The shape memory effect and superelasticity in two-phase polycrystalline α/β brasses

Abstract

The shape memory effect (SME), superelasticity (SE), and cyclic deformation behavior of two-phase α/β brasses have been investigated at various temperatures, using tensile tests andin situ optical microscopic observations. The morphology and characteristics of the (thermoelastic) martensitic transformation and the mechanism of the SME are similar to those for single-phase β-brass, but the amount of irrecoverable strain is larger in the two-phase alloys due to plastic deformation of the α particles. After unloading and heating, the slipbands in the discrete a particles remain, whereas the martensite almost disappears; thus, the higher the volume fraction of α particles, the larger the amount of irrecoverable strain. The deformation behavior of alloy A at temperatures above the martensite start (Ms) temperature (with 26 pct α phase) is dominated by deformation of the α phase, so complete SE cannot be obtained after cyclic deformation, both at room temperature and at -40 °C. While in alloy B (containing 15 pct α phase), the deformation behavior is dominated by the formation of stress-induced martensite (SIM). The α particles are deformed before SIM formation on loading at room temperature, but on the contrary, SIM forms before the α particles are deformed on loading at -40 °C (>Ms). Complete SE can be obtained in alloy B after cyclic deformation at room temperature to a given strain but does not occur at -40 °C because the a particles are deformed along with the growth of pre-existing SIM under larger strain during cycling at this temperature.