Microstructure and mechanical properties of severely deformed Al-Mg-Sc-Zr alloy and their evolution during annealing

The ultrafine-grained structure developed here after severe plastic deformation of Al-Mg and Al-Mg-Sc-Zr by equal channel angular pressing (ECAP) and cold rolling (CR) is presented to consist of dense dislocation walls and cell structure of dimension of similar to 300 nm. Detailed characterization by traditional transmission electron microscopy (TEM), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD) has been carried out to reveal microstructure evolution during deformation and annealing. A unique bimodal grain structure, which consists of ultrafine grains (less than 150 nm) and fine grains (in the range of 200-500 nm) having high dislocation density, was developed after 16 ECAP passes and CR. It was also found that microstructure inhomogeneity exits even after high ECAP passes and annealing at high temperature (280 degrees C) for 1 h. High yield strength (YS) of similar to 550 MPa and tensile strength (TS) of similar to 560 MPa were obtained after 16 ECAP passes and CR, however, the corresponding elongation dropped to similar to 8.2%. Subsequent optimum annealing was found to be 240 degrees C x 1 h, by which comprehensive mechanical properties were maintained (similar to 481 MPa for YS, similar to 512 MPa for TS and similar to 13.7% for elongation) in Al-Mg-Sc-Zr (16 ECAP passes). The reasons of inhomogeneous microstructure and the origins of high strength, i.e. high solute Mg content, dense dislocations, ultrafine grains and nanoscale Al-3 (Sc, Zr) particles are discussed.