Superplastic deformation mechanism of the friction stir processed fully lamellar Ti-6Al-4V alloy

The rolled Ti-6Al-4V alloy sheets were subjected to friction stir processing (FSP) using high heat-input parameters at a tool rotation speed of 325 rpm with a traverse speed of 50 mm/min, and a fully lamellar microstructure was obtained in the stir zone (SZ). The high-temperature tensile tests were then conducted on this fully lamellar microstructure in the temperature range of 850-900 degrees C at the strain rates of 3 x 10(-2)-3 x 10(-4) s(-1). The superplasticity with elongations of above 400% was achieved at all the testing temperatures with the appropriate strain rates, and the maximum elongation of 553% was achieved at the temperature of 875 degrees C and 1 x 10(-3) S-1, which was attributed to the dynamic globularization at the low strain stage and subsequent boundary sliding (BS) at the high strain stage. The main dynamic globularization mechanisms were considered as discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) accompanied with the beta phase growing towards the inside of the groove along the low angle grain boundaries (LAGBs). The present spheroidized microstructure shows an excellent thermal-mechanical stability because the co-existence of the two phases of alpha and beta could effectively inhibit the severe grain growth and facilitate the continuous operating of the BS. In addition, the beta phase transferring from compressive boundaries to tensile boundaries and the strain/stress induced the alpha to beta phase transformation can act as the additional accommodation mechanisms to relax the stress concentration and inhibit the formation of the cavities, which can facilitate the achievement of the enhanced superplasticity.