Materials Science Foundations Vols. 81-82

Abstract: Basic directions in the theory of martensitic transformations are briefly listed. Within the framework of the dynamic theory based on the synthesis of concepts of heterogeneous nucleation and wave growth of martensite crystals, the possibilities of description of morphological parameters during the В2→B19, В2→B19′, В2→R transformations are analyzed. It is demonstrated that the calculated and observed habit planes and orientation relationships can be matched.

Abstract: This microstructural model of the functional-mechanical behavior of shape memory alloys (SMA) includes a description of the reversible phase deformation, microplastic deformation due to the accommodation of martensite and the evolution of the deformation defects. The laws of these phenomena are formulated in terms of the generalized thermodynamic forces. The microplastic flow rule accounts for isotropic and kinematic hardening, which are related to the accumulation of the deformation defects. The model gives a good description both of reversible and irreversible deformation under one-side or cyclic thermomechanical loading of SMA and opens a way for the fatigue life prediction. The model can be applied for solving of the mechanical problems for SMA parts such as dampers or base isolators used in seismic protection devices.

Abstract: Nowadays, ferromagnetic shape memory Heusler alloys are ones of famous multifunctional materials exhibiting many interesting features in the temperature interval of the martensitic transformation due to the strong interrelation between crystal structure and magnetic order. The multiferroic, magnetoresistive, martensitic and related magnetic shape-memory behavior as well as magnetocaloric properties are examples of these unique features. Generally, tuning of both structural and magnetic transition temperatures can be useful to achieve better functional properties. Today, the optimization problem of Heusler compounds is of a great importance. In this chapter, we review the most important features of ternary and quaternary ferromagnetic shape memory Ni-Mn-In and Ni-Mn-In-Z materials, which are experimentally and theoretically obtained in the last three years. We discuss the experiments devoted to the study of phase diagrams, thermomagnetizations, magnetic field and stress induced strains, magnetoresistance and magnetocaloric effects. The theoretical investigations of magnetic and structural properties are reviewed in the framework of the phenomenological approach, first-principles and Monte Carlo methods.

Abstract: This chapter proposes an efficient approach to thermomechanical constitutive modeling for shape memory alloys using a novel separation of a martensite internal variable. This approach assumes that the martensitic internal variable has two components corresponding to two contrary martensite ensembles, which are separated by their opposite signs of contributions to a quantity of transformation strain. The constitutive models, based on a new separating concept, are presented in this chapter for uniaxial and three-dimensional proportional loading. The kinetic relations of these models are constructed using the Brinson model ideas concerning the interphase transformation processes in shape memory alloys. A number of illustrative numerical examples are presented here for one-dimensional modeling. The new separation of the internal variable has prospects for the description of material behavior in cases when mechanical loading may change sign and the phenomena are caused by martensitic reorientation. The conceptual and methodological solutions stated in the present work may be useful for subsequent modeling.

Abstract: The thermoelastic martensitic transformations, shape memory effect and superelasticity in high-strength single crystals of ferromagnetic FeNiCoAlX (X = Ta, Nb, Ti), CoNiGa, NiFeGaCo alloys and TiNi alloy in monophase and heterophase states with nanoscale dispersed particles are investigated. The dependences of the thermal and stress hysteresis, superelasticity temperature range, reversible transformation strain on the size of the dispersed particles, crystal orientation, stress state, level of applied stress and test temperature are obtained. The criteria of high-temperature superelasticity and the conditions for narrow thermal and stress hysteresis, large value of reversible transformation strain, which exceeds the theoretical lattice strain, are established. The thermodynamic description of the effect of particles on the stress-induced martenstic transformation in single crystals of new high-ferromagnetic alloys are elaborated.

