Effects of Solution Heat Treatment Temperatures on Microstructures and Mechanical Properties of SP-700 Ti Alloys

The aim of this study is to explore the effects of solution heat treatment temperatures on the microstructures and mechanical properties of water-quenched SP-700 Ti alloys. The main objective is to determine whether the tensile strength and ductility of the SP-700 Ti alloys conform to the quality requirements of armor materials for aircraft, spacecraft, other transportation vehicles, sporting goods, and turbine blades. Therefore, we have experimented with different solution heat treatment temperatures in order to obtain alloys with more excellent tensile strength and ductility. The results showed that after SP-700 Ti alloys (β transus temperatures is 900°C) were subjected to solution heat treatment and then quenched with water, the microstructures of the high-temperature single-phase region (950°C and 900°C) were composed of the soft residual β_r base phase and distributed, hard, and brittle needle-like α" martensite phase. The tensile strength and ductility of this material were not good. In the comparatively lower-temperature dual-phase region (750°C), the microstructure of the water-quenched alloy was composed of a residual β base phase and a pro-eutectoid α phase. Because no martensite phase occurred in the alloy, we observed no obvious work hardening characteristics. However, it exhibited the highest impact toughness and hardness of all the fabricated materials. By contrast, in the high-temperature dual-phase region (850°C and 800°C), because the room temperature was still between the martensite start (M_s) and martensite finish (M_f) temperatures, the microstructures of the alloys were composed of the softer βr base phase, hard brittle pro-eutectoid α phase (α_p) and α" martensite phase. With the “stress-induced martensite phase transformation” that occurred during the tensile test, the SP-700 Ti alloys experienced a high strength increase and high ductility characteristics. Especially, when the solution heat treatment temperature was 800°C, the alloy had the highest strength and ductility, and therefore best met the requirements of armor material standards (MIL-DTL-46077).