Effect of β phase stability at room temperature on mechanical properties in β-rich α plus β type Ti-4.5Al-3V-2Mo-2Fe alloy

The stability of the beta phase at room temperature in various microstructures of a beta-rich alpha+beta type Ti-4.5Al-3V-2Mo-2Fe alloy and its relationship with the fracture toughness, hardness and tensile properties were investigated. A variety of microstructures were established by varying solution treatment temperatures in alpha+beta field, cooling rate after solution treatment and the condition of subsequent second-step annealing treatment after air-cooling treatment. These microstructures have beta phase with lattice parameters of beta phase ranging between 0.3244 nm and 0.3221 nm. The stability of beta phase, which is indicated by decreasing lattice parameter of beta phase, is increased by either lowering cooling rate or formation of diffusional transformation products (secondary phases) in the beta phase. The beta phase with lattice parameter of beta phase around 0.3242 nm is the minimal instability of unstable beta phase at room temperature for attaining deformation-induced martensite in tensile specimens. There exists a proper degree of beta phase stability for increasing the fracture toughness, J(IC). The relatively higher fracture toughness is obtained at low or high stability of beta phase. The high fracture toughness at low stability of beta phase (unstable beta) is mainly due to the deformation-induced martensite. While, the high fracture toughness at high stability of beta phase (stable beta) is mainly due to the secondary phase in the phase that produces a prominent crack deflection toughening mechanism. However, the relatively lower fracture toughness is obtained at high stability of beta phase when the beta phase contains small amount or no secondary phase, This leads to conclude that, if only the phase stability is taken into account for explaining fracture mechanism, the fracture toughness would decrease monotonously with increasing stability of beta phase. The Vickers hardness is nearly independent of stability of beta phase.