Cryogenic deformation behavior of Al–Zn–Mg–Cu alloy and its sensitivity to temperature and strain rate

Abstract

The flow behavior and deformation mechanism of solution quenched Al 7075 alloys at different cryogenic temperatures and strain rates are systematically investigated in the present work. Uniaxial tensile experiments were conducted at five different temperatures (room temperature (RT), −80 °C, −120 °C, −160 °C, −190 °C) and three different strain rates (0.00025 s−1, 0.001 s−1, 0.01 s−1). Deformation characteristics and microstructure of the sample were studied by optical microscope (OM), scanning electron microscopy (SEM), electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results showed that the deformation behavior of solid solution quenched 7075 aluminum alloy is synergistically affected by the deformation temperature and strain rate. The strength and plasticity of the sample are simultaneously elevated at −190 °C and −160 °C compared to that at RT with strain rates of 0.00025 s−1 and 0.001 s−1, respectively. The critical temperatures for plastic enhancement are −160 °C, −120 °C and −80 °C with strain rates of 0.00025s−1, 0.001s−1 and 0.01s−1, respectively. The proportion of increase in tensile strength and plasticity at −190 °C is more significant with the increase of strain rate. Different strain rates can lead to different Portevin-Le Chatelier (PLC) types of stress-strain curves. The PLC effect exhibits serrated fluctuations of type C, type B and type A at RT with strain rates of 0.00025s−1, 0.001s−1 and 0.01s−1, respectively. When the strain rates are 0.001s−1 and 0.01s−1, the PLC effect is completely suppressed at cryogenic temperatures. Interestingly, the PLC effect reappears at −160 °C and −190 °C with a strain rate of 0.00025s−1. The pinning effect of clusters formed by solute atom diffusion on dislocations as well as the grid-like entanglement formed by dislocations can result in the unstable plastic flow of aluminum alloy. In summary, the tensile properties and the PLC type of the 7075 aluminum alloys are determined by the combination of deformation temperature and strain rate.