Effect of the alpha grain size on the deformation behavior during isothermal compression of Ti-6Al-4V alloy

The effects of alpha grain size on the flow stress, the apparent activation energy for deformation (Q) and the processing maps of Ti-6Al-4V with an equiaxed microstructure are thoroughly investigated using isothermal compression tests, and detailed explanation is given based on the microstructure observation and quantitative analysis. The shapes of flow curves are dependent on the microstructure characteristic of the alloy before deformation and during the deformation process. The flow stress increases with increasing equiaxed alpha phase, but decreases with increasing alpha grain size. The Q-values for d_r1 and d_r2 are smaller than those for d_r3 and d_r4, respectively, which is possibly attributed to that Ti-6Al-4V alloy for d_r1 and d_r2 which exhibits a very strong grain-boundary sliding (GBS) mode besides dominant dislocation glide/climb mechanism. The local efficiency maxima and unstable regions in processing maps change with the alpha grain size, which implies that proper hot-working domains should be modified in different grain size range so as to meet the precision forging process.