Effect of .BETA. Phase Stability at Room Temperature on Mechanical Properties in .BETA.-Rich .ALPHA.+.BETA. Type Ti-4.5Al-3V-2Mo-2Fe Alloy.

Abstract

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

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