Effect of Heat Treatment on the Microstructure Development of TC21 Alloy

The microstructure development of TC21 alloy after double α/β solution and aging heat treatment was investigated. Results show that primary α phase morphology is determined by the first solution treatment at the top of α/β portion. Varying the cooling rates can significantly influence the volume fraction of retained β phase and secondary α phase. Higher cooling rate results in greater retention of the β phase and transformation of the phase into secondary α phase upon aging. This higher amount of fine secondary α phase causes higher strength and lower toughness. Second α/β heat treatment temperature remarkably influences the heat treatment response of the microstructure. Higher heat treatment temperature keeps greater amounts of β with fine transformed α in the final microstructures. Lower α/β heat treatment temperature results in less responsive retained β during aging process. Aging treatment leads to decomposition of retained β phase, particularly in larger retained β regions that exhibit lower stability.