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Microstructure, nanohardness, and special features of fracture of three-phase titanium alloy and stainless steel joint through a nanostructural nickel foil are investigated. Uniformly distributed microcracks are observed in Ti2Ni and TiN3 layers joined at temperatures above T = 700°C, whereas no microcracks are observed in the TiNi layer. This suggests that the reason for microcracking is an anomalously large change in the linear expansion coefficient of the TiNi layer during austenitic-martensitic transformation. Specimens subjected to mechanical tests at T = 20°C are fractured along different layers of the material, namely, in the central part of the specimen they are fractured along the Ti2Тi/TiNi interface, whereas at the edge they are fractured along the TiNi/TiNi3 interface.
For the example of two-phase VT8 titanium alloy, the correlation between the rate sensitivity of the plastic stress, which is the basic index of the appearance of superplasticity, and increased solid-state weldability of the material is established. It is shown experimentally that depressing the lower boundary of superplastic deformation, by changing the structural state of the material, permits adequate reduction in the temperature at which a high-quality solid-phase joint is obtained. The relation established between the superplasticity and the weldability of the alloy in the solid state permits a deeper understanding of the formation of a solid-phase joint in superplastic deformation and probably applies universally to materials in which superplasticity is possible.