Results of investigations of structural and phase transformations that occur in the titanium nick elide based alloy Ti 49.5 Ni 50.5 with a shape memory effect during severe plastic deformation by torsion under high pressure (HPT) are reported. The studies were performed using transmission and scanning electron microscopy, neutron and X ray diffraction, and measurements of temperature dependences of electrical resistivity. The martensitic B2 → B19' transformation was found to be induced in the alloy when applying a high pressure. After unloading, the martensitic B19' phase is retained in the alloy. The fine structure of the B19' martensite and its evolution into nanocrystalline and, subsequently, amorphous state during HPT with 1/4, 1/2, 1, 5, and 10 rev have been studied. It was shown that, after HPT, all nanosized crystallites whose sizes are less than 30-50 nm have a B2 type structure and, therefore, the reverse martensitic B19' → B2 trans formation is realized in the alloy at room temperature after unloading.

Investigation of the thermoelastic martensitic transformation is of high interest nowadays because of the numerous applications of the materials with such structural peculiarities. Thermodynamics, kinetics, structure, morphology of martensitic transformation still remain unclear in many respects. From this point of view, the effective way to study various properties of metallic crystals on atomistic level is molecular dynamics simulation, for which good qualitative agreement with the experiment can be achieved even with simple Morse or Lennard-Jones interatomic potentials. In this paper, the effect of dislocations on the direct and reverse martensitic transformation is studied by molecular dynamics simulation in a two-dimensional model of the ordered alloy with the AB stoichiometry. The three dimensional analog to this structure is B2 superstructure based on bcc lattice, which is characteristic for intermetallic NiTi alloy. It is found, that the dislocations can be considered as the nucleation centers for martensite phase, increasing the temperature of the direct martensitic transformation in comparison with the homogeneous martensitic transformation. The martensite domains found in the structure after transformation and the reverse martensitic transformation takes place in the presence of the domain boundaries, meaning that the austenite nucleates heterogeneously. At the reverse transformation, splitting of perfect dislocations into partials dislocations took place. Thus, it was established in the present study that, on the one hand, dislocations affect the direct martensitic transformation as the nucleation centers, and from the other hand, reverse martensitic transformation changes the dislocation structure of the modeled alloy.

The results of the comparative analysis of the Ti 50 Ni 25 Cu 25 alloy structures produced in the initial amorphous state by rapid quenching from the melt (RQM), after severe plastic deformation by torsion under high pressure (HPT), and postdeformation heat treatment (PHT) are presented. The study was carried out using neutron and X ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. The initially amorphous alloy has been established to nanocrystallize after torsion under a pressure of 7 GPa to 0.5 revolutions of the anvil. Then, after 1, 5, 10, and 15 rev, the alloy again undergoes the strain induced amorphization even with the retention, even after 5-15 rev, of a large number of highly dispersed nanocrystals less than 3-4 nm in size with a distorted B2 lattice in the amorphous matrix. Their crucial role as nuclei of crystallization provides the total low temperature nanocrystallization during subsequent annealing starting from 250-300°C. It is shown that PHT of the alloy amorphized by HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) austenitic B2 type structures in it. A complete diagram of thermoelastic martensitic transformations in the region of B2 austenite states, from nanostructured state to conventional polycrystalline one, has been constructed. The size effect on the stabilization of martensitic transformation in nanocrystalline B2 alloy has been established.

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The amorphous melt-spun (MS) Ti 50 Ni 25 Cu 25 alloy is subjected to high-pressure torsion (HPT) at temperatures of 20-150 C. The bright-field (BF) TEM images of the alloy change as a result of HPT processing, the view of the bright-field image depends on the temperature of HPT processing. HPT processing leads to a decrease in the energy of structural relaxation during heating up to 400 C, in comparison with the initial MS ribbons. The temperature of crystallization of the MS Ti 50 Ni 25 Cu 25 alloy decreases after HPT processing by 40-20 C (depends on the temperature of HPT processing). In the MS Ti 50 Ni 25 Cu 25 alloy, HPT processing and subsequent annealing produces noticeably finer grains than annealing at 450 C of the initial MS alloy. Consequently, HPT processing alters the crystallization kinetics of the MS Ti 50 Ni 25 Cu 25 alloy.

Two-dimensional model of diatomic crystal is proposed in the present work for the investigation of martensitic transformation under thermomechanical treatment, i.e. the proposed modell allows to simulate temperature effect as well as external loading, like tension, plastic deformation, ets. The model, based on the Morse potential, which are used for the simulation of the diatomic crystal of NiTi type, which is well known as an alloy with the martensitic transformation. This model allows one to define the main characteristics of different phases as the function of one of the potential parameter at 0 K. Potential parameters for the realization of forward and reverse martensitic transformation at finite temperatures are found by the careful checking of all the values. The appearance of one preferable martensite phase is shown despite there are two possible martensite phases. The appearance of the domain boundaries is shown for the martensite phase because of the realization of two opposite direction of martensite growth. The starting and finishing temperatures of the martensitic transformation are obtained. The effect of external stresses on the course of the martensitic transformation is investigated. The potential present in this work can be perspectively used for the investigation of such processes as phase hardening in diatomic crystals.

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