Crystal structures and shape-memory behaviour of NiTi

Shape-memory alloys (SMAs) are a unique class of metal alloys that after a large deformation can, on heating, recover their original shape. In the many practical applications of SMAs, the most commonly used material is NiTi (nitinol). A full atomic-level understanding of the shape-memory effect in NiTi is still lacking, a problem particularly relevant to ongoing work on scaling down shape-memory devices for use in micro-electromechanical systems. Here we present a first-principles density functional study of the structural energetics of NiTi. Surprisingly, we find that the reported B19' structure of NiTi is unstable relative to a base-centred orthorhombic structure that cannot store shape memory at the atomic level. However, the reported structure is stabilized by a wide range of applied or residual internal stresses. We propose that the memory is stored primarily at the micro-structural level: this eliminates the need for two separate mechanisms in describing the two-way shape-memory effect.     

NiTi合金主动脉支架弯曲变形行为的研究

利用Pro/E和Ansys软件建立了胸主动脉支架轴向柔顺性分析模型,对27种NiTi合金支架的柔顺性进行了模拟分析与实验验证。在此基础上,又进一步系统地分析了弯曲变形时支架结构尺寸变化对中央部位横截面变形行为的影响。结果显示,减少圆周方向上的连接体的数量,增加支撑体的长度或数量,都能够提高支架的柔顺性,但是减少连接体的数量比增加支撑体的长度或数量对支架柔顺性的影响程度要大。支架弯曲变形时连接体的数量对支架中央部位横截面的变形具有显著的影响。     

Very high strain-rate response of a NiTi shape-memory alloy

The compressive response of a NiTi shape-memory alloy is investigated at high strain rates, using UCSD’s modified split Hopkinson pressure bar and a mini-Hopkinson bar with specially designed striker bars. To obtain a constant strain rate during the formation of the stress-induced martensite phase in a Hopkinson test, a copper-tube pulse shaper of suitable dimensions or a stepped striker bar is employed, since without a pulse shaper or with a uniform striker bar, the strain rate of the sample will vary significantly as the material’s microstructure changes from austenite to martensite, whereas with proper pulse shaping techniques a nearly constant strain rate can be achieved over a certain deformation range. At a very high strain rate, the yield stress and the stress-induced martensite formation process are significantly different from those at moderately high strain rates, suggesting that, correspondingly, different microstructural changes may be involved in the phase transition regime. The material’s yield stress appears lower when measured in a mini-Hopkinson bar (with very small samples) as compared with that measured by a 1/2-in. Hopkinson bar (with relatively large samples), possibly due to the sample size that may produce different deformation mechanisms within the superelastic strain range. The transition stress from the austenite to the martensite phase shows strain-rate sensitivity. This may be explained by considering the interfacial motion of the formed martensite phase, based on the thermally activated and dislocation-drag models. There exists a certain critical strain-rate level, at which the transition stress for the stress-induced martensite formation equals the yield stress of the austenite phase. Therefore, the shape-memory alloy deforms by the formation of stress-induced martensites, accompanied by the yielding of the martensite phase at this critical strain rate, while the material deforms plastically by the dislocation-induced plastic slip at strain rates above this critical level.     

Tensile behaviour of superelastic NiTi alloys charged with hydrogen under applied strain

ABSTRACT

The tensile behaviour of NiTi alloys is investigated after hydrogen charging during the austenite, half-transformation and martensite phases. The specimens are charged with different current densities and charging durations. During the tensile tests, the strain of the plateau transformation decreases due to hydrogen-induced residual martensite variants. This decrease becomes important when the charging happens during the martensite phase. Accordingly, the hydrogen ensures the stability of the phase in which the charging process occurs. Moreover, a heightening of transformation stress is noticed during the plateau. The transformation stress increases when the current density grows and the charging duration rises. This occurrence is caused by the interaction between the hydrogen and NiTi structures, where hydrogen delays the NiTi martensite transformation.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

