Influence of the pouring temperature ranging from 680 to 780 ℃ on the solidification behavior, the microstructure and mechanical properties of the sand-cast Mg-10Gd-3Y-0.4Zr alloy was investigated. It was found that the nucleation undercooling of the a-Mg phase increased from 2.3 to 6.3 ℃. The average a-Mg grain size increased from 44 to 71 μm, but then decreased to 46 μm. The morphology of the eutectic compound transformed from a continuous network into a discontinuous state and then subsequently into an island-like block. The volume fraction of β-Mg_24RE_5 phase increased and its morphology transformed from particle into rod-like. The increase in pouring temperature increased the solute concentration. YS increased from 138 to 151 MPa, and UTS increased from 186 to 197 MPa. The alloy poured at 750 ℃ had optimal combining strength and ductility. The fracture surface mode transformed from quasi-cleavage crack into transgranular fracture, all plus the dimple-like fracture, with the micro-porosity and the re-oxidation inclusion as major defects. The average a-Mg grain size played a main role in the YS of sand-cast Mg-10Gd-3Y-0.4Zr alloy, besides other factors, i.e. micro-porosity, morphology of eutectic compounds, re-oxidation inclusion and solute concentration.
The effect of the cooling rate ranging from 1.4 °C/s to 3.5 °C/s on the solidification behavior of the sand-cast Mg?10Gd?3Y?0.4Zr alloy was studied by computer aided cooling curve analysis (CA-CCA). With the increase in cooling rate, the nucleation temperature (Tα,N) increases from 634.8 °C to 636.3 °C, the minimum temperature (Tα,Min) decreases from 631.9 °C to 630.7 °C, the nucleation undercooling (ΔTN) increases from 2.9 °C to 5.6 °C, the beginning temperature of the eutectic reaction (Teut,N) increases, the time of the eutectic reaction shortens, solidus temperature decreases from 546.0 °C to 541.4 °C, and solidification temperature range (ΔTS) increases by 6.1 °C. The increased nucleation rate (N&) is supposed to be the main reason for the increased?TN. Increased value (Teut,N?Teut,G) and shortened time of the eutectic reaction cause the change in the volume fraction and morphology of the second phase.
Influence of multi-cycle cryogenic treatment and tensile temperature on microstructure, mechanical properties and fracture mechanism of as-extruded Mg-10Gd-3Y-0.5Zr magnesium alloy was investigated. The results show that there have no significant changes in tensile properties of the tested alloy after 10 d in liquid nitrogen immersion or 10 cycles of high-low temperature treatment at all test temperatures. The room temperature ultimate tensile strength increases from 398 MPa to 417 MPa after 20 cycles of high-low temperature treatments. Compared with the room temperature, the tested alloys exhibit higher tensile properties at low temperatures. At -196 °C, the yield strength and ultimate tensile strength of the as-extruded-T5 Mg-10Gd-3Y-0.5Zr alloy are 349 MPa and 506 MPa, respectively, increasing by about 18% and 27%, respectively. The transgranular cleavage fracture mechanism is observed at room temperature, while at low temperatures both ductile fracture and cleavage fracture behaviors coexist.
The microstructures and mechanical properties of Mg-4.2Zn-1.5RE-0.7Zr alloy were investigated under different heat treatment conditions. The as-cast alloy consisted ofα-Mg phase, T-phase and Mg51Zn20 phase. After aging treatment （single-step （325 °C, 10 h） and two-step （325 °C, 4 h）＋（175 °C, 14 h））, neither T-phase nor Mg51Zn20 phase dissolved into the matrix and the coarsening ofα-Mg phase was not significant. When peak-aged at 325 °C for 10 h, dense short rod-likeβ′1 phase precipitated in the matrix. Further ageing at 325 °C led to coarsening ofβ′1 phase and a decrease in number density. Alloy aged at 325 °C for 10 h achieved the highest yield strength （YS） and ultimate tensile strength （UTS） of 153.9 MPa and 247.0 MPa, which were increased by 48 MPa and 23 MPa from as-cast condition, respectively. While the elongation slightly decreased to 15.6%. Comparatively, the YS and UTS of alloy two-step aged by （325 °C, 4 h）＋（175 °C, 14 h） showed little difference from those of single-step aged alloy, but with a lower elongation of 13.4%. In addition, the fracture surfaces of Mg-4.2Zn-1.5RE-0.7Zr alloy under different thermal conditions were mainly characterized by quasi-cleavage feature, but with differences in the details.
The corrosion behaviors of low-pressure sand cast Mg-10Gd-3Y-0.5Zr(wt.%)alloys in as-cast,solution treated(T4)and aged(T6)conditions were studied by means of immersion test and electrochemical measurements in 5wt.%NaCl solution saturated with Mg(OH)_(2).It was observed that the corrosion rate in the T4 condition was lower than that of the as-cast and T6 conditions by both sand casting and permanent mold casting with the same order of as-cast>T6>T4;while the corrosion resistance of the permanent mold casting is superior to the sand casting.The morphologies of the corrosion products are similar porous structures consisting of tiny erect flakes perpendicular to the corroded surface of the alloy,irrespective of the heat treatment conditions.Especially,the corrosion film in T4 condition is more compact than that in the other two conditions.In addition,the severer corrosion happening to the as-cast condition is correlated with the galvanic corrosion between the matrix and the eutectic compounds;while improved corrosion resistance for the T4 and T6 conditions is ascribed to the dissolution of the secondary eutectic compounds.The measured corrosion current densities of Mg-10Gd-3Y-0.5Zr alloys in as-cast,T4,and T6 conditions are 36μA·cm^(-2),10μA·cm^(-2),and 33μA·cm^(-2),respectively.The proposed equivalent circuit[Rs(CPE_(1)R_(t)R_(f)CPE_(2))]by Zview software matches well with the tested electrochemical impedance spectra(EIS)data.
In this study,the effect of rotating gas bubble stirring treatment on the microstructures of semi-solid AZ91-2Ca alloy was investigated.The semi-solid slurry was produced by injecting fine gas bubbles into the melt through a rotating steel diffuser during solidification,and the samples of semi-solid slurry were taken by the rapid quenching method.The results show that fine and sphericalα-Mg particles can be obtained under rotating gas bubble stirring treatment.The process parameters such as gas flow rate,cooling rate and rotation speed have significant influence on the morphology of primary solid particles.After rotating gas bubble stirring treatment,the higher the particle density,the finer and rounder the primaryα-Mg particles.The formation of numerous solid particles is due to the combination mechanisms of copious nucleation and dendrite fragmentation.
