Equal channel angular sheet extrusion (ECASE) produces twinning heterogeneity in commercially pure titanium

Commercially pure grade 2 titanium in sheet form was processed by equal channel angular sheet extrusion (ECASE) up to two passes at room temperature. The ECASE process generated a heterogeneous state of deformation, mainly affecting the sheet metal edges' vicinity. The deformation heterogeneity gave rise to notable microstructural changes throughout the sheet thickness with high densities of twins located around 600 mu m from the sheet edges. At the same time, the middle zone was kept free of twins. The highest twin density generated with the ECASE process corresponds mostly to tension twins of type {10 (1) over bar2}(10 (1) over bar1) oriented towards the extrusion direction. The heterogeneous structure produced a material strength increase of 200 MPa, maintaining a uniform deformation zone of 10%. The good strength-ductility combination was related to a heterogeneous distribution of geometrically necessary dislocations (GNDs) between the hardest and softest areas. The edge areas presented greater GNDs of type < a > on the twins' exterior than in their interior while in the middle zone, the highest GND densities grouped around triple points and grain boundaries. It was found that the grains with the most significant capacity to nucleate twins and store GNDs were those with the c-axis of the crystals parallel to the normal direction (ND). According to the Hall-Petch relationship, the strength improvement of the heterogeneous material comes from the grain size reduction mainly due to twins induced by deformation around the edge areas.

Automated summary

Commercially pure grade 2 titanium sheets were processed using equal channel angular sheet extrusion (ECASE) up to two passes at room temperature, resulting in a heterogeneous microstructure with high twin densities near the sheet edges. This heterogeneity increased material strength by 200 MPa while maintaining a 10% uniform deformation zone. The strength improvement was mainly attributed to grain size reduction induced by twins near the edge areas during deformation.