Shape memory behavior of Ti–Ta and its potential as a high-temperature shape memory alloy

In this study, the effect of Ta content on shape memory behavior of Ti–Ta alloys was investigated. The shape memory effect was confirmed in Ti–(30–40)Ta alloys. The martensitic transformation start temperature (M_s) decreased by 30 K per 1 at.% Ta. The amount of ω phase formed during aging decreased with increasing Ta. A stable high-temperature shape memory effect was confirmed for Ti–32Ta (M_s = 440 K) during thermal cycling between 173 and 513 K. On the other hand, the high-temperature shape memory effect of Ti–22Nb, which has a similar M_s to Ti–32Ta, exhibited poor stability due to the large amount of ω phase formed during thermal cycling. It is suggested that Ti–Ta is an attractive candidate for the development of novel high-temperature shape memory alloys.     

Shape memory effect of TiNi-based springs trained by constraint annealing

This study investigated shape memory effect (SME) behaviors of TiNi-based shape memory alloys springs trained by constraint annealing treatments. The results indicated that SME behaviors were determined by constraint annealing sequences and the chemical composition of alloys. Springs made by Ni-rich TiNi binary alloys showed both single-way SME (SW-SME) and two-way SME (TW-SME) depending on the constraint annealing sequences, while those made by TiNiCu alloys only showed SW-SME. The mechanism and stability of TW-SME induced by constraint annealing were discussed. The occurrence of TW-SME was found to be associated with oriented internal coherent strain caused by preferential Ti₃Ni₄ precipitates. A special method, which could achieve a stable post-inherent TW-SME and size selectivity, was proposed to provide an effective approach for devising actuators in Ni-rich alloys.     

In situ evaluation of the transformation behaviour of NiTi-based high temperature shape memory alloys

Fine grained polycrystalline NiTi shape memory alloys containing 15 at.% Hf and Zr and zero or 3 at.% Cu fabricated by ingot metallurgy were investigated using in situ synchrotron X-ray diffraction in order to examine the viability of producing stable and affordable high temperature shape memory alloys. The alloys produced had a high thermal hysteresis, in excess of 70 °C but A_f temperatures of over 250 °C were obtained for Ni₅₀Ti₃₅Hf₁₅. 3 at.% Cu additions did not significantly reduce the per-cycle degradation of transformation temperatures but did reduce the transformation temperatures. The evolution of the lattice parameters during the first five thermal cycles was observed. Negative thermal expansion was found in the b_B19′ cell direction in all the alloys examined and significant deviations in the lattice parameters in the region of transformation were found. A per-cycle evolution in the end-point B19′ lattice parameters was observed, but no such evolution was found for the B2 phase, which is rationalised by appealing to the increase in population of interface dislocations.     

Martensitic transition and shape memory effect of Ti-Zr-Mo series alloys

Development of shape memory alloys is always one of the most important directions for functional Ti alloys. The Ti-Zr-Mo series alloys with various Mo contents were prepared. The main aim of the current work is to investigate the effects of Mo on martensitic transition and shape memory effect of Ti-Zr alloy. The X-ray diffraction and transmission electron microscope results indicate that the phase constitution of the examined alloys is greatly dependent on Mo content. The Ti-Zr-Mo alloy with 2 wt% Mo is composed mainly of α′ martensite and a few β phase. As the Mo content increased to 4 wt%, the Ti-50Zr-4Mo alloy consists of α″ martensite and β phase. As the Mo content further increased to 8 wt%, the alloy consists mainly β phase and a barely detectable amount of α″ martensite. Thermal analysis shows that the reverse martensitic transition temperature of the examined alloys decreases with the increasing of Mo. The reverse martensitic transition start, A_s, temperature is approximately 584 °C for Ti-50Zr-2Mo alloy and 519 °C for Ti-50Zr-4Mo, respectively. And the martensitic transition start, M_s, temperature is approximately 553 °C and 501 °C for that two alloys, respectively. But no obvious exothermic and/or endothermic peak can be observed in DSC curve of Ti-50Zr-8Mo alloy. Furthermore, the effect of Mo content on shape memory recovery ratio, η, of the examined alloys was also investigated. Results show that the η first increases and then decreases with the increasing of Mo. The alloy with 4 wt% Mo has the maximum η approximately 13.8%. The influencing mechanism of Mo content on shape memory effect of the examined alloys was also discussed. This findings not only supplied a series of shape memory TiZr-based alloys, but also enriched and deepened the theory of shape memory effect.     

