(A) study on the mechanical properties and microstructures of grain refined Cu-Al-Ni shape memory alloys

Abstract

The effects of grain refinement on the mechanical and shape memory properties of β Cu-Al-Ni alloys were investigated by optical microscopy, tensile testing, X-ray analysis, and SEM fractography. Various microalloying elements, such as 0.3Ti, 0.3Ti-0.2Mn, 0.3Ti-0.6Zr, were added to Cu13.2Al-3Ni shape memory alloy in order to obtain grain-refined shape memory alloy. It was found that the simultaneous addition of Ti and Zr are the most effective in reducing β grain size and suppressing grain growth rate; the minimum grain size about 110㎛ was obtained by 0.3Ti-0.6Zr additions. Influence of the β grain size on the mechanical properties was investigated by tensile testing. Fracture strength increases from 380 MPa to 903MPa, and fracture strain increases from 4% 50 8.6% by the simultaneous addition of 0.3%Ti and 0.6Zr to the Cu-13.2Al-3Ni base. It was found that $σ_t$ and dσ/dε increased with grain size according to the Hall-Petch relationship. The ultimate tensile strength and strain to fracture also followed the similar Hall-Petch relationship. The scanning electron microscope observation of the tensile-fracture specimens revealed that the grain boundary cracking was remarkably suppressed by the microalloying additions, and the fracture mode changed from the brittle fracture to the ductile fracture with the decreasing β grain size. It was also found that the shape memory and pseudoelastic recovery properties were not affected significantly by the presence of second phase particles in the decreased grain. X-ray and transmission electron microscopy analyses showed that all alloys were fully martensitic with a mixture of internally faulted M18R and internally twinned N2H types.