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

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