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

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 2012-08, Vol.43 (8), p.2932-2938
• Main Authors: Kim, Yeon-Wook, Chung, Young-Soo, Choi, Eunsoo, Nam, Tae-Hyun
• Format: Article
• Language: English
• Subjects: Alloys; Applied sciences; Characterization and Evaluation of Materials; Chemistry and Materials Science; Exact sciences and technology; Materials Science; Metallic Materials; Metallurgy; Metals. Metallurgy; Microstructure; Nanotechnology; Powder metallurgy; Structural Materials; Surfaces and Interfaces; Symposium: Physical and Mechanical Metallurgy of Shape Memory Alloys for Actuator Applications; Thin Films
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-011-0980-9

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. Ti 50 Ni 30 Cu 20 shape memory alloy powders were prepared by gas atomization, and the sieved powders with the specific size range of 25 to 150  μ 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  μ m was 304.5 K (31.5 °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  μ m was only 274 K (1 °C). A dense cylindrical specimen of 10 mm diameter and 15 mm length were fabricated by spark plasma sintering (SPS) at 1073 K (800 °C) and 10 MPa for 20 minutes. Then, this bulk specimen was heat treated for 60 minutes at 1123 K (850 °C) and quenched in ice water. The M s of the SPS specimen was 310.5 K (37.5 °C) whereas the M s of conventionally cast ingot is found to be as high as 352.7 K (79.7 °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.