallized and exhibits one-stage B2?B19 phase transformation with the temperature hysteresis of 14 °C. The annealed ribbon is composed of B2, B19 and B11-TiCu phase with (001) preferential orientation. On the stress―strain curves, the rearrangement of the martensite variants and stress-induced martensitic transformation are observed below the M_f temperature and above the A_f temperature, respectively. The annealed ribbon exhibits up to 1.6% superelastic shape recovery with small stress hysteresis of 25 MPa. No flat stress-plateau is associated with the superelasticity. The annealed ribbon shows a well-defined shape memory effect during thermal cycling from ?60 to 100 °C. The transformation strain and recovery strain increase with increasing the applied external stress. Under the external stress above 150 MPa, the shape recovery strain is not sensitive to it and keeps stable at about 1.74%.     

Comparative study on microstructure and martensitic transformation of aged Ni-rich NiTi and NiTiCo shape memory alloys

In this article the influence of aging heat treatment conditions of 250, 350, 450 and 550 °C for 3 h on the microstructure, martensitic transformation temperatures and mechanical properties of Ni₅₁Ti₄₉Co₀ and Ni₄₇ Ti₄₉Co₄ shape memory alloys was investigated. This comparative study was carried out using X-ray diffraction analysis, scanning electron microscope, energy dispersive spectrometer, differential scanning calorimeter and Vickers hardness tester. The results show that the microstructure of both aged alloys contains martensite phase and Ti₂Ni in addition to some other precipitates. The martensitic transformation temperature was increased steadily by increasing the ageing temperature and lowering the value of valence electron number (e_v/a) and concentration. Moreover, the hardness measurements were gradually increased at first by increasing the aging temperature from 250 to 350 °C. Further elevating in aging temperature to 450 and 550 °C decreases the hardness value.     

时效对TiNiCr形状记忆合金马氏体相变与超弹性的影响(英文)

通过X射线衍射分析(XRD)、透射电子显微观察(TEM)、差式扫描量热分析(DSC)与拉伸实验研究时效处理对Ti48.4Ni51.1Cr0.5合金显微组织、马氏体相变与超弹性的影响规律与机制。经400°C时效处理30 min后,合金中形成Ti3Ni4析出相。当时效温度介于400°C和500°C之间时,合金表现出两步马氏体相变。经时效处理的Ti48.4Ni51.1Cr0.5合金在室温下表现出优异的超弹性。随时效温度自300°C升高到450°C,超弹性恢复率增加。继续升高时效温度,恢复率下降。超弹性应力滞后表现出相反的变化趋势。通过分析Ti3Ni4析出相随时效处理的演化规律解释了时效处理与马氏体相变和超弹性之间的关系。     

Superelastic behavior of nanostructured Ti50Ni48Co2 shape memory alloy with cold rolling processing

Effects of cold rolling followed by annealing on microstructural evolution and superelastic properties of the Ti₅₀Ni₄₈Co₂ shape memory alloy were investigated. Results showed that during cold rolling, the alloy microstructure evolved through six basic stages including stress-induced martensite transformation and plastic deformation of martensite, deformation twinning, accumulation of dislocations along twin and variant boundaries in martensite, nanocrystallization, amorphization and reverse transformation of martensite to austenite. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold-rolled specimens was completely crystallized and an entirely nanocrystalline structure was achieved. The value of stress level of the upper plateau in this nanocrystalline alloy was measured as high as 730 MPa which was significantly higher than that of the coarse-grained Ni₅₀Ti₅₀ and Ti₅₀Ni₄₈Co₂ alloys. Moreover, the nanocrystalline Ti₅₀Ni₄₈Co₂ alloy had a high damping capacity and considerable efficiency for energy storage.     

Accumulation of Residual Strain in TiNi Alloy During Thermal Cycling

Residual strain accumulation during thermal cycling of the Ti₅₀Ni₅₀ alloy under a constant stress of 200 MPa through the temperature range of complete and incomplete forward martensitic transformation was studied. The temperature range of the forward martensitic transformation during thermal cycling was chosen as 25, 50, 75, or 100% of the M _s-M _f interval measured in the first cycle. It was shown that intensive accumulation of residual strain took place in the last stage of the forward transformation. It was observed that resistivity increased more rapidly with an increase of the fraction of the temperature range of forward martensitic transformation.