FEM analysis of localized deformation behaviour in pseudoelastic NiTi shape memory alloys

This paper takes into account the localized deformation behaviour of a pseudoelastic NiTi shape memory alloy with finite element method. A three‐dimensional micromechanical model has been developed, in which the difference between the elastic properties of austenite and martensite is considered. The model is implemented as User MATerial subroutine (UMAT) into ABAQUS. Then, a polycrystalline NiTi shape memory alloy with [1 1 1] texture under tensile loading is simulated. The main features of the propagation of the deformation band reported in literatures are captured in the simulation. It is also shown that the initiation and propagation of the deformation bands are strongly affected by the geometry of the specimens.     

Plastic deformation of CuAlMn shape-memory alloys

CuAlMn alloys transforming below room temperature were cold rolled after quenching up to 93% reduction. Alloys deformed only 10% were already completely martensitic and did not show reverse transformation to ₁ upon heating. With increasing deformation, refinement of martensitic plates occurred due to repeated internal twinning and injection of new orientation variants. This was accompanied by decreasing size of ordered domains down to that of shortrange ordered microdomains at 93% reduction. Deformation bands appearing at 60% reduction consisted of extremely small grains, giving an electron diffraction pattern resembling that of an amorphous structure.     

Training effect in Fe-Mn-Si shape-memory alloys

The training effect in Fe-Mn-Si shape-memory alloys has been examined by length change and electrical resistivity measurements. After 13 deformation-heating cycles, it was found that the major recovery took place at a temperature lower by 30 K than the first cycle. Simple thermal cycling also lowered the starting temperature of the reverse transformation and increased the finishing temperature. At the same time, the martensitic transformation temperature was found to increase significantly, for example by 35 K, at the 14th thermal cycle. The characteristics of shape-memory effect affected by development of the homogeneous and fine deformation structure by the thermal cycling are discussed in the light of the training effect.     

R-phase transformation in NiTi alloys

Abstract

In near equiatomic NiTi alloys, the thermoelastic transformation between austenite and the R-phase shows unique properties, which make the R-phase transformation very promising for applications. In the present paper, the fundamental issues related to the R-phase transformation, especially the effects of different thermomechanical treatments, are reviewed. Inspired by the literature review, recent work on controlling the R-phase transformation temperature by low temperature aging treatments is summarised.     

爆炸合成NiTi／NiTi合金的相变行为

采用爆炸焊将两块原子百分比不同的NiTi合金制成大块成分不均匀的NiTi／NiTi复合板，并在此基础上对其相变行为以及相应的回复应变特征进行了研究。结果表明焊后的复合板具有良好的界面结合和可逆马氏体相变；冷变形后NiTi／NiTi合金的逆相变温度及其相变温度范围随预应变量的增大而升高；同时，在较宽的温度范围内得到两步回复应变。     

Fracture of precipitated NiTi shape memory alloys

The fracture mechanisms in single crystal and polycrystalline Ti-50.8at%Ni shape memory alloys containing Ti₃Ni₄ precipitates are studied using the scanning electron microscope (SEM). Aged materials with three different precipitate sizes (50 nm, 150 nm, and 400 nm), which have interfaces ranging from semi-coherent to incoherent, are considered. The mechanisms of material fracture identified in the single crystal NiTi are: 1. Nucleation, growth, and coalescence of voids from the Ti₃Ni₄ precipitates, 2. Cleavage fracture on {100} and {110} crystallographic planes, 3. Nucleation, growth, and coalescence of voids from fractured Ti-C inclusions. Cleavage and ductile tearing mechanisms also operate in polycrystalline NiTi, however, since the Ti-C inclusions are an artifact of single crystal growth processes, mechanism 3 was not discovered in the polycrystalline materials. Cleavage fracture and ductile tearing are found to act in conjunction, with the relative dominance of one over the other depending on the local precipitate size and concentration. As the Ti₃Ni₄ precipitate size increases to about 400 nm, the overall fracture is dominated by failure mechanism 1, and the cleavage markings become diffuse. Finally, we assert that the high tensile ductility of drawn NiTi polycrystals is due partially to the fact that drawn bar and wire stock usually have a strong {111} fiber texture. Such a texture promotes the initiation of the transformation at low stresses and concurrently prevents primary cleavage on the {100} or {110} planes.     

