Micro-nano twins appearing in ultrafine-grained Ti–6Al–4V alloy induced by high-pressure water jet technology

Abstract In this study, the surface of an ultrafine-grained Ti–6Al–4V alloy was treated with high-pressure water jet technology (HPWJT). Then, the microstructure of the alloy was investigated using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Three obviously different microstructural characteristics were observed from the treated surface to the matrix: ultrafine equiaxed grains with diameters from hundreds of nanometers to a few micrometers, micro-nano twins inside ultrafine equiaxed α grains, and an initial undeformed microstructure with ultrafine equiaxed grains. Formation of micro-nano twins in the subsurface was related to the special misorientation angle of neighboring grains. We proved the dependence of twinning on the crystallographic orientation and grain size of the α phase. In addition, the quantitative contributions of the micro-nano twins to the surface hardness of the alloy were calculated; micro-nano twins significantly improved the surface hardness of the alloy. This study provides a new easy method for introducing micro-nano twins on the surfaces of Ti and its alloys, and it may further improve the mechanical properties of these alloys.