Microstructural changes occurring during superplastic deformation of Ti-6AI-6V-2Sn alloy

Abstract

Microstructural changes occurring during superplastic deformation of Ti-6Al-6V-2Sn alloy with an initial microstructure consisting of mixed fine lamellar and equiaxed α grains were investigated. Uniaxial tensile tests with constant strain rate were conducted at temperatures ranging from 775 to 925°C and at strain rates rangingfrom 7 × 10-5 to 1 × 10-3 S-l. To investigate the microstructural changes occurring during deformation, some of the tests were terminated at preprogrammed true strains of 0.5, 0.9, and 1.5 for subsequent metallographic investigation. The effects of high temperature exposure on the microstructural changes and on the superplastic deformation behaviour were also evaluated. It was found that both static and dynamic recrystallisation were initiated under certain test conditions and could be related to the flow stress behaviour during the superplastic deformation tests. For tests at low temperature and high strain rate, the flow stress increased quickly at the very beginning of the deformation without significant microstructural change. After the flow stress reached its maximum value, dynamic recrystallisation occurred at a lamellae accompanied by a decrease of the flow stress, known as strain softening. Raising the test temperature or decreasing the deformation strain rate provided the opportunity for thermal energy to initiate static or semidynamic recrystallisation. Thereafter, the flow stress behaviour at the beginning of the test changed to a slow strain hardening type. There also existed a transition temperature; soaking before tensile testing above this temperature would result in static recrystallisation, and the superplastic deformation characteristics would be affected.