Domain walls and transverse shock waves in shape-memory alloys

The martensitic phase transition in shape-memory alloys which is responsible for their ferroelastic and pseudoelastic deformation behaviour is described by a one-dimensional Landau theory. Domain walls between Martensite variants and between Austenite and Martensite are modelled as transverse shock waves. From the balance of momentum and of energy the jump of temperature across and the speed of the domain walls are calculated. The direction of motion follows from the entropy principle. Under certain conditions a moving domain wall acts like a heat engine converting free energy into mechanical work.     

Size effects on magnetic actuation in Ni-Mn-Ga shape-memory alloys

The off-stoichiometric Ni(2)MnGa Heusler alloy is a magnetic shape-memory alloy capable of reversible magnetic-field-induced strains (MFIS). These are generated by twin boundaries moving under the influence of an internal stress produced by a magnetic field through the magnetocrystalline anisotropy. While MFIS are very large (up to 10%) for monocrystalline Ni-Mn-Ga, they are near zero (<0.01%) in fine-grained polycrystals due to incompatibilities during twinning of neighboring grains and the resulting internal geometrical constraints. By growing the grains and/or shrinking the sample, the grain size becomes comparable to one or more characteristic sample sizes (film thickness, wire or strut diameter, ribbon width, particle diameter, etc), and the grains become surrounded by free space. This reduces the incompatibilities between neighboring grains and can favor twinning and thus increase the MFIS. This approach was validated recently with very large MFIS (0.2-8%) measured in Ni-Mn-Ga fibers and foams with bamboo grains with dimensions similar to the fiber or strut diameters and in thin plates where grain diameters are comparable to plate thickness. Here, we review processing, micro- and macrostructure, and magneto-mechanical properties of (i) Ni-Mn-Ga powders, fibers, ribbons and films with one or more small dimension, which are amenable to the growth of bamboo grains leading to large MFIS, and (ii) "constructs" from these structural elements (e.g., mats, laminates, textiles, foams and composites). Various strategies are proposed to accentuate this geometric effect which enables large MFIS in polycrystalline Ni-Mn-Ga by matching grain and sample sizes.     

Shape deterioration and reversible shape-memory effect in Ni-Ti alloys

The effect of constraint heating on shape-memory properties in Ni-Ti alloys is studied. Specimens which memorized linear shape showed remarkable shape deterioration under constraint conditions above 1% bending surface strain and 100° C heating temperature. A reversible shape-memory effect was obtained after constraint heating, and shape change due to this was about 1.4% bending surface strain. These effects strongly depend on ageing temperatures during shape-memory treatment.     

Influence of heat treatment on the transformation hysteresis of CuAlNiMnTi shape-memory alloys

The electrical resistance method, metallograph analysis, TEM observation and X-ray diffraction analysis have been employed to investigate the influence of heat treatment on the transformation hysteresis (A_f-M_s) of CuAlNiMnTi shape-memory alloys (SMAs). It was found that when the quenching specimens were subjected to ageing treatment, the transformation temperature was decreased and the transformation hysteresis was initially increased, but then tended towards stability; when the quenched specimens were subjected to moderate-temperature treatment at 600C, the transformation temperature decreased and the amount of transformation also decreased. The type of thermoelastic martensitic transformation changed from type I and type II and then to type I again, and the transformation hysteresis initially decreasing and then increasing, accompanying the change of the type of thermoelastic martensitic transformation. The reason for this lies in the fact that ageing treatment leads to the precipitation of NiAl-based phase, while moderate-temperature treatment leads to the precipitation of -phase.