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As most Mg alloy products are now produced by a casting process, the development of high strength cast Mg alloys can promote their further applications and has already become one of the hot research areas of Mg alloys. The present paper reviews the strengthening mechanisms, tensile properties and modification results of commercial high strength cast Mg alloys; as well as the development of Mg-Gd, Mg-Nd and Mg-Sn based alloys. It concludes that precipitation strengthening is the most important strengthening mechanism in high strength cast Mg alloys, which contributes more than 60% of yield strength in solution & peak-aged (T6) cast Mg alloys. For the yield strength, the alloys follow the sequence of Mg-Gd(Y)-Ag > Mg-Gd(Y)-Zn > Mg-Gd-Y/Sm/Nd > Mg-Y-Nd (WE series) > ZK61 > Mg-Nd > AZ91 > Mg-Sn. Mg-Gd(Y)-Ag based alloys are the strongest cast Mg alloys at present, followed by Mg-Gd(Y)-Zn based alloys. The high yield strengths of Mg-Gd(Y)-Ag and Mg-Gd(Y)-Zn cast alloys are due to the co-precipitation of basal and prismatic meta-stable phases.

In order to design a high strength sand cast Mg–Gd–Zn alloy, the microstructures and mechanical properties of cast Mg–12Gd– x Zn–0.4Zr (x = 0, 0.4, 0.8) (wt%, signed as GZ1200K, GZ1204K and GZ1208K) alloys were studied. Zn addition changes the constitution of the strengthening phases in Mg 12Gd alloy. Small amount of Zn addition leads to the formation of basal precipitates of γ″ and γ′, accelerated-precipitation of the β 1 phases and lower number density of the prismatic β′ phases, and modifies the morphology of the prismatic β′ precipitates. Compared with the Zn-free alloy, the Zn-containing alloys (GZ1204K & GZ1208K) having higher yield strength in peak-aged and over-aged conditions are mainly due to the extra prismatic β 1 precipitates, which do not precipitate in the Zn-free alloy. Under sand casting condition, GZ1208K alloy strengthened by the co-precipitates of the prismatic β′ and β 1 phase indicates a good combination of strength and ductility at room temperature: yield strength of 270 MPa, ultimate tensile strength of 348 MPa and elongation of 2.6%. • Zn addition changes the constitution of the strengthening phases in the Mg Gd alloy. • Zn addition reduces the number density and modifies the morphology of the β′ precipitates. • Zn addition leads to the extra strengthening effects by the β 1 phases. • Sand cast Mg-12Gd-0.8Zn alloy shows a better yield strength with elongation. [ABSTRACT FROM AUTHOR]

Mg-10Gd-3Y-0.4Zr (wt%) (GW103K) Mg-RE based alloy was manufactured in Laser melting deposition (LMD) process and its microstructure and mechanical properties were studied in as-fabricated and T6-treated conditions. As-fabricated LMDed GW103K alloy consists of equiaxed grains about 19 µm in size and Mg 3 Gd, GdH 2 particles. Under conventional solution and aging treatment parameters, grains of LMDed alloy coarsen significantly, from 19 to 158 µm. Plenty of GdH 2 particles can be observed after solution treatment. Yield strength (YS), ultimate tensile strength (UTS) and elongation of as-fabricated and T6-treated LMDed GW103K alloy at room temperature are 118–232 MPa-13.9% and 191–298 MPa-8.9%, respectively. Compared with sand cast alloy, as-fabricated LMDed GW103K alloy has comparable YS (−5 MPa), much higher UTS (+51 MPa) and elongation (+12.1%). Unfortunately, after T6 treatment under conventional heat treatment parameters, YS and UTS were about 40 MPa lower than those of sand cast alloy, with relatively higher elongation (+5.8%). Reduction of Gd, Y content during LMD process, grain growth and formation of GdH 2 particles during heat treatment are believed to be the main reasons of the reduced YS and UTS. [ABSTRACT FROM AUTHOR]

