Microstructure and martensitic transformations in a dual-phase α/β Cu-Zn alloy

The martensitic transformations in a dual-phase α/β Cu-Zn shape-memory alloy, containing 15 pct by volume of α particles, were studied during subcooling and deformation. The crystal structure and characteristics of the martensitic transformation of a dual-phase Cu-Zn alloy were found to be similar to those of a single-phase alloy. Both the thermal martensite formed by subcooling and the stress-induced martensite (SIM) formed by loading possessed an M9R long-period stacking-order (LPSO) structure, with internal stacking faults on the (001) basal plane. Upon subcooling, the α particles were deformed in order to accommodate the shape strain accompanying the martensitic transformation. Although most of them are deformed by slip, deformation twins have, nevertheless, been found in a few α particles. Upon loading, the SIM with an M9R structure nucleates and grows at a given temperature; subsequently, another martensite phase (α _s ) possessing an fct structure is formed, with a shear developing on the basal plane of the initial M9R SIM during further loading. However, during unloading, both the α _s and SIM are transformed and follow the reverse sequence back to the parent phase. However, some residual SIM and α _s were found at zero load, due to a constraint effect of the deformed α particles and grain boundaries. The α _s martensite may be formed by two intersecting plates of SIM or by advanced deformation on a single plate of SIM. In addition to the residual SIM and α _s martensite, an SIM/α _s lamellar martensite was found in the deformed specimen.