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 Mg3Gd, GdH2 particles. Under conventional solution and aging treatment parameters, grains of LMDed alloy coarsen significantly, from 19 to 158 µm. Plenty of GdH2 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 GdH2 particles during heat treatment are believed to be the main reasons of the reduced YS and UTS.

•Fatigue morphology evolution was studied by Laser Scanning Confocal Microscopy.•3D morphology of persistent slip markings and twins was characterized.•Non-uniform deformation among grains, the PSMs and twins were quantified.•Initiations of fatigue crack were clearly investigated.Laser scanning confocal microscopy (LSCM) and Electron back-scattered diffraction (EBSD) were applied to the study of surface morphology variation of as-cast Mg–3.0Nd–0.2Zn–Zr (NZ30K) (wt.%) alloy under tension-compression fatigue test at room temperature. Two kinds of typical damage morphologies were observed in fatigued NZ30K alloy: One was parallel lines on basal planes led by the cumulation of basal slips, called persistent slip markings (PSMs), and the other was lens shaped, thicker and in less density, led by the formation of twinning. The surface fatigue damage morphology evolution was analyzed in a statistical way. The influences of stress amplitude and grain orientation on fatigue deformation mechanisms were discussed and the non-uniform deformation among grains and the PSMs, within twinning were described quantitatively.

Different solution treatments at 480, 500 and 520 °C were carried out on the semi-continuously cast Mg96.34Gd2.5Zn1Zr0.16 alloy. It was found that different solution temperatures lead to formation of different phases. Besides α-Mg matrix and Zn–Zr compounds, the secondary phases along the grain boundaries change with solution temperature. At 480 °C and 500 °C, long period stacking ordered (LPSO) structured X phases and residual eutectic compounds (Mg, Zn)3Gd are observed, while at 520 °C, only residual eutectic compounds (Mg, Zn)3Gd exist. The different microstructures solution treated at different temperatures lead to different aging response and mechanical properties. Higher solution temperature results in higher aging response and better mechanical properties. The alloy solution treated at 520 °C for 8 h and aged at 200 °C for 64 h shows the best tensile properties at room temperature: ultimate tensile strength of 405 MPa, yield strength of 292 MPa and elongation of 5.3%. The influences of existing forms of Gd and Zn elements on tensile properties of Mg–Gd–Zn alloy were discussed, which indicates that the existing of Gd and Zn elements as precipitates leads to better strengthening effect than that in the form of LPSO structured X phase.

•The thickness of diffusion layer in Gd-Mg diffusion couple was largely increased by an intermediate frequency magnetic field (IFMF).•The solution treatment of Mg-Gd alloy was also remarkably promoted by the IFMF.•These phenomena are mainly due to the enhanced atomic diffusion ability in Mg–Gd alloy under IFMF.•The results are new for Mg-RE based alloy and can be used to accelerate the solution treatment.An intermediate frequency magnetic field (IFMF) was imposed upon Gd–Mg diffusion couples that were annealed at 773 K. 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.

High cycle fatigue behavior of casting Mg96.34Gd2.5Zn1Zr0.16 alloy was investigated for its improvement by heat-treatment. After solution treatment (T4, 10 h@773 K) or solution treatment plus artificial aging (T6, 10 h@773 K+128 h@473 K), fatigue strength of this alloy was found to be enhanced. The T6-treated alloy achieved the highest fatigue strength, 130 MPa, being 25 MPa and 18 MPa 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.

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±8 MPa. 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.

The microstructure, tensile properties and high cycle rotary bending fatigue behavior of low pressure sand cast (LPSC) AM-SC1 Mg alloy, solution-treated at 525, 540 and 560 °C respectively and then aged at 215 °C, were studied. The solution temperature has little influence on the average grain size of LPSC AM-SC1 alloy, but decreases the size and content of the residual secondary compounds when solution temperature increases. Compared with the alloy solution treated at 525 and 540 °C, the alloy treated at 560 °C shows obviously higher yield strength (+19 MPa) and ultimate tensile strength (+24 MPa), comparable elongation, and higher rotary bending fatigue strength (+8 MPa). However, when the fatigue stress amplitude is higher than the fatigue strength, the 560 °C solution treated alloy shows an obvious lower fatigue life. The fatigue fracture behavior of LPSC AM-SC1 alloy was discussed as well in this paper.