Three as-cast and as-extruded Mg-5Zn-x Y-0.6Zr(x=5 wt%, 8 wt%, 11 wt%) alloys were prepared, and the effects of Y content on the microstructures and mechanical properties of the alloys were investigated. The results show that the investigated Mg-Zn-Y-Zr alloys mainly consist of a-Mg, X-Mg12 YZn and minor amount of W-Mg_3Y_2Zn_3 phases. The volume fraction of X-Mg_(12) YZn phase increases and that of W-Mg_3Y_2Zn_3 phase decreases with the rising of Y content in the alloys. The as-extruded Mg-5Zn-11Y-0.6Zr alloy owns the optimal ultimate tensile strength and yield strength of 429 and 351 MPa, respectively. Mg-5Zn-5Y-0.6Zr alloy owns the maximum elongation of 13.6%.
The micromorphology and the concentration o massive precipitates produced by twin-roll casting(TRC processes without and with multi-electromagnetic fields fo Al–Zn–Mg–Cu alloy at 670 °C were investigated in detai by means of optical microscopy(OM) and electron probe micro analyzer(EPMA). The results clearly show tha under a 0.2 T static magnetic field, the macro-segregation bands are remarkably alleviated according to the order o uniform static magnetic field, half-wave oscillating elec tromagnetic field as well as alternating oscillating elec tromagnetic field, as compared with the non-field TRC process(B = 0). Moreover, under the alternating oscillat ing electromagnetic TRC process, almost all segregation bands disappear. Additionally, through the observation on a smaller scale, the netlike precipitates elongated and broken by electromagnetic force, and replaced by numerous bulk depositions. EPMA analysis shows that the added atoms are diffused from deposition to a(Al) matrix, resulting in that the solute concentration in and around the precipitates is tending toward uniformity and stability.
The effects of the extrusion speed on the microstructures, mechanical properties and aging hardening behaviors of Mg-9Gd-3Y-1.5Zn-0.8Zr alloy have been investigated. The microstructure evolution during hot extrusion has also been discussed. The microstructures of the extruded alloys mainly consist of the equiaxed dynamically recrystallized grains and fiber-like long period stacking ordered （LPSO） phase. The increasing extrusion speed results in grain coarsening and the mechanical properties deterioration. Mean- while, it could also retard the aging hardening response. The extrudability of the investigated alloy is limited. Cracks nucleate and grow up rapidly when the extrusion speed is over 0.3 m/min. It should be closely related to the large amount of LPSO phase and the unique microstructure evolution. The rapidly precipitating fine lamella during extrusion could pin the dislocations and grain boundaries effectively. It could strengthen the alloy but also limit the extrudability of the investigated alloy.
Xuan LiuZhiqiang ZhangWenyi HuQichi LeLei BaoJianzhong Cui
