Martensitic phase transformations in IMI 550 (Ti-4Al-4Mo-2Sn-0.5 Si)

The influence of solution-treatment temperature on the martensitic phase transformations observed in IMI 550 (Ti-4Al-4Mo-2Sn-0.5Si) has been investigated. When solution treatment is conducted at temperatures above 1233 K, a hexagonal martensite (alpha ') is formed on rapid cooling. However solution treatment at temperatures between 1233 and 1123 K results in the formation of an orthorhombic martensite (alpha ") on rapid cooling. Finally, below 1123 K, the beta phase is stable-no martensitic transformation occurs on rapid cooling. This transition from alpha --> alpha (primary) + (alpha ' + beta (retained)) --> alpha (primary) + (alpha " + beta (retained)) --> alpha (primary) + beta (metastable) + omega, with decreasing solution-treatment temperature, is shown to be a result of alloy partitioning during solution treatment. Crystallographic analysis indicates that the transition in the martensite crystal structure with decreasing solution-treatment temperature is related to chemical short-range ordering (CSRO) in the high-temperature beta phase.