Shape memory properties of a two-phase NiAl plus Fe alloy

The mechanical properties, shape memory properties, microstructure, and phase stability were studied in a two-phase alloy, Ni-25.5Al-16Fe-0.14B (at.%). Boron improved the grain-boundary strength, while iron improved the cleavage strength resulting in about 7% room temperature ductility. Two-way shape memory cycling from 100 to 200 °C (Ap = 137 °C as measured by differential scanning calorimetry) under a constant load was done as a function of load and tensile prestrain and typically resulted in a recoverable length change of 0.6%. However, after approximately 2000 cycles the shape memory length changes decreased to a level approximately offsetting thermal expansion (about 0.15%). Some of this loss could be recovered by restraining. After this cycling treatment or after 10 days at 100 °C or 200 °C, the room temperature ductility decreased, and transmission electron microscopy examination at 200 °C showed a tweed appearance in some B2 regions apparently due to the formation of a precursor to Ni5Al3 phase is necessary for producing a practical high transformation temperature shape memory alloy based on NiAl.