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 diff raction, and differential scanning calorimetry. The effects of alloyin g additions (B, Fe, and Mn) on martensite stability, shape recovery, a nd tensile ductility were also studied. NiAl-based SMAs can be made du ctile by alloying with B for enhanced grain boundary cohesion and Fe f or improved bulk properties. Iron has the undesirable effect that it d ecreases the martensite --> austenite transformation temperatures (A(p )). Fortunately, A(p) can be increased by decreasing the ''equivalent' ' Al content of the alloy. In this way, a high A(p) temperature of app roximately 190-degrees-C has been obtained without sacrificing ductili ty. Recoverable strains of approximately 0.7% have been obtained in a Ni-Al-Fe alloy with A(p) temperature of approximately 140-degrees-C. M anganese additions (2-10%) lower A(p), degrade hot workability, and de crease room temperature ductility. Good-quality, ductile SMA ribbons h ave been produced by melt spinning. However, additional alloy design i s required to suppress the aging-induced embrittlement caused by Ni5Al 3 formation.