Relationship between martensitic reversibility and different nano-phases in a FeMnAlNi shape memory alloy

Abstract

In order to suppress the crack formation, water or air cooling after solution treatment is employed to introduce γ phase in a Fe43.61Mn34.74Al13.38Ni8.27 alloy. In addition to the γ phase, nano-sized B2 and β-Mn phases with an average diameter of 7.8nm are precipitated in α phase for water cooled samples, while nano-sized β-Mn phase of 11.5nm is mainly found in the α phase for air cooled samples. This nano-sized coherent B2 phase is beneficial to improving the martensitic reversibility, while the nano-sized incoherent β-Mn phase has no positive effect on achieving the good martensitic reversibility. Furthermore, the nano-sized coherent B2 phase is more effective to avoid the formation of hairpin-shaped dislocations at γ′/α interface than the nano-sized incoherent β-Mn phase. The above two reasons are the main causes of that the reversibility of stress-induced γ′ martensite in the water cooled samples is better than that in the air cooled samples. As a result, the water cooled samples exhibit strong shape memory effect while the air cooled samples have weak one. For the purpose of obtaining the good shape memory effect in FeMnAlNi alloys, the nano-sized coherent B2 phase is expected to be precipitated.