Microstructure and transformation behavior of Ni50Ti45Ta5 shape memory alloy
(Department of Materials Engineering, Dalian University of Technology, Dalian 116024, China)
Abstract: The microstructure and transformation behavior of Ni50Ti45Ta5 shape memory alloy were investigated using optical microscope, EPMA, X-ray diffraction and DSC methods. The microstructures are primary NiTi dendrite and net-like eutectic. The room temperature phases of this alloy are B2 austenite with lattice parameter a=0.302 5 nm, B19′ martensite with lattice parameters a=0.290 nm, b=0.412 nm and c=0.473 nm, β=98.2° and a body center cubic β-Ta with lattice parameter a′＝0.330 4 nm. A maximal shape memory recovery strain of 5.6% is obtained after deformation to 6.4%. The reverse transformation temperatures increase and the forward transformation temperatures decrease in the first heating/cooling cycle after deformation. The reverse transformation heat also increases with increasing deformation. The phenomenon disappears in the second heating/cooling cycle. It is closely related to the variation of elastic energy and irreversible energies during deformation.
Key words: microstructure; transformation behavior; Ni50Ti45Ta5 shape memory alloy; deformation