Characterization, Processing, and Alloy Design of NiAl-Based Shape Memory Alloys

The microstructures and phase transformations in binary Ni-al, ternary Ni-Al-Fe, and quaternary Ni-Al-Fe-Mn shape memory alloys (SMAs) were investigated by light and electron microscopy, electron and X-ray diffraction, and differential scanning calorimetry. The effects of alloying additions (B, Fe, and Mn) on martensite stability, shape recovery, and tensile ductility were also studied. NiAl-based SMAs can be made ductile by alloying with B for enhanced grain boundary cohesion and Fe for improved bulk properties. Iron has the undesirable effect that it decreases the martensite → austenite transformation temperatures (Ap). Fortunately, Ap can be increased by decreasing the “equivalent” Al content of the alloy. In this way, a high Ap temperature of ∼ 190°C has been obtained without sacrificing ductility. Recoverable strains of ∼0.7% have been obtained in a Ni-Al-Fe alloy with Ap temperature of ∼140°C. Manganese additions (2–10%) lower Ap, degrade hot workability, and decrease room temperature ductility. Good-quality, ductile SMA ribbons have been produced by melt spinning. However, additional alloy design is required to suppress the aging-induced embrittlement caused by Ni5Al3 formation.