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     

Nature of nanocrystallization in shear bands created by megaplastic deformation in amorphous alloys

A theoretical study is made of the process of nanocrystallization upon the formation of shear bands created by megaplastic deformation in amorphous metallic alloys. Such nanocrystallization is shown to be caused by a considerable increase in temperature inside the shear bands, which in turn is associated with the stored energy of megaplastic deformation. The temperature increment depends on the degree of deformation, the rate of propagation of the shear band, and the physical parameters that determine the thermal characteristics of an amorphous matrix in the range of the shear band.     

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.

     

Metastable states in ternary ordered alloys

The Gorskii-Bragg-Williams approximation is employed in a theoretical study of certain features in the atomic distribution in ternary ordered alloys having fcc lattices. The effect of various parameters in the interatomic interactions and of the concentration of the third alloying element are studied for their influence on the formation of metastable states in these alloys.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 31–34, June, 1971.     

Self-blocking of shear bands and the delocalization of plastic flows in amorphous alloys upon megaplastic deformation

Features of the formation of shear bands and nanocrystalline phases upon the megaplastic deformation of amorphous alloys based on iron, nickel, and titanium at room temperature in a Bridgman chamber are analyzed via transmission electron microscopy. It is shown that the transition from strongly localized to quasi-homogeneous plastic deformation occurs at a definite stage of the inhomogeneous plastic flow. Mechanisms based on the self-blocking of propagating shear bands by particles of the nanocrystalline phase that emerge due to a dissipative increase in the temperature along the front of shear bands are proposed for the delocalization of plastic flow.     

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.     

Pseudo-elastic hysteresis in shape memory alloys

Observations of pseudo-elastic hysteresis loops in the shape memory alloy CuAlNi are presented. Particular emphasis is laid on the interior of the overall loop and the phenomena of internal yield and recovery and internal loops are discussed. A thermodynamic argument is presented which may afford an interpretation of the observed phenomena in terms of interfacial energies.     

Specific features of martensitic transformations, microstructure, and mechanical properties of nanostructured shape memory TiNi-TiFe alloys

The microstructure, martensitic transformations, and properties of quasibinary shape memory TiNi-TiFe alloys produced by superrapid melt spinning (SMS) have been studied using transmission electron microscopy and X-ray diffraction analysis and by measuring some properties     

Formation of self-accommodation complexes of martensite crystals during transformations of distortion type in alloys with the shape memory effect

This work discusses the formation of self-accommodation complexes of martensite crystals during transformations of distortion type and experimentally observed shape memory effects in unordered solid solutions. A method for the analysis of orientation relationships between the lattices of austenite and marten-site in terms of the possibility of formation of self-accommodation complexes is given.     

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.     

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.     

Physics of megaplastic (Severe) deformation in solids

The main regularities of structural and phase transformations occurring in solids have been analyzed experimentally and theoretically within the framework of the concept of manifestation of additional channels providing the dissipation of an elastic energy introduced into a solid under megaplastic deformation. It has been demonstrated that an active participation of low-temperature dynamical recrystallization processes, phase transitions of the type crystal ai amorphous state, and thermal effects under the conditions of an insufficient efficiency of the dislocation and disclination relaxation modes can consistently explain almost all the experimental results obtained for very severe plastic deformations.     

Influence of deformation on the force characteristics of shape memory effects in alloys based on titanium nickelide

Studies of the influence of plastic deformation and the straining temperature on the temperature dependence of the stresses developed in titanium nickelide shape memory alloys are described. Zh. Tekh. Fiz. 68, 141–142 (July 1998)     

Low-temperature specific heat of Ni–Mn–Ga ferromagnetic shape memory alloys

Results of the low-temperature specific heat measurements (2–80 K) for one austenitic and three martensitic Ni–Mn–Ga ferromagnetic alloys are presented. The alloy compositions are chosen to comprise a wide span of valence electron concentrations e/a=7.3–7.78. Debye temperature (261–345 K) is found to be an increasing function of e/a while the experimental values of the Sommerfeld coefficient (2.9–3.4 mJ/mol K²) appear to be increasing in the martensitic region only. Observation of those trends rekindles the discussion about the role of vibrational and electronic contributions to the lattice instability and transformation mechanism of studied alloys.     

Theory of stripe domains in magnetic shape memory alloys

The evolution of multivariant patterns in thin plates of magnetic shape memory materials with an applied magnetic field was studied theoretically. A geometrical domain-model is considered composed of straight stripe-like martensite variants with constant internal magnetization (high anisotropy limit) and magnetic domain wall orientation fixed by the twin boundaries. Through integral transforms of the demagnetization energy, the micromagnetic energy is cast into a form convenient for direct numerical evaluation and analytical calculations. The equilibrium geometrical parameters of multivariant patterns with straight and oblique twin boundaries have been derived as functions of the applied field and the material parameters of a plate. It is shown that the oblique multivariant states exist only in plates with thicknesses L larger than a certain critical value L₀. In samples with L<L₀ a magnetic-field-driven transformation occurs directly between single variant states.     

A macro-mechanical constitutive model of shape memory alloys

It is of practical interest to establish a precise constitutive model which includes the equations describing the phase transformation behaviors and thermo-mechanical processes of shape memory alloy (SMA). The microscopic mechanism of super elasticity and shape memory effect of SMA is explained based on the concept of shape memory factor defined by the author of this paper. The conventional super elasticity and shape memory effect of SMA are further unified as shape memory effect. Shape memory factor is redefined in order to make clear its physical meaning. A new shape memory evolution equation is developed to predict the phase transformation behaviors of SMA based on the differential relationship between martensitic volume fraction and phase transformation free energy and the results of DSC test. It overcomes the limitations that the previous shape memory evolution equations or phase transformation equations fail to express the influences of the phase transformation peak temperatures on the phase transformation behaviors and the transformation from twinned martensite to detwinned martensite occurring in SMA. A new macro-mechanical constitutive equation is established to predict the thermo-mechanical processes realizing the shape memory effect of SMA from the expression of Gibbs free energy. It is expanded from one-dimension to three-dimension with assuming SMA as isotropic material. All material constants in the new constitutive equation can be determined from macroscopic experiments, which makes it more easily used in practical applications. Supported by the National Natural Science Foundation of China (Grant No. 95505010), the National High Technology Research and Development Program of China (Grant No. 2006AA03Z109), the China Postdoctoral Science Foundation (Grant No. 20080430933), the Open Foundation of Institute of Engineering Mechanics of National Seism Bureau of China (Grant No. 2007B02), and the Harbin Talent Foundation of Scientific and Technical Innovation (Grant No. RC2009QN-017046)