Baroelastic shape memory effects in titanium nickelide alloys subjected to plastic deformation under high pressure

The effects of a high pressure and torsional plastic deformation in Bridgman anvils on the structure and phase transformations in titanium nickelide-based shape memory alloys are studied by electron microscopy, neutron diffraction, and X-ray diffraction. The physical properties of the alloys are measured. It is found that the baroelastic effects related to the highly reversible B2 ? B19?? martensitic transformation can occur in metastable austenitic titanium nickelide alloys in both the standard polycrystalline and nanocrystalline states under high pressure.     

Plasticity and shape memory effects under martensitic transformations in titanium nickelide

It has been established that in titanium nickelide the rate of development of plastic deformation and, therefore, the effectiveness of the manifestation of the properties of shape memory and superplasticity depend on the mechanical properties of the material and the character and conditions under which the martensitic transformation occurs. The main condition for ensuring high memory parameters is a premartensitic instability of the lattice and the existence of a soft modulus at the moment of the transformation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 88–92, April, 1981.     

Mechanism for the bidirectional shape memory effect in titanium nickelide crystals

A mechanism for the bidirectional and all-round shape memory effects observed in titanium nickelide crystals is discussed quantitatively by using the theory of diffuse martensitic transformations (DMTs). These effects are associated with an anisotropic distribution of Ti₃Ni₄ particles, which arises in bent crystals subjected to annealing followed by relaxation of coherent microstresses produced by the particles. Using the DMT theory, the influence of the stepwise B2 → R → B19′ phase transition on the magnitude and sign of the radius of curvature of a thin strip of titanium nick-elide is calculated and the conditions are determined under which the bidirectional and all-round shape memory effects occur depending on structural factors and the geometrical parameters of the strip.     

Euler instability of bidirectional shape memory effect in a titanium nickelide strip

The mechanism for the Euler instability of the bidirectional shape memory effect in a titanium nickelide strip is discussed in terms of the diffuse martensitic transformations. The shape instability and a plastic jerk of a strip that are observed in the temperature range of a forward and the reverse martensitic transformation are due to the additional bending moment that arises in the case where the ends of the strip are fixed (with the use of hinges) and the strip can only rotate about them.     

Two-way reversible deformation in titanium nickelide

A systematic study has been made of the shape-memory effects which arise in titanium nickelide after prestraining under isothermal conditions. It has been found that under thermal cycling in the free state a broad spectrum of phenomena is observed — repeatedly reversible shape memory, reversible deformation, deformation of an oriented transformation, etc., each of which can be realized independently of the others in the temperature range of the B2 R and R B19' transformations. When summing up the observed laws of the mechanical behavior of the material we used concepts of heterogeneous development of deformation in crystals, structurally hereditary properties of alloys with a shape-memory effect, as well as the principle of independent initiation of various channels of deformation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 71–76, August, 1988.     

Micromemory effects in shape memory alloys

Summary Thermoelastic martensitic transformations (TMT) have been on the stage for several years in connection with shape memory alloys. Interest has recently grown in partial cycling or involving the incomplete reverse transformation in a pre-programmable way. Attention is here focussed on the hysteresis cycle of several TMT (NiTi, NiTiFe, AgCd) related either to a complete transformation or to incomplete cycling as required to activate stimulated stepwise martensite-to-austenite reversible transformation (SMART). The modifications of the hysteresis cycles are discussed in the light both of built-in kinetics barriers and of the hierarchy of symmetries between parent and product phase. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

NiTi形状记忆合金形变机制的应变率相关性研究

利用高速拉伸实验机在宽的应变率范围内(0.001–1200 s^-1), 研究了NiTi形状记忆合金的宏观力学性能随应变率的变化规律, 并借助透射电子显微镜深入研究了微观结构在不同应变率下的演变机制. 研究发现: NiTi合金马氏体(B19’相)孪晶的解孪晶应力随应变率的升高而近乎线性增大, 表明NiTi 合金解孪晶应力具有正向应变率相关性. 在拉伸应变率为10 s^-1的样品微观结构中发现了大量的解孪晶区域, 而当应变率进一步增大到100 s^-1和 1200 s^-1时, 在样品中没有发现解孪晶区域的存在, 样品微观组织以孪晶形式存在. 该结果表明, NiTi合金的马氏体解孪晶速率应在 10–100 s^-1范围内. 在高应变率下(≥qslant10 s^-1)均发现了热引发奥氏体相(B2)的存在, 表明随应变率的增加, 拉伸过程由等温过程逐渐变为绝热过程. 此外, 在1200 s^-1 的样品差示扫描热量曲线中还发现了一个小肩峰, 表明相变过程由一步相变变为两步相变. 关键词： NiTi形状记忆合金 高速拉伸 应变率相关性 透射电子显微镜     

Modification of structural states in titanium nickelide under severe deformation

X-ray diffraction, electron microscopy, and thermal analysis are used to study the effects of high degrees of deformation on the structural state of titanium nickelide. It is established that nearly amorphous regions are formed along with defective fine-grained structure in strongly deformed titanium nickelide.Tomsk Construction Engineering Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–7, December, 1992.     

