Grain refinement of CuZnAl shape memory alloys

Abstract

Various dopants, such as boron and its compounds and pure elements such as Zr, Ti, V and Cr, were added to two commercial, copper-based shape memory alloys (70.7% Cu-25.7% Zn-3.58% Al [alloy X] and 65.6% Cu-31.4% Zn-2.95% Al-0.1% Zr [alloy Y]) in order to identify their grain refining abilities. Different annealing schedules were given to certain of the grain-refined alloys to observe their grain growth rate. Boron and various borides provided some degree of grain refinement, although not extensive. Zr and Ti produced the most grain refinement. Moreover, Zr suppressed the grain growth rate to virtually zero even after 6 h annealing at 800°C. The Ti-doped alloys had a relatively larger grain growth rate. Zr-rich second-phase particles, observed at the grain boundaries of Zr-doped alloys, are believed to be responsible for grain refinement. The formability of all alloys investigated was studied by hot-rolling with intermediate annealing. A 98% reduction, leading to a final thickness of 0.254 mm could be achieved for all the alloys. For the 0.91% Zr-doped alloys, a 98% reduction did not increase the grain growth rate. Zr-, Ti-, V-, and Cr-doped alloys were tensile-fractured as well as impact-fractured in situ in an Auger vacuum system. These alloys all showed a ductile transgranular fracture mode; no grain boundary segregation was detected. An equation to predict the M s temperature in CuZnAl alloys was obtained by analyzing previous data using a first-order linear regression computer program.