EFFECT OF COOLING RATE DURING SOLUTION HEAT TREATMENT ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF SP-700 TITANIUM ALLOYS

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 (α_p), α〞-martensite and residual β (β_r) 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 α_p, 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.