Reversible martensite transformations in thermal cycled polycrystalline Cu-13.7%Al-4.0%Ni alloy

A polycrystalline copper alloy with 13.7 wt% aluminum and 4.0 wt% nickel, produced by plasma fusion followed by injection molding, was investigated for the reversible martensitic transformations (RMT) occurring in association with load-free thermal cycling treatments (TCT's). The as-received quenched alloy was subjected to distinct TCT's up to 500 cycles within the temperature interval from 258 K (below Mf) to 373 K (above Af). Each distinct TCT was finished at room temperature (RT) either by half cooling cycle (258 K to RT) or by half heating cycle (373 K to RT). Transformations were characterized by X-ray diffraction, differential scanning calorimetry, optical microscopy and microhardness. The results revealed a complex sequence of transformations involving the high temperature β1 phase, the metastable γ1′ and β1′ martensitic phases, and the intermediate Al7Cu4Ni phase, known as the R phase. Depending on the number of cycles and the finishing way, the sequence of transformations was partial or complete. The RMT with TCT promotes different changes in the microstructure, including an apparent pull out of grains. Microhardness was also sensibly affected owing to the complex participation of different phases.