Ti-10V-2Fe-3Al合金の逆形状記憶効果と靭性回復

Ti-10V-2Fe-3Al alloys exhibit shape memory (SM) and reverse shape memory (RSM) effects. When an alloy sample that has been strained by external force at room temperature is heated, the strain recovers and SM effect develops at around 300℃, but as the temperature increases further, the shape changes in the opposite direction due to RSM effect at around 450℃. This RSM effect has potential applications in forming processes such as thin-walled pipes, but has the disadvantage that the RSM treatment makes the material very brittle. Therefore, in this study, a heat treatment to restore toughness while maintaining the shape after forming was investigated. The alloy quenched from 1050℃ had a microstructure consisting of a β matrix phase and α′′-martensite (α′′Mq). Differential scanning calorimetry (DSC) results showed that the continuous heating process occurred in the following order: α′′Mq → β reverse transformation, ω formation, ω disappearance, thermally induced α′′iso phase formation, α precipitation and α → β transformation. Ageing at 300℃, where the SM effect appears, caused significant embrittlement due to the formation of the ageing ω phase. Ageing treatment at 450℃, where the RSM effect is obtained, resulted in the formation of a fine α phase, which also caused significant embrittlement. On the other hand, additional aging at 600℃ for 1.8 ks after RSM treatment significantly improved the toughness and produced material properties comparable to aerospace material specifications. It was found that the embrittlement in the RSM treatment was due to the precipitation of fine α phase, and that the growth of α phase with a width of about 0.2 µm or more was required for toughness recovery. It was also found that the specimen shape formed by the RSM treatment hardly changed after the additional heat treatment of 1.8 ks at 600℃.