Formation behavior of subcrystals and its strengthening and toughening mechanism by coupling with α phase in titanium alloys during forging at various temperatures

Abstract

Subgrain formation and its coupling with the α phase significantly improve the strength and toughness of titanium alloys. To elucidate the effect of temperature changes on the formation mechanism of nano-subgrains in Ti-6V-5Al-5Mo-5Cr-3Nb-2Zr-0.2Si alloy, multi-directional forging was conducted at various temperatures, and finally quantified the contribution of subgrains and phase evolution to mechanical properties. The results revealed that the equiaxed α phase precipitates at the β grain boundary. As the temperature rises, the aspect ratio of the α phase increases, while its content decreases. The formation mechanism of nano-subgrains during multi-directional forging at 690 ℃ involves dislocation accumulation and the separation of torsional bands within the β grains. When the temperature increases, the deformation resistance decreases, eliminating the need for crystal torsion to reduce this resistance. Subsequently, high-density dislocations form dislocation walls, which delineate the boundaries of fine nano-subgrains. This random orientation of subgrains significantly enhances both the strength and toughness of the titanium alloy forged at 770 ℃. Therefore, the tensile strength and fracture toughness of the alloy reach peak values of 1084.8 MPa and 54.18 MPa·m^1/2, respectively. Microstructural analysis of the cracks reveals that the nano-subgrains effectively hinder their rapid propagation. Due to the coupled strengthening effect of subgrains and α phases, the tensile strength and fracture toughness of the titanium alloy forged at 770 ℃ are increased by 26 % and 40 % respectively compared with the as-cast alloy.