This paper studied the tensile crack initiation behaviors in cast NZ30K alloys under as-cast, T4- and T6-treated conditions. Results indicate that tensile failure of cast magnesium alloys mainly nucleates from the tensile-sensitive microstructural constituent such as eutectic particles, grain boundaries and twin grain boundaries. For the as-cast alloy, the micro-cracks form mainly by cracking of the eutectic phases (~83%). In addition, twin grain boundaries are also found to be preferential sites for crack nucleation (~17%). The tensile failure in the T4- and T6-treated alloys is mainly caused by twin nucleation (T4~60% and T6~44%) and grain boundaries cracking (T4~28% and T6~52%). Interactions among dislocation-slip, twinning and grain boundaries as well as eutectic phases determine the crack initiation behavior of cast magnesium alloys. [ABSTRACT FROM AUTHOR]

Abstract: This article presents the tension–compression high cycle fatigue behavior of as-cast Mg96.34Gd2.5Zn1Zr0.16 alloy produced by semi-continuous casting at ambient temperature. The relationship between stress amplitude and cycles to failure is established, which indicates that fatigue strength of this alloy is approximately 105±8MPa. Fracture surface of specimens were examined using a scanning electron microscope, indicating that the fatigue cracks all initiate from the oxides located at the surface. Different from other cast Mg alloys, there exist two kinds of unique fatigue morphologies at the fatigue propagation region, which consists of fine steps. Meanwhile, there is a fatigue life gap between 105 and 107 cycles on the S–N curve, which probably demonstrates that the growth rate of the fatigue cracks of as-cast Mg96.34Gd2.5Zn1Zr0.16 alloy is relatively large, and once the fatigue cracks form, the samples could fails in less than 105 cycles. [Copyright &y& Elsevier]

Abstract: High cycle fatigue behavior of casting Mg96.34Gd2.5Zn1Zr0.16 alloy was investigated for its improvement by heat-treatment. After solution treatment (T4, 10h@773K) or solution treatment plus artificial aging (T6, 10h@773K+128h@473K), fatigue strength of this alloy was found to be enhanced. The T6-treated alloy achieved the highest fatigue strength, 130MPa, being 25MPa and 18MPa greater than those of the as-cast and T4-treated alloy, respectively. The average fatigue life of the heat-treated alloys is longer than that of the as-cast alloy a given stress amplitude. For the distribution of fatigue life, a fatigue life gap spanned from 105 to 107 can be observed in the as-cast and T4-treated alloy. Such a gap is absent after the alloy received artificial aging. The mechanism for the high cycle fatigue behavior of the casting alloy after heat-treatment was also discussed. [Copyright &y& Elsevier]

Abstract: In present work, the NdH2 precipitates in Mg–2.5%Nd (wt.%) alloy were identified by various microstructural analyzing techniques including OM, XRD, TEM with EELS, SEM with EDS, etc. It was revealed that the as-cast alloy contained α-Mg solid solution and eutectic phase Mg12Nd, while there were NdH2 particles at or near the grain boundaries in 540°C/6h solution-treated state. As widely acknowledged, they worsen its mechanical property, especially, the cracking and fracture behaviors. Synchronously, the formation of hydride NdH2 is proved to be favored and practicable based on thermodynamics theory. [Copyright &y& Elsevier]

Abstract: An intermediate frequency magnetic field (IFMF) was imposed upon Gd–Mg diffusion couples that were annealed at 773K. It was found that the average thickness of diffusion layer under the magnetic field was much wider than that without the magnetic field, indicating that the atomic diffusion ability was enhanced. The magnetic field was then applied to the solution treatment of Mg–16.66Gd–0.088Zr (wt%) alloy. Compared with the samples without magnetic field, the area fraction of the eutectic phase (Mg5Gd phase) was evidently lower at each time point when the IFMF was imposed during the solution treatment, i.e. the solution treatment process was accelerated by the magnetic field. These two phenomena are mainly due to the enhanced atomic diffusion ability in Mg–Gd alloy under IFMF. [Copyright &y& Elsevier]