| Summary: | 博士 === 國立中央大學 === 機械工程學系 === 104 === The relationship between the microstructures and the mechanical properties of solution-treated SP-700 titanium alloys, as obtained with different cooling rates, was investigated. The results indicate that the water-quenched alloy contains the primary α (αp), α”-martensite and residual β (βr) phases. Aging heat treatment can convert both α”-martensite and the βr phases to the fine-grained α + β equilibrium phases, resulting in a significant increase in tensile strength and hardness. Both the air-cooled and furnace-cooled alloys consist of the αp, α and β phases. The air-cooled alloy containing the fine-grained α phase has relatively higher hardness. Aging heat treatment causes only a slight enhancement in tensile properties because it cannot convert the phases in both alloys. Stress-induced martensitic phase transformation occurs in the water-quenched alloy under applied stress, after which the alloy exhibits higher tensile strength, higher ductility, and lower yield strength.
This study investigates the effects of cold working prior to aging on the microstructure and mechanical properties of SP-700 titanium alloy. The results indicate that the microstructure of the quenched alloy comprises blocky primary α, retained β, and acicular α” martensite distributed in the β matrix. The retained β is transformed to denser and finer brittle acicular martensite α” by stress-induced martensitic transformation and the quantity of retained β decreases with higher degrees of cold working. The quenched alloy exhibits not only low yield strength, but the stress-induced martensite leads to a distinct increase in strength with good ductility. Plastic deformation prior to the aging treatment produces a great increase in the yield strength due to refinement of the precipitate microstructure, leading to the ratio of strength increment and ductility being very low.
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