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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 36 μA·cm-2, 10 μA·cm-2, and 33 μA·cm-2, respectively. The proposed equivalent circuit [Rs(CPE1(Rt(RfCPE2))] by Zview software matches well with the tested electrochemical impedance spectra (EIS) data.

Cyclic deformation and low-cycle fatigue behavior of Mg–10Gd–3Y–0.5Zr alloy in sand-cast and aging treatment conditions (sand-cast-T6) were investigated by carrying out full reversed strain-controlled tension-compression tests at the strain amplitude ranging from 0.25 to 0.7%. The results show that stress–strain hysteresis loops of the studied alloys display near tension-compression symmetry, which is dominated by microstructure and strain amplitude. Both sand-cast and sand-cast-T6 alloys exhibit cyclic hardening and softening phenomenon with increasing loading cycles. Meanwhile, the fatigue life of the aged alloy is higher than that of the sand-cast alloy at all applied strain amplitudes. The theoretical strain fatigue limits (ε0) of sand-cast and sand-cast-T6 alloys are 2.1% and 2.3%, respectively. In addition, the low-cycle fatigue behavior of the studied alloy at different strain amplitudes was also investigated. [ABSTRACT FROM PUBLISHER]

In this present study, the influence of different casting processes on high cycle fatigue behavior of Mg–10Gd–3Y–0.5Zr magnesium alloy was investigated by using porosity-free low-pressure sand-casting (LPS) bars and gravity permanent mold casting (GPM) ingots. The results show that the fatigue properties of both LPS and GPM Mg–10Gd–3Y–0.5Zr alloy in as-cast condition are determined by Mg matrix and eutectic phase. However, the fatigue property improvement for LPS alloy by T6 heat treatment is significantly superior to that of GPM alloy. The different degree of enhancement of fatigue properties for two conditions of the alloy is related to different crack initiation mechanism. The fatigue crack of the LPS alloy initiates from the free surface of the sample, while the crack of the GPM alloy initiates from porosities or inclusions near the surface of the sample. Meanwhile, the crack of slip band has a crucial effect on the fatigue crack initiation of both as-cast and T6 conditions for LPS alloy. [ABSTRACT FROM PUBLISHER]

In this study, the microstructure evolution of semi-solid Mg–10Gd–3Y–0.5Zr alloy during isothermal heat treatment has been investigated. The results show that primary particles coarsen continuously during the holding. Coarsening rate decreases with the increase of isothermal temperature. When isothermal temperature increases from 600 °C to 620 °C, the dominant mechanism for coarsening changes from particle coalescence to Ostwald ripening. Equiaxed as-cast microstructure is beneficial to the semi-solid microstructure after isothermal heat treatment, which brings about the refinement and spheroidization of primary particles, and shortening of holding time. Significant modification of second phases can also be achieved after isothermal heat treatment, due to its unique solidification process. The optimum processing parameters for Mg–10Gd–3Y–0.5Zr alloy in isothermal heat treatment are isothermal temperature of 610 °C–620 °C and holding time of 20–40 min. [ABSTRACT FROM AUTHOR]

Abstract: The present study investigated the effect of cooling rate on the microstructure and mechanical properties of sand casting Mg–10Gd–3Y–0.5Zr alloy with a cooling rate range of 0.7–3.6°C/s. When the cooling rate was increased, the average grain size of α-Mg decreased from 59μm to 39μm, the volume fraction of the second phase increased from 17.6% to 24.5%, and the eutectic compound exhibited continuous network instead of coarsening discontinuous network of plate-shaped microstructure. Energy-dispersive X-ray test results suggested that solute content of RE elements (Gd and Y) was low within the area near the grain boundary of α-Mg with high cooling rate, and solid-solution reaction occurred in the sand casting process. In addition, Vickers hardness (HV) testing indicated that HV increased with the increasing cooling rate, and the volume fraction of second phase (V) played a main role in hardness of the alloys. There was a linear relationship between them, the fitting function was HV=1.25V+63.05, R 2=0.9989. Tensile strength test showed that both of ultimate tensile strength (UTS) and tensile yield strength (TYS) first increased and then decreased with the increasing cooling rate. Based on fracture observations, the types of fracture surface characteristics are correspondingly classified into three modes, transgranular, dimple-like fracture and intergranular fracture with the increasing cooling rate. [Copyright &y& Elsevier]