Microstructure and Shape Memory Characteristics of Powder-Metallurgical-Processed Ti-Ni-Cu Alloys

Abstract

Even though Ti-Ni-Cu alloys have attracted a lot of attention because of their high performance in shape memory effect and decrease in thermal and stress hysteresis compared with Ti-Ni binary alloys, their poor workability restrains the practical applications of Ti-Ni-Cu shape memory alloys. Consolidation of Ti-Ni-Cu alloy powders is useful for the fabrication of bulk near-net-shape shape memory alloy. Ti50Ni30Cu20 shape memory alloy powders were prepared by gas atomization, and the sieved powders with the specific size range of 25 to 150 mu m were chosen for this study. The evaluation of powder microstructures was based on a scanning electron microscope (SEM) examination of the surface and the polished and etched powder cross sections. The typical images showed cellular/dendrite morphology and high population of small shrinkage cavities at intercellular regions. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis showed that a B2-B19 one-step martensitic transformation occurred in the as-atomized powders. The martensitic transformation start temperature (M-s) of powders ranging between 25 and 50 mu m was 304.5 K (31.5 A degrees C). The M-s increased with increasing powder size. However, the difference of M-s in the as-atomized powders ranging between 25 and 150 mu m was only 274 K (1 A degrees C). A dense cylindrical specimen of 10 mm diameter and 15 mm length were fabricated by spark plasma sintering (SPS) at 1073 K (800 A degrees C) and 10 MPa for 20 minutes. Then, this bulk specimen was heat treated for 60 minutes at 1123 K (850 A degrees C) and quenched in ice water. The M-s of the SPS specimen was 310.5 K (37.5 A degrees C) whereas the M-s of conventionally cast ingot is found to be as high as 352.7 K (79.7 A degrees C). It is considered that the depression of the M-s in rapidly solidified powders is ascribed to the density of dislocations and the stored energy produced by rapid solidification.