Mechanical properties and thermal stability of nanocrystallized pure aluminum produced by surface mechanical attrition treatment

By means of surface mechanical attrition treatment (SMAT), nanocrystalline surface layers were generated in pure aluminum (Al) sheet. The microstructure of the nanocrystalline surface layer was systematically characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that dislocation slip is the main mechanism for grain refinement of Al with SMAT. The mechanical properties were tested by uniaxial tensile and micro-hardness tests. After SMAT, the yield strength of the nanostructured Al was about two times higher than that of the coarse-grained counterpart, with compromise of elongation. The relationship between microstructure and mechanical properties of Al with SMAT was discussed. Moreover, the Al sample after SMAT exhibits a much faster strain hardening rate reduction and a weaker strain hardening than the one without SMAT. Furthermore, the thermal stability of the nanocrystalline surface layer was studied by isothermal and isochronal annealing treatments at the temperature range from 155 °C to 355 °C. It reveals that such nanocrystalline surface layer of Al exhibits high thermal stability, which can be retained up to 275 °C. The growth kinetics of nano-sized grain of Al subjected to SMAT was investigated.