Achieving grain refinement and enhanced mechanical properties in Ti-6Al-4V alloy produced by multidirectional isothermal forging

This study investigated the principle of multidirectional isothermal forging (MDIF) and determined the major microstructural evolution features and unique room-temperature mechanical properties of extra-low interstitial-grade Ti-6Al-4V alloy. The grain refinement mechanism, grain boundary characteristics, and phase transformation during MDIF were explored. After three-step MDIF, a homogeneous microstructure with a grain size of about 0.5 µm was produced. The ultrafine grained Ti-6Al-4V alloy exhibited high yield strength (1170 MPa), high ultimate tensile strength (1190 MPa), and good ductility (10.4%). The mechanism of grain refinement during MDIF included continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). Grain subdivision resulted from CDRX or the transformation of cellular dislocation substructures into new ultrafine grains. The necklace of new DDRX grains formed along the initial grain boundaries of the Ti-6Al-4V alloy. The main strengthening mechanisms were grain boundary and dislocation strengthening. The strength and grain size followed the typical Hall-Petch relationship.