Papers by Keyword: Thermoelastic Martensitic Transformation

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: In the present study the effect of second γ'-phase particles which do not undergo martensitic transformation on the functional properties – shape memory effect and superelasticity, in ferromagnetic FeNiCoAlX (X = Ta, Nb, Ti) and NiFeGaCo single crystals are investigated. Dispersed γ'-phase particles allow to control both mechanical and functional properties due to variation of chemical composition, volume fraction and size of nanoparticles, and to obtain the nanocomposites with complex of necessary properties.

Abstract: Using Co₃₅Ni₃₅Al₃₀ (at.%) single crystals the effects of crystals axis orientation and ageing at 673 K for 0.5 h in free state on superelasticity responses in compression are investigated. Ageing of these single crystals is found to result in strengthening of the high-temperature В2 phase and L1₀ martensite, twofold increasing of temperature interval of SE response and weakening of the orientation dependence and improving the cycle stability of superelasticity response. In [011]- and [123]-oriented single crystals the values of the stress hysteresis decrease a nearly threefold as compared to the quenched state. The combination of high-strength [001] crystal axis orientation and precipitation of coherent nanoscale particles leads to the best stability of SE response in aged Co₃₅Ni₃₅Al₃₀ single crystals.

Abstract: In the present study the effects of thermomechanical treatment on the stress-induced martensitic transformation and superelasticity of [001]-oriented Ni₅₄Fe₁₉Ga₂₇ (at.%) single crystals were investigated. It is shown that high-temperature superelasticity is observed up to 453 K in the as-grown Ni₅₄Fe₁₉Ga₂₇ single crystals. Thermomechanical treatment result in increasing of the martensite yield stress, and so the SE interval, which is observed up to 523 K.

Abstract: The present article is dedicated to the study of thermoelastic martensitic transformation in the TiNi equiatomic composition by low-frequency internal friction. Considered the influence of oscillation frequency, strain amplitude and heating rate on the internal friction peak of the martensitic nature.

Abstract: The temperature and amplitude dependence of internal friction fcc Mn45Cu55 alloy aged at 400 °C were studied. Two low-temperature internal friction peak observed in the quenched state. Physical mechanism of the peaks was determined by the effect of frequency and strain amplitude on the temperature dependence of internal friction. The influence of the heat treatment to the internal friction of the investigated alloy was shown.

Abstract: Methods of X-ray diffraction, transmission and scanning electron microscopy, and electron diffraction have been used to study phase and chemical compositions and structure of Ti₂NiCu alloys. The alloys of the quasi-binary section TiNi–TiCu, which exhibit in the initial as-cast state thermoelastic martensitic transformations B2↔B19 and related shape memory effects, have been produced by rapid quenching of the melt (melt spinning technique). The chemical composition of the Ti₂NiCu alloys was varied with respect to titanium and nickel within x ≤ ±1 at. %. The mechanical properties of the alloys have been measured in the initial state and after subsequent heat treatment. The kinetics of the crystallization from the amorphous state, devitrification processes and the forward and reverse thermoelastic martensitic transformations were investigated. Their characteristic temperatures have been determined by measuring temperature dependences of the electrical resistivity of the alloys. The diagram of the dependence of the critical temperatures on the chemical composition has been constructed.

Abstract: Results of investigations of structure and phase transformations and properties of the TiNi-based alloys with a shape memory effect (SME) after severe plastic deformation (SPD) by cold rolling, cold drawing, high pressure torsion and subsequent annealing are reported. It is found that the baroelastic effects related to the highly reversible martensitic transformations can occur in alloys, subjected to high pressure. The evolution of fine structure of the alloys into nanocrystalline and then amorphous state during SPD and after subsequent annealing have been studied. The effect of grain size on the martensitic transformations and properties of the alloys is discussed.

Abstract: Spinodal decomposition of solid solution in the 60 at.% Mn - 40 at.% Cu alloy was observed at the temperatures of decomposition (380 – 420 °C) using the neutron diffraction method in situ. The contribution of residual stresses of third type introduced by further cooling of alloy and, correspondingly, martensitic tranformation is estimated. The relatively small value of stresses of third type introduced by martensitic transformation is revealed. Appearance of magnetic superstructural reflexes typical for antiferromagnetic ordered structure is noted.