Improved superplasticity and microstructural evolution of Ti2AlNb alloy sheet during electrically assisted superplastic gas bulging

The electrically assisted forming has been a potential technique in the field of metallic sheet processing. Due to the demand of aerospace industry, Ti2AlNb alloy sheet combined with electrically assisted bulging method is studied. The results indicate that the superplasticity characterized by height/diameter ratio of the electrically assisted sample is 0.64 whereas that of the furnace-heating sample is 0.18. Because of the decrease in temperature during electrically assisted bulging and uneven temperature distribution, the lamellar O-phase precipitates from the matrix B2-phase with different sizes at typical positions of bulging sample, which is considered to simultaneously give rise to the transformation induced superplasticity. Subject to the high heating rate and short elevated-temperature exposure time compared with furnace-heating bulging, the O-phase is not coarse attributed to the deficient diffusion, while the O-phase has markedly grown up in the furnace-heating bulging sample. Moreover, no cavity has been found in the electrically assisted bulging sample. The impacts of temperature, strain, and electric current during electrically assisted bulging on precipitation and growth of O-phase were individually discussed in this work.