Mechanism of considerable strain hardening in ultrafine-grained pure aluminum sheets at cryogenic temperatures

Abstract

Research on the deformation behavior of ultrafine-grained (UFG) pure aluminum (Al) sheets was conducted at a temperature of −196 °C to provide a new strategy for forming complex components with UFG sheets. In this work, a UFG pure Al sheet with an average grain size of 0.9 μm was prepared by cold rolling (CR) and recovery annealing. Then, the mechanical behaviors of the UFG pure Al sheet were evaluated by uniaxial tensile tests and bulging tests. The fracture morphology, microstructure evolution and surface topography were observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The UFG pure Al exhibits considerable strain hardening and superior ductility at −196 °C, significantly different from the obvious strain softening behavior that occurs at room temperature (RT). As the temperature is decreased from RT to −196 °C, the uniform elongation (UEL) is improved 24.2 times from 1.2% to 30.2%, and the total elongation (TEL) is improved 3.3 times from 11.8% to 51.2%. The limiting dome height (LDH) and the ultimate bulging rate, K, of the UFG pure Al sheet at −196 °C are 1.6 times and 2.9 times greater than those at RT, respectively. Microstructure observations show that many dislocation substructures such as dislocation pile-ups, dislocation cells and dislocation networks are formed in UFG pure Al deformed at −196 °C, that would otherwise be absent in its counterpart deformed at RT. This unusual phenomenon is attributed to the remarkable transition of the dominant deformation mechanism of UFG pure Al from significant dynamic recovery accompanied by grain boundary sliding (GBS) to dislocation multiplication and accumulation with decreasing temperature.