Laws of stress and strain change in alloys based on titanium nickelide under loading and unloading

In the present work, laws of stress and strain change in alloys with phase transitions based on titanium nickelide are investigated. A hysteresis character of the alloy behavior versus temperature, strain, and stress is established. A formula for calculating stress versus strain under loading-unloading is suggested. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 38–46, April, 2008.     

In situ X-ray investigation of deformation martensite in titanium nickelide

X-ray structural analysis was used to carry out an investigation of the martensitic transformation in titanium nickelide during mechanical loading. A diffractometer with filtered cobalt radiation was used, along with an attachment that allowed the sample to be loaded by bending. No R-martensite was observed during loading. The structure of deformation martensite was identified as monoclinic B19. When deformation is increased, the martensite has a more complicated structure and was identified as a mixture of monoclinic B19 and triclinic B19 phases. When the load is removed, the triclinic martensite disappears without hysteresis, whereas a small amount of monoclinic martensite remains and disappears only when heated to 40–60C.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 39–41, May, 1991.     

Nanoscopic size effects on martensitic transformations in shape memory alloys

The effect of the grain size and transverse film size in nano-and micrometer ranges on the parameters of martensitic transitions in shape memory alloys is theoretically considered in the framework of the theory of diffuse martensitic transitions. A quantitative analysis of the size effects is performed including not only the thermodynamic aspect of the martensitic transformation but also its kinetic aspect, which is particularly sensitive to structural and size factors. This complex approach makes it possible to explain the following three basic facts associated with the influence of a decreased grain size or transverse film size on the parameters of the martensitic transition in shape memory alloys: a decrease in the critical (characteristic) transition temperature, an increase in the transition temperature smearing, and the existence of a critical grain size or film thickness below which the martensitic transformation in alloys is blocked.     

Study of the heat treatment influence on the formation of nanostructured state in bulk titanium nickelide alloys subjected to severe plastic deformation

The influence of annealing on bulk samples of Ti_49.4Ni_50.6 alloy subjected to severe plastic deformation by torsion under high pressure has been studied by transmission electron microscopy and X-ray diffractometry. It is found that a homogeneous nanocrystalline state is formed in the bulk samples after annealing.     

Theory of magnetic shape memory effect and pseudoelastic deformation in Ni-Mn-Ga alloys

The effect of the magnetic field on the deformation behavior of magnetic alloys of the Heusler type under different loading conditions is discussed in terms of the theory of diffuse martensitic transitions. The effects of magnetic shape memory, pseudoelastic deformation, and generation of reactive stresses in response to the magnetic field are considered. The theoretical relationships are compared with the experimental data available in the literature.

     

Characterization of phase transformation in shape memory alloys by atomic force microscope

An atomic force microscope operated at various temperatures is introduced to evaluate phase transformation temperature and to observe microstructure for a shape memory alloy at same time in this paper. A commercial hot-rolled TiNi shape memory alloy bar is ground, polished and etched. The specimen is then observed by atomic force microscopy at the temperature range of 20–100 C in nitrogen gas. The topographies of a TiNi specimen show twinning martensite with rough surface and smooth austenite at various temperatures. The shape memory effect of the TiNi alloy is analyzed based on the shifts of the topographies obtained at various temperatures, which are used to evaluate the phase transformation temperature between martensite and austenite. The phase transformation temperature is also confirmed in a differential scanning calorimeter (DSC) experiment.     

Effect of severe plastic deformation by torsion on the structure and properties of TiNi-based alloys with shape memory effects

The microstructure and properties of TiNi-based shape memory alloys subjected to severe plastic deformation by torsion have been studied using transmission electron microscopy, X-ray diffraction analysis, and mechanical measurements.     

Atomic force microscopy study on microstructural changes by 'training' in Fe-Mn-Si-based shape memory alloys

Microstructural changes after several cycles of the thermomechanical treatment consisting of a small deformation by stress-induced martensitic transformation (fcc to hcp) and subsequent reversion to austenite by heating (referred to as 'training') have been studied by atomic force microscopy in Fe-Mn-Si based shape memory alloys. Well-trained samples contain a uniform distribution of thin martensite plates of the same variant, the widths of which decrease with increasing number of the training cycles, and their distribution becomes more uniform. Such microstructural development by training originates mainly from extremely thin plates (about 1 nm thick) of hcp phase that are still retained together with stacking faults in the austenite even after heating far above the reverse transformation temperature. In the reverse transformation on heating, a martensite plate that looks as though it is apparently one plate is, in fact, split into very thin plates, which indicates that the plate actually consists of extremely thin martensite plates and these thin plates are reverse-transformed one after another by reverse movement of the Shockley partial dislocations at their tips. This mode of reverse transformation ensures a perfect shape memory effect.