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Description
The Ti49.2Ni50.8 alloy has a bcc lattice ordered according to the B2 type and a phase enriched with nickel Ti2Ni, was chosen as the research material. Thermal cycling of samples in a free state in various initial states was carried out by sequential immersion of the samples in liquid nitrogen, which is obviously lower than the temperature Ms of the direct transformation, followed by heating to a temperature of 150 °C, which is higher than the Af temperature of the reverse martensitic transformation. The number of thermal cycles "heating - cooling" ranged from 0 to 250. In this work, the ultrafine-grained structure was formed by equal-channel angular pressing (ECAP) on a hydraulic press using tooling of the USATU design. A mode was chosen in which cylindrical TiNi alloy billets were subjected to 8 passes along the BC route at a temperature of 450 °C, the channel intersection angle (φ) - 120 °. In the Ti49.2Ni50.8 alloy in the coarse-grained and ultrafine-grained states, thermal cycling makes it possible to increase the strength of the material due to the accumulation of defects. With multiple martensitic transformations, an increase in the values of the ultimate strength, yield strength, and phase yield occurs, the most intensive increase is observed up to n=100 thermal cycles, then, starting from n = 150 cycles, the values of mechanical characteristics stabilize in the coarse-grained and ultrafine-grained states. Generation and accumulation of dislocations in the structure occurs; in addition, compound nanotwins (001)B19' at n=100, which were formed during thermal cycling, were found. In the ultrafine-grained state, a similar trend in structural changes is observed; however, nanotwins are observed only at n=250.
The study was supported by a grant from the Russian Science Foundation (project No. 20-72-00075).
| Speaker Country | Russia |
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