Enhanced ductility in a bimodal ultrafine-grained Ti-6Al-4V alloy fabricated by high energy ball milling and spark plasma sintering

Ultrafine-grained and nanocrystalline alloys have strengths several times higher than those of their coarse-grained counterparts, but they often exhibit limited ductility at room temperature. Enhancement of ductility is critical to the practical application of those kinds of materials. In the present study an ultrafine-grained Ti-6Al-4V alloy with a bimodal microstructure were fabricated by means of spark plasma sintering of a mixture of ball milled and unmilled powders. The sintered sample showed a microstructure consisting of equiaxed ultrafine grains (74%), lamellar-structured coarse grains (20%) and recrystallized coarse grains (6%). The bimodal-structured alloy exhibited high compressive strengths (σy=1368 MPa, σmax=2306 MPa) and a large plastic strain to failure (εp=24%) simultaneously. The high strength primarily results from the contribution of equiaxed ultrafine grains, while the enhanced ductility may be attributed to (1) the improved strain hardening capability by the presence of coarse grains, (2) the α/β interfaces in lamellar structure that allow for easy slip transmission, (3) the high fraction of high angle grain boundaries in the ultrafine-grained matrix and (4) occurrence of crack blunting and deflecting due to the introduction of lamellar-structured regions.