Quantitative analysis on boundary sliding and its accommodation mode during superplastic deformation of two-phase Ti-6Al-4V alloy

A study has been made to investigate boundary sliding and its accommodation mode with respect to the variation of grain size and alpha/beta volume fraction during superplastic deformation of a two-phase Ti-6Al-4V alloy. A load relaxation test has been performed at 600 degrees C and 800 degrees C to obtain the flow stress curves and to analyze the deformation characteristics by the theory of inelastic deformation. The results show that grain matrix deformation (GMD) is found to be dominant at 600 degrees C and is well described by the plastic state equation. Whereas, at 800 degrees C, phase/grain boundary sliding (P/GBS) becomes dominant and is fitted well with the viscous flow equation. The accommodation mode for fine-grained microstructures (3 mu m) well agrees with the isostress model, while that for large-grained structures (11 mu m) is a mixed mode of the isostress and isostrain-rate models. The sliding resistance analyzed for the different boundaries is lowest in the alpha/beta boundary, and increases on the order of alpha/beta much less than alpha/alpha approximate to beta/beta, which plays an important role in controlling the superplasticity of the alloys with various alpha/beta phase ratios.