Effect of microstructure on dry sliding wear behaviour in CuZnAl shape memory alloys

The main objective of this work has been the characterization of the wear behaviour of the CuZnAl shape memory alloys in their different phases. The weight losses for the different alloys in function of the present phase, and of the M_s transformation temperature are studied. Adhesive wear tests, Pin-on-Disk, according to the ASTM-G99 standard have been carried out. After this a characterization of the wear surfaces by means of optical microscopy, scanning electron microscopy, differential scanning calorimetry and X-ray diffraction have also been carried out. We can observe that the wear behaviour of this alloy depends on the shape memory properties. For the β shape memory alloys a lineal relation of weight loss with M_s transformation temperatures are obtained. For the martensitic phase and β+ martensitic phase the relation weight loss–M_s is not evident.     

Microstructures and shape memory characteristics of dual-phase Co–Ni–Ga high-temperature shape memory alloys

The influence of microstructure on mechanical properties and shape memory characteristics of Co–Ni–Ga high-temperature shape memory alloys were investigated in this study. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were employed to detect the microstructures. We found that these alloys were composed of dual phases, a non-modulated tetragonal L1₀ martensite and a face-centered cubic (fcc) γ phase. The martensite was twinned and well self-accommodated. The γ phase was a Co-based solid solution with 30% lower hardness than martensite. Although the fracture mode was intergranular, the strength and plasticity of the alloys increased markedly with the increasing volume fraction of the γ phase. The presence of the γ phase in grain boundaries rather than in the martensite is favorable to shape memory recovery. This was revealed by the maximum shape recovery strain over 5.0% that was obtained in the Co₄₆Ni₂₅Ga₂₉ alloy, with the γ phase formed mainly in grain boundaries.     

Comparison of high temperature shape memory behaviour for ZrCu-based,Ti–Ni–Zr and Ti–Ni–Hf alloys

New ZrCu-based high temperature shape memory alloys with M_s close to 500 K are under development. The shape memory behaviour of this material is compared to those of Ti–Ni–Zr and Ti–Ni–Hf alloys. The optimal compositions show a shape recovery of not less than 3% at temperatures above 470 K.     

Microstructure and martensitic transformation behavior of Ni56-xMn25FexGa19 shape memory alloys简

Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni56-x Mn25 Fex Ga19(x = 0, 2, 4, 6, 8) shape memory alloys were investigated using optical microscopy(OM), X-ray diffraction analysis(XRD), differential scanning calorimeter(DSC), and compressive test. It is found that these alloys are composed of single non-modulated martensite phase with tetragonal structure at room temperature, which means substituting Fe for Ni in Ni56 Mn25 Ga19 alloy has no effect on phase structure. These alloys all exhibit a thermoelastic martensitic transformation between the cubic parent phase and the tetragonal martensite phase. With the increase of Fe content, the martensitic transformation peak temperature(Mp) decreases from 356 °C for x = 0 to 20 °C for x = 8, which is contributed to the depressed electron concentration and tetragonality of martensite. Fe addition remarkably reduces the transformation hysteresis of Ni–Mn–Ga alloys. Substituting Fe for Ni in Ni56 Mn25 Ga19 alloy can decrease the strength of the alloys and almost has no influence on the ductility and shape memory property.     

高熵形状记忆合金的研究进展

高熵形状记忆合金是在等原子比NiTi合金的基础上,结合高熵合金的概念,逐渐发展起来的一种新型高温形状记忆合金。近年来,已开发出了综合性能优异的(TiZrHf)50(NiCoCu)50系和(TiZrHf)50(NiCuPd)50系高熵形状记忆合金,引起了广泛的关注和研究兴趣。本文从物相组成、微观组织、马氏体相变行为、形状记忆效应和超弹性等角度出发,综述了高熵形状记忆合金的研究进展,并对高熵形状记忆合金未来的研究重点进行了展望。     

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.     

