Strengthening mechanism in MP159 Co-base alloy has been investigated. The stability of properties after prolonged aging and its relationship with phase transformation has also been Studied. Cold work (CW) of Solution treated (ST) alloy produces a large number of thin FCC twins and a few HCP ε-platelets. A deformation-induced strengthening is mainly attributed to the twins. The Subsequent age-strengthening effect at 660℃/4h is due to the transformation of additional γ_(fcc)→ε_(hcp). The γ' phase precipitated in alloy during 650℃ prolonged exposure hardens significantly ST alloy, but no effect on 48%CW alloy which can be explained by the formation of a zone depleted in γ' precipitates in 48%CW alloy. The wide precipitate-free zone (PFZ) impairs the strengthening effect. Because of the formation of secondary ε、γ'and possibly μ、σ-phase during prolonged aging, the 48%CW alloy still has high strength, hardness and ductility. γ' precipitate in ST alloy is nucleated and grown rapidly at 800 ℃ with the hardness increase from 8HRC to 35HRC、μ-phase is occurred on the boundary of ST alloy after 800℃/70h treatment. At the beginning, the additional γ_(fcc)→ε_(hcp) takes place in 48%CW alloy at 800 ℃, that makes the hardness increase quickly. Afterward, the hardness drops down slowly due to the effect of recovery and recrystallization. The properties of 48%CW alloy tends to be stable because of the existence of twins and the pinning effect of precipitation phase on the grain boundary and subgrain interface, that is the reason why the strengthening effect can be maintained after high temperature aging.
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