Abstract: In the article there is presented a brief overview of combined systematic investigations of the alloys exhibiting thermoelastic martensitic transformations (TMTs). As is known, such alloys are distinguished by a whole number of specific one-and multi-fold-reversibility shape-memory (SM) effects occurring upon changing temperature, pressure, magnetic field, at the background of superplasticity and highly reversible deformation of transformation under a load or its relieving.There have been considered the alloys classification, the thermodynamical and kinetic aspects of TMTs, the origin and structural mechanisms of realization of multivarious pre-transition phenomena and TMTs proper, the influence of complex alloying, the peculiar features of a structure, the physico-mechanical properties and methods of production of the alloys most promising from the viewpoint of application. For this purpose there were employed structural methods of the X-ray diffraction (XRD) structure-phase analysis, neutron diffraction analysis, transmission and scanning electron microscopy techniques of high resolution, together with studying texture and atomic composition, as well as measuring a number of physical properties and SM-effect characteristics.The effect of the alloying and external actions of different origin on the magnetic and structural phase transformations and properties of the studied alloys with the magnetically, thermally, and mechanically controlled manifestations of the shape memory is discussed. A sequence of the TMTs with the formation of different martensitic phases, as well as the crystallographic structure and crystal-geometry specific features of the formation of these phases is described. The phase diagrams of the magnetic and martensitic transformations in a number of binary, ternary, and quaternary alloys synthesized via different schemes of alloying are presented.On the example of the binary and doped alloys TiNi it has been shown that an employment of thermo-mechanical treatments of the alloys via multiple or repeated torsion under pressure, rolling or drawing leads to their high-level strengthening and grain refinement (up to amorphization). In this case the use of low-temperature annealing provides for both the creation in the alloys of the homogeneous nanostructured state with a controllable grain size already in the interval of 50–200 nm and efficient regulation of physico-mechanical properties with retaining comparably high values of the parameters of SM, including deformation-, temperature-, and force-related.There has been revealed the effect of the grain size on the critical temperatures of TMTs and, as a consequence, on the TMT-stipulated SMEs. On this basement, in dependence of the grain size, the poly-packet, mono-packet twinned or single-crystal structures of martensite can be realized.There are discussed an experimentally revealed deformation-induced atomic disordering in Heusler alloys with self-forming a nanocrystalline fcc (A1) structure and amorphization in the alloys of titanium nickelide, as well as long-range order recovery in them taking place in the course of low-temperature annealing in conditions of retaining of the nanostructured state of austenite and at feasibility of cascade occurrence of TMTs and SMEs. It has been shown that the use of super-rapid quenching (SRQ) via employment of a melt spinning technique makes it possible to produce ductile submicrocrystalline ribbons of the Heusler Ni₂MnGa-based alloys with magnetically controllable TMTs. For a number of alloys based on titanium nickelide the effect of SRQ on the internal structure, grain refinement, and amorphization has been considered. The reasons of the amorphization and ways of subsequent nanostructurization of the alloys under investigation are discussed.

Abstract: Nowadays, martensitic transformation and shape memory effect, superelasticity, high damping capacity and other effects associated with this type of structural phase transitions are still in the focus of scientists and engineers, especially once these phenomena are taking place at elevated temperatures. The list of the materials undergoing this kind of transformation is constantly widening. Yet, industrial application of these materials, called high temperature shape memory alloys, is still hindered due to the lack of understanding of the peculiarities of the high temperature martensitic transformation and shape memory effect. Present work summarizes results of scientific studies of these high temperature phenomena oriented onto the development of physical principles suitable for industrial high temperature shape memory alloys design.

Abstract: We reviewed structural and magnetic properties of Ni-Mn-Al Heusler alloys which are a representative of ferromagnetic shape memory alloy system. Special attention was given to a variety of crystal structures of the low-temperature martensitic phase formed in this alloys and to the influence of disorder and Co alloying on magnetic properties of Ni-Mn-Al. Magnetic properties strongly depends on the chemical composition, crystal structure and heat treatment. In present review works concerning magnetic properties dependence on treatment are made out. A two way shape memory effect observed recently in Ni-Mn-Al melt spun ribbons is briefly described. The aim of the review is to sum up the information about Ni-Mn-Al family.

Abstract: In the work, we studied the regularities and mechanisms of microstructure formation in binary TiNi alloys with 50.2 and 50.8 at.% Ni under warm abc pressing with a stepped decrease in strain temperature (873, 673, 623, and 573 К) and isothermal (723 К) multipass caliber rolling. In the TiNi alloy with 50.2 at.% Ni at all abc pressing stages, microstructures inhomogeneous in grain size were formed due to faster dynamic recrystallization and hence faster formation of finer grains and subgrains in strain localization bands compared to microvolumes bounded by these bands. After final abs pressing at 573 К with a total true strain е = 7.7, a microstructure composed of submicro-and nanocrystalline grains and subgrains was found. In the TiNi alloy with 50.8 at. % Ni subjected to warm rolling, three stages of grain structure evolution were revealed. At the first stage with low strains, the average grain size ‹d› increased due to collective dynamic recrystallization. At the second and third rolling stage, the average grain size ‹d› decreased steeply due to discontinuous and continuous dynamic recrystallization. On rolling at е = 2.0, a microstructure composed of micro-and submicrocrystalline grains was formed. An algorithm was proposed for estimating the critical strain for the onset of dynamic recrystallization from dependences of grain sizes on true strain. The sequences and temperatures of martensite transformations from a cubic В2 phase to rhombohedral R and monoclinic В19′ martensite phases were studied depending on the strain accumulated in abc pressing and rolling.