Deformation heterogeneity of Ti under cryogenic channel-die compression

Commercial purity Ti was subjected to channel die compression in liquid nitrogen for the purpose of studying its deformation characteristics from the viewpoint of the grain refinement induced by severe plastic deformation. Deformed specimens showed microstructural heterogeneity in that the initial blocky grains of about 50 gm in size turned into a mixture of the easily deforming soft grains and the hard grains revealing local concentration of dislocations and deformation twins. Using electron back scattered diffraction analysis the in-grain misorientation-axis distribution was studied, from which it was found that the deformation heterogeneity resulted from the anisotropy of individual grains with respect to the compression die: while the soft grains were those oriented favorably for the prism < a > slip, the hard ones were oriented for non-prism slips and deformation twinning. The difference in the response of individual grains, therefore, led to a remarkable effect of the crystal texture of specimens on the maximum strain that can be imposed on specimens without cracking. Based on the present results, a way to achieve expedient grain refinement through the cryogenic plane-strain compression was suggested.