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

• Water cooled alloy has strong shape memory while air cooled alloy exhibits weak one.
• Nano-sized coherent B2 and incoherent β-Mn phases exist in water cooled alloy.
• Nano-sized incoherent β-Mn phase is mainly precipitated in air cooled alloy.
• Nano-sized coherent B2 phase has a positive effect on improving shape memory effect.
• Nano-sized incoherent β-Mn phase exhibits no positive effect on shape memory effect.

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 7.8 nm are precipitated in α phase for water cooled samples, while nano-sized β-Mn phase of 11.5 nm 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.