Study On The Microstructure And Mechanical Properties Of In-situ Re_xSi_y/GW Magnesium Matrix Composites

Based on the Mg-Gd-Y(-Si) system, in-situ RExSiy/GW magnesium matrix composites were successfully prepared by traditional stir casting processes. The microstructures and mechanical properties of as-cast, cast-T4 and cast-T6 Mg-10Gd-3Y(wt.%) and Mg-12Gd-4Y-1.0Si(wt.%) alloys were studied by inductively coupled plasma analyzer(ICP), optical microscope(OM),X-ray diffraction(XRD), scanning electron microscope(SEM), energy dispersive X-Ray spectroscopy(EDX), nanoindentation, dynamic mechanical analysis(DMA) and compression testing. The microstructures, phase compositions of Gd5 Si3, YSi and Mg2 Si intermetallic compounds and their effection to the alloy hardness, elastic modulus and mechanical properties were mainly investigated. The Gibbs free energy on the reactions of Mg-Gd-Y-Si system were calculated by thermodynamic analysis. The elastic modulus and hardness of Gd5 Si3 intermetallic compounds were tested by nanoindentation for the first time.There are a lot of Gd5 Si3, YSi and Mg2 Si intermetallic compounds in the grain boundaries of Mg-12Gd-4Y-1.0Si(wt. %) alloy. The microstructures of Mg2 Si intermetallic compounds were changed from polygon and Chinese script type to tiny particles by Gd and Y. Thermodynamic analysis suggests that Si is inclined to react with Gd and Y in the normal magnesium alloy metallurgy temperature(700℃≤T≤780℃). Nanoindentation shows that the elastic modulus and hardness of Gd5 Si3 intermetallic compounds are 148.8GPa and 12.2GPa, respectively. The elastic modulus of Mg-12Gd-4Y-1.0Si(wt. %) alloy is 51.2GPa, which is 2.4 GPa higher than Mg-10Gd-3Y(wt. %) alloy. However the ultra-compression strength of Mg-12Gd-4Y-1.0Si(wt. %) alloy is 331 MPa, which is about 30 MPa lower than Mg-10Gd-3Y(wt. %) alloy.After solid solution treated at 500℃ for 6h, the volume fraction, chemical composition, microstructure and size of Gd5 Si3, YSi and Mg2 Si intermetallic compounds remained the same, which suggested that they were very stable at 500℃ and they had outstanding heat-resistance properties. Ultra-compression strength of cast-T4 Mg-12Gd-4Y-1.0Si(wt. %) alloy is 281 MPa, which is about 40 MPa lower than Mg-10Gd-3Y(wt. %) alloy.The hardness of both the Mg-10Gd-3Y(wt.%) and Mg-12Gd-4Y-1.0Si(wt.%) alloys increase about 30 HV after aging treatment(500℃×6h+225℃×16h).The hardness of cast-T4 and peak aging Mg-10Gd-3Y(wt.%) alloy are both 3HV higher than that of Mg-12Gd-4Y-1.0Si(wt.%) alloy. Ultracompression strength of cast-T6 Mg-12Gd-4Y-1.0Si(wt. %) alloy is 372 MPa, which is about 43 MPa lower than Mg-10Gd-3Y(wt. %) alloy.