Microstructure and strengthening mechanisms in an Al-Mg-Si alloy processed by equal channel angular pressing (ECAP)

The microstructure, mechanical properties, and strengthening mechanisms of an Al-Mg-Si alloy (AA6060) subjected to severe plastic deformation using equal channel angular pressing (ECAP) were investigated. Samples were passed through a die with an inner angle of I broken vertical bar = 90A degrees and outer arc of curvature of psi = 37A degrees at room temperature up to 12 passes via route Bc. Electron backscatter diffraction (EBSD) was used to evaluate the microstructure and misorientation boundaries. The microstructure showed a large fraction of low-angle boundaries associated with subgrain formation in the first ECAP pass, while after eight and 12 passes, a heterogeneous ultrafine grain structure with an average grain size around 0.57 and 0.47 mu m, respectively, was obtained. In order to characterize the mechanical properties, microhardness and tensile tests were carried out. Results of mechanical property tests show that microhardness, yield stress, and ultimate tensile strength increase as ECAP pass number increases up to a maximum value of 120 HV, 344 MPa, and 355 MPa, respectively, after five passes. The Hall-Petch effect, dislocation, solid solution, and precipitation strengthening were evaluated to determine the dependence of the yield stress on the ECAP pass number. The results show that the strength effect arises from the subgrain microstructure rather than from the high-angle grain boundaries developed.