Fabrication of high-strength Mg-Gd-Zn-Zr alloys via differential-thermal extrusion

A high-strength Mg-Gd-Zn-Zr alloy developed by a novel extrusion method called differential-thermal extrusion (DTE) was detailed characterized, to reveal the effects of the DTE on microstructure and mechanical properties. Different from traditional isothermal extrusion (ITE) with the same billet and mold temperature, the billet temperature for the DTE is remarkably higher than the mold temperature. The finer grains and less long period stacking ordered (LPSO) structure in the DTE sample resulted from its higher cooling speed after deformation. Meanwhile, the stronger precipitation strengthening effect was caused by the higher billet preheat temperature which reserved the precipitation ability before extrusion. As a result, the finer grains and stronger precipitation strengthening led to the strong tensile yield strength (380 MPa) and ultimate tensile strength (461 MPa) of the aged DTE sample, while those of the aged ITE sample are only 338 MPa and 420 MPa, respectively. The higher strengths indicate the DTE is a better extrusion process for producing high-strength Mg alloys than the traditional ITE.