Microstructural,mechanical and shape memory characterizations of Ti–Mo–Sn alloys

As a β stabilizing element in Ti-based alloys, the effect of Mo on phase constitution, microstructure, mechanical and shape memory properties was investigated. Different compositions of Ti–xMo–3Sn alloys (where x=2, 4, 6, at.%) were prepared by arc melting. A binary composition of Ti–6Mo alloy was also prepared for comparison. Ti–xMo–3Sn alloys show low hardness and high ductility with 90% reduction in thickness while Ti–6Mo alloy shows high hardness, brittle behavior, and poor ductility. Field emission scanning electron microscopy (FESEM) reveals round morphology of athermal ω (ω_ath) precipitates. The presence of ω_ath phase is also confirmed by X-ray diffraction (XRD) in both as-cast and solution-treated and quenched conditions. The optical microscopy (OM) and FESEM show that the amount of martensite forming during quenching decreases with an increase in Mo content, which is also due to β→ω transformation. The hardness trends reinforce the presence of ω_ath too. The shape memory effect (SME) of 9% is the highest for Ti–6Mo–3Sn alloy. The SME is trivial due to ω_ath phase formation; however, the increase in SME is observed with an increase in Mo content, which is due to the reverse transformation from ω_ath and the stress-induced martensitic transformation. In addition, a new and very simple method was designed and used for shape memory effect measurement.     

Phase composition and plasticity of alloys with shape memory of the system Ni-Mn-Ti

In the alloys of the systems Ni-Ti, Cu-Al-Ni, Cu-Zn-Al, Cu-Al-Mn shape memory is found in small intervals of concentration of the existence of intennetallic phases with the crystalline structures B2, DO₃, L2₁. A deviation from the optimal range of concentrations or the segregation of phases in heat treatment impairs the characteristics of shape restoration. In the elaboration and introduction of alloys with shape mennory based on intennetallic phases it is therefore particularly important to study stable and metastable phase diagrams. The plotting of such diagrams is the main task of the present work.I. P. Bardin Central Research Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 31–34, January, 1994.     

Analysis of Variant Orientation Before and After Compression in Polycrystalline Ni50Mn29Ga21 MSMA

This paper deals with the microstructure and plastic deformation of Ni₅₀Mn₂₉Ga₂₁ ferromagnetic shape memory alloys. In contrast to conventional shape memory alloys, plastic deformation in the martensitic phase, which is due to twin boundary motion, may be caused not only by mechanical stress but also by an external magnetic field. The polycrystalline sample was prepared by directional solidification with a texture parallel to the heat flow. Afterwards, a heat treatment follows for chemical homogenization and stress relaxation in the austenitic state. The configuration of the twin boundaries was analyzed before and after compressing the samples. The microstructure after compression was related to the magnetic properties.     

Smart Control Systems for Smart Materials

Shape memory alloys (SMAs) are thermally activated smart materials. Due to their ability to change into a previously imprinted shape by the means of thermal activation, they are suitable as actuators for microsystems and, within certain limitations for macroscopic systems. Most commonly used SMAs for actuators are binary nickel-titanium alloys (NiTi). The shape memory effect relies on the martensitic phase transformation. On heating the material from the low temperature phase (martensite) the material starts to transform into the high temperature phase (austenite) at the austenite start temperature (A _s). The reverse transformation starts at the martensite start temperature after passing a hysteresis cycle. To apply these materials to a wide range of industrial applications, a simple method for controlling the actuator effect is required. Today??s control concepts for shape memory actuators, in applications as well as in test stands, are time-based. This often leads to overheating after transformation into the high temperature phase which results in early fatigue. Besides, the dynamic behavior of such systems is influenced by unnecessary heating, resulting in a poor time performance. To minimize these effects, a controller system with resistance feedback is required to hold the energy input on specific keypoints. These two key points are directly before transformation (A _s) and shortly before retransformation (M _s). This allows triggering of fast and energy-efficient transformation cycles. Both experimental results and a mechatronical demonstrator system, exhibit the advantages of systems concerning efficiency, dynamics, and reliability.     

