Development of Superplastic Deformation in Ultrafine-Grained α–β-Titanium Alloys with a Large Content of β-Phase

The features of the development of superplastic deformation and the evolution of structural-phase states of ultrafine-grained VT22 and VT35 alloys under tension in the temperature range of 823−973 K and strain rate 6.9·10⁻³ s⁻¹ are studied. It is shown that for the VT35 alloy, the structural-phase state evolution under tension at the temperatures of 873 and 973 K has a qualitatively different character. After deformation at 873 K, a stable microduplex α-β structure with an average grain-subgrain size of about 0.2 μm is formed in the alloy. In this case, the main mechanism of deformation under the conditions of the experiment is grain boundary sliding. At the same time, after deformation of the VT35 alloy at 973 K, there is a significant increase in the grain size with an increasing strain degree and an increase in the β-phase volume fraction to 95%. In this case, the α-phase is observed mainly in the form of individual particles along the grain boundaries of the β-phase. It is found out that the structural state evolution of the VT22 alloy at the deformation temperature of 973 K is qualitatively similar to that of the structural state of the VT35 alloy at 873 K. Based on a comparative analysis of the structural-phase state evolution during the deformation of VT22 and VT35 alloys at 973 K, it is hypothesized that the relatively low ductility of the VT35 alloy samples under the conditions is due to the rapid grain growth, the alloy transition to an nearly single-phase state, which results in the hindered development of grain boundary sliding.