SHAPE-MEMORY PROPERTIES OF A 2-PHASE NIAL PLUS FE ALLOY

The mechanical properties, shape memory properties, microstructure, an d phase stability were studied in a two-phase alloy, Ni-25.5Al-16Fe-0. 14B (at.%). Boron improved the grain-boundary strength, while iron imp roved the cleavage strength resulting in about 7% room temperature duc tility. Two-way shape memory cycling from 100 to 200 degrees C (A(p)=1 37 degrees C as measured by differential scanning calorimetry) under a constant load was done as a function of load and tensile prestrain an d typically resulted in a recoverable length change of 0.6%. However, after approximately 2000 cycles the shape memory length changes decrea sed to a level approximately offsetting thermal expansion (about 0.15% ). Some of this loss could be recovered by restraining. After this cyc ling treatment or after 10 days at 100 degrees C or 200 degrees C, the room temperature ductility decreased, and transmission electron micro scopy examination at 200 degrees C showed a tweed appearance in some B 2 regions apparently due to the formation of a precursor to Ni5Al3 pha se is necessary for producing a practical high transformation temperat ure shape memory alloy based on NiAl.