A high-strength binary Mg-1.2Ce alloy with ultra-fine grains achieved by conventional one-step extrusion during 300-400 °C

Ultra-fine grained magnesium alloys are usually prepared by severe deformation and medium-low temperature deformation. In this work, it has been found that the binary Mg-1.2Ce alloy with the uniform distributed Mg12Ce precipitates smaller than 3 mu m is a good candidate to obtain ultra-fine grains by conventional one-step extrusion at high temperatures. All the three Mg-1.2Ce samples extruded at 300 degrees C, 350 degrees C and 400 degrees C show a bimodal microstructure with high-density dislocations, whose average grain sizes are 1.19 +/- 0.92 mu m, 1.32 +/- 1.30 mu m, and 1.44 +/- 1.33 mu m, respectively. A linear relation of ln(d) = -0.05ln(Z)+1.27 was determined relating the average grain size (d) to the Zener-Hollomon parameter (Z). The alloy extruded at 300 degrees C exhibits an exceptionally high ultimate tensile strength (UTS) of 412.3 +/- 5.3 MPa and yield strength (TYS) of 387.6 +/- 3.2 MPa, but a low elongation after fracture (El) of 4.9 +/- 0.8 %. With the extrusion temperature increasing, the tensile strength gradually decreases. For the 1.2E-400 sample, the TYS and the UTS drop to 347.2 +/- 2.1 MPa and 349.4 +/- 2.8 MPa, while the El increases to a more acceptable value of 12.6 +/- 1.4 %. The microstructure analysis reveals that Ce atoms segregate along grain boundaries and dislocations in the Mg-1.2Ce alloy, which can limit the gliding of GBs or the slipping and climbing of dislocations. Additionally, dislocations can be pinned by Mg12Ce precipitates, thereby also restricting dislocation and grain boundary mobility. Consequently, the dynamic recrystallization (DRX) process is delayed, leading to the formation of a bimodal microstructure.