Alexander Razov

Influence of neutron irradiation on the martensitic transformations and shape-memory effect in a TiNi alloy

The paper reports measurements of the strains and electrical resistance of a TiNi shape-memory alloy under irradiation in the low-temperature helium circuit of a nuclear reactor. Irradiation of the alloy in martensitic state at 170 K revealed that the transition temperatures from cubic to rhombohedral and from rhombohedral to monoclinic phase decrease exponentially with increasing dose. No change in the shape-memory effects and transformation plasticity was observed up to a dose of 6.7×1022 n/m2. Keeping the sample at 340 K without irradiation restores (increases) partially the transition temperatures. The relations observed can be assigned to a change in the degree of long-range order in the lattice caused by neutron irradiation.

Functional Properties of TiNi Shape Memory Alloy after High Strain Rate Loading

The review on effect of a high strain rate on the properties of TiNi-shape memory alloys is presented. The study of thermo-mechanical and functional properties of SMA after high strain rate loading was carried out. The object of study was an equiatomic TiNi shape memory alloy. The samples were tensioned at a strain rate of about 103 s-1 at various temperatures in martensitic, austenitic, and two-phase state, using the Split Hopkinson Pressure Bar technique. Two-way shape memory effects were investigated. Two-way shape memory after high strain rate loading was less than after the quasi-static one for all cases, except for straining in martensitic state.

Neutron-irradiation-induced shape memory effect in a TiNi alloy

The recovery of inelastic strains in Ti-Ni alloy samples irradiated in a nuclear reactor under isothermal conditions was studied. Before irradiation, the cylindrical samples were compressed to a residual strain of 3–6% in the martenstici state at room temperature. The samples were irradiated at a temperature of 45°C, which does not exceed the temperature of the onset of the reverse martensitic transformation AS. Irradiation with a fastneutron fluence of 5 × 1020 cm−2 is established to result in the recovery of the residual strain. The value of the recoverable strain is comparable to that observed under the conditions of the shape memory effect on heating of the deformed alloy and even somewhat exceeds it. The obtained data show that neutron irradiation can induce the shape-memory effect in the TiNi alloy. This is due to a decrease in the temperatures of the martensitic transformations under irradiation.

Mechanical and structural aspects of high-strain-rate deformation of NiTi alloy

The mechanical behavior of the binary polycrystalline NiTi alloy with a quasi-equilibrium structure has been considered in the course of the high-strain-rate extension in a temperature range of 20–300°C. The quasi-equilibrium structure, which is necessary to ensure the long-term stability of special properties of the alloy, was achieved using aging, after which both the forward and reverse martensitic transformations exhibited a multistage character and the phase composition at room temperature was characterized by the presence of R and B19′ martensites. To separate the contributions that come from the equilibrium structure and from the high rate of tension to the mechanical behavior of the alloy, a comparative analysis of the diagrams of high-strain-rate and quasi-static tension has been performed. It has been shown that the action of several mechanisms of reversible deformation is determined by the specific features of the equilibrium structure, and the level of stresses at which these mechanisms are developed is controlled by the rate of tension. The results of the X-ray diffraction study of the phase composition of the alloy samples after high-strain-rate tension, which make it possible to conclude that the mechanical behavior of martensite and austenite upon the dynamic tension of the alloy is determined by the development of stress-induced R → B19′, B2 → R, and B2 → B19′ transformations and by the processes of the detwinning and reorientation of crystals of B19′ martensite, are given.

X-ray determination of the volume fraction of the B19′ martensite in nickel-titanium alloys

An X-ray method has been considered for determining the volume fraction of the B19′ martensite in semi-product NiTi alloys, which is based on comparing the total integrated intensity of its strongest reflections with the total integrated intensity of reflections from the matrix B2 phase. It has been shown that the method enables one to determine the volume fraction of martensite with an error of about 1% both in textured and textureless samples.

Kinetics of radiation damage and martensitic transformations in TiNi alloys irradiated with neutrons

The variation of the temperatures of martensitic transformations and the rate of radiation damage in TiNi alloys were studied upon irradiation with reactor neutrons. The irradiation was performed at temperatures of 120 and 335 K. In the process of irradiation, electrical resistance of the alloys was measured continuously and thermal cycling through the temperature range of martensitic transformations was carried out. The transformation temperatures were shown to decrease at different rates with increasing irradiation fluence. The electrical resistance increases linearly with increasing neutron fluence to 6.7×1018 cm−2 irrespective of the irradiation temperature. Deviation from a linear dependence is only observed when the irradiation leads to a change in the phase state of the alloy. The rate of the resistance increase only slightly depends on the irradiation temperature. In martensite, it is greater by a factor of 2–4 than that in austenite. Mechanisms of irradiation-induced modification of the structure of TiNi alloys that explain the experimental data obtained are discussed.

Delay of reversible deformation in titanium nickelide after an incomplete transformation cycle

Characteristic features of the appearance of reversible shape memory in an equiatomic TiNi alloy were studied after a cycle in an incomplete range of martensitic transformation temperatures. It was observed that the establishment of reversible shape memory is accompanied by the SMART effect (Stepwise Martensite to Austenite Reversible Transformation) which is manifest as a temperature delay of the deformation under heating. The delay occurs at that temperature at which the transformation terminated in the preliminary thermal cycle and is 3 °C.