Slip, twinning and twin-twin interaction in a gradient structured titanium

Quantitative analysis on the deformation mechanisms of a rotationally accelerated shot peening (RASP) processed gradient structured Ti was studied by using electron back-scattering diffraction technique. A 2000 urn thick gradient structure was introduced from the strain-free center to the treated surface. The low angle grain boundary fraction evolution showed that, with decreasing distance from the surface, dislocation generated continuously in the core region, then became saturated in the twin transition region, and finally decreased in the nano-structured region. Three twinning systems were identified: {11 (2) over bar1}, {10 (1) over bar2} extension twin (ET) and {11 (2) over bar2} compression twin (CT). The volume fraction of {10 (1) over bar2} ET increased firstly and then decreased when approached to the surface, while {11 (2) over bar2} CT showed an opposite tendency. The volume fraction of {11 (2) over bar1} ET decreased gradually form the center to the surface. Both ET <-> ET and CT <-> CT twin-twin interactions were found to accommodate the strain and result in grain refinement. Glissile or sessile dislocations could be generated as a result of twin-twin interactions. With twin-twin interactions proceeding continuously, twin lamellae became narrower and even de-twinning was triggered. Due to different grain refinement levels, a hardness gradient was also found in RASP-processed Ti.