Influence of Predeformation on Tempering Structure of Ti-5Al-2Fe-3Mo Alloy

A near-β-type Ti-5Al-2Fe-3Mo alloy (Super-TIX®523AFM) was developed as a low-cost and high-strength material for automotive components such as intake valves. Its mechanical properties, such as strength, elongation and Young's modulus, are adjustable via the solution treatment temperature. Furthermore, a unique phenomenon different from the shape memory effect that appears after tempering has recently been discovered in the alloy. In this study, the influence of predeformation on the microstructural evolution attributed to the tempering of the alloy quenched from 900℃ (α+β) was investigated to clarify the cause of the unique phenomenon. Although the phase constitution of the quenched specimen was α+β, the effective Mo equivalency (6.5Moeffeq) of the β phase was lower than the minimum β composition (10Moeffeq) in common β alloys. Work-induced α″ martensite plates consisting of single α″ variant including nanosize β domains were formed in the β-phase by cold-rolling. Upon heating the rolled specimen to around 250℃, the plates vanished owing to the α″→β inverse transformation and the hardness decreased. However, when the rolled specimen was heated to 450℃, the hardness markedly increased as a result of the β→α″ transformation, and the plates were formed again. The plates consist of a single α″ variant, but the α″ lattice is not extensively continuous and is divided into nanosize domains by boundaries containing many faults. It is considered that the evolution of single-α″-variant plates generates a large distortion due to the transformation strain and leads to the unique shape change. Such plates were not formed in undeformed specimen by aging at 450℃, and the β-phase was transformed into a nanosize domain structure composed of multiple α″ variants. Consequently, these fine structure in the undeformed specimen led to its higher hardness than that of the rolled specimen.