Effect of ternary alloying elements on the shape memory behavior of Ti–Ta alloys

The effect of ternary alloying elements (X = V, Cr, Fe, Zr, Hf, Mo, Sn, Al) on the shape memory behavior of Ti–30Ta–X alloys was investigated. All the alloying elements decreased the martensitic transformation temperatures. The decrease in the martensitic transformation start (M_s) temperature due to alloying was affected by the atomic size and number of valence electrons of the alloying element. A larger number of valence electrons and a smaller atomic radius of an alloying element decreased the M_s more strongly. The effect of the alloying elements on suppressing the aging effect on the shape memory behavior was also investigated. It was found that the additions of Sn and Al to Ti–Ta were effective in suppressing the effect of aging on the shape memory behavior, since they strongly suppress the formation of ω phase during aging treatment. For this reason the Ti–30Ta–1Al and Ti–30Ta–1Sn alloys exhibited a stable high-temperature shape memory effect during thermal cycling.     

On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys

In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0}_B2 planes parallel to {0 0 1}_B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys.     

Electron-irradiation-induced nano-crystallization in quasicrystal-forming Zr-based metallic glass

Phase selection in electron-irradiation-induced crystallization and crystal-to-amorphous-to-crystal (C–A–C) transition at 298 K in quasicrystal-forming Zr–Pt metallic glass alloys were investigated. Two types of f.c.c. nano-crystalline precipitates were formed in amorphous Zr₈₀Pt₂₀ and Zr_66.7Pt_33.3 alloys under electron irradiation; such unique nano-crystalline structures were not observed during thermal annealing. It was inferred that unique phase selection in electron-irradiation-induced crystallization and thermal crystallization can be explained by the large negative chemical mixing enthalpy (ΔH_chem) in Zr–Pd and Zr–Pt alloys.     

Self-accommodation of crystals of martensitic phases in titanium and zirconium based alloys

The orientation relationships between austenite and ???? and ???? martensite lattices in titanium and zirconium alloys is performed in view of the possibility of the formation of self-accommodating complexes, which is an important component of the shape memory effect. Different diffraction patterns calculated for various matrices of the orientation relationship were compared with the experimental patterns for Ti₄₅Zr₄₅Nb₁₀ alloy using a formula developed for the optimal expression of the orientation relationship for ????. The generalization and analysis of the literature data for titanium- and zirconium-based solid solutions performed using the developed algorithm lead us to conclusions regarding the tendency of ???? martensite in titanium-based solid solutions to self-accommodate, which occurred more strongly than that in zirconium-based alloys, and the best results in shape memory realization should be expected in Ti-Ta and Ti-Nb systems. Possible reasons for the absence of the shape memory effect for ???? martensite in titanium- and zirconium-based alloys, with regard to the necessary crystallographic conditions for the formation of self-accommodating processes are analyzed.     

A new ferromagnetic shape memory alloy system

It is shown that the alloys Co₂Ni_1−xGa_1+x, x = 0.06, 0.09, 0.12, 0.15, are ferromagnetic shape memory alloys. In the as-solidified state their martensite start temperatures vary in the range 20 °C<T<60 °C as the concentration parameter x decreases. The high and low temperature phases are body centered cubic and orthorhombic and/or monoclinic, respectively, The transformation hysteresis, i.e. the difference between the martensite and austenite start temperatures, equals approximately 30 degrees. The saturation magnetization of the alloys resembles that of nickel while their coercive force is of the order of 100mT.     

Work Production and Variation in Shape Memory Effects during Thermal Cycling of Equiatomic TiNi Alloy

The influence of thermal cycling under a symmetric scheme for work performance on the functional properties and work output of equiatomic TiNi shape memory alloys was studied. It was found that the Ti₅₀Ni₅₀ alloy produced an effective work output of 9.7 MJ/m³ and an efficiency of 1.47% in a symmetrical scheme for work production where the stress acting on cooling was 50 MPa, and the stress acting on heating was 200 MPa. It was observed that the Ti₅₀Ni₅₀ alloy demonstrated the monotonic dependence of plastic strain on the number cycle, and 3.1% of the residual strain was accumulated over 30 cycles. The data have shown that using a ‘symmetric’ scheme for work production allows one to reduce a plastic strain 13-fold in comparison with an ‘asymmetric’ scheme. Thus, the symmetric scheme for work production provides for the high stability of the functional properties and work output of TiNi-based shape memory alloys.