| As the lightest structural metal, magnesium alloys are attractive for aerospace,automobile and electronic applications. Among various Mg alloys, the Mg–Gd base alloysare particularly promising because they possess higher strength and creep resistance thanWE54(Mg–5Y–4RE (rare earth)), the prevailing high-strength commercial Mg alloy.Hence, Mg–Gd base alloys are most promising in developing advanced Mg alloys. Grainrefinement often leads to a uniform distribution of secondary phases and improves themechanical properties and castability of alloys. Thus far, Zr is regarded as the most potentgrain refiner for Al free magnesium alloys and usually introduced through commercialbinary Mg–Zr master alloys. However, Mg–Zr master alloys are expensive because themanufacturing process of them is complicated and consumes a significant amount ofenergy, and the utilization rate of the master alloys is low. Therefore, cheaper but effectiveand stable grain refiners are still under investigation. Furthermore, the solute content andcooling rate would affect the grain refinement effect, and with the presence of nucleantparticles, the combined addition of solute was found to further improve the grainefficiency of these nuclei. Recently, it has been reported that a kind of Al2RE particles,Al2Y is a very potent grain refiner. some other Al2RE particles also tend to be effectivenuclei for the Mg matrix.The effect of Al additions on grain refinement of Mg–10Gd(–3Y) alloys has beeninvestigated in the present work. It was found that Al additions led to significant grain refinement, and the grain refinement mechanism was attributed to the in situ formation of Al2Gd and Al2(Gd0.5Y0.5) particles which were potent nucleants. Based upon above results, the relationship between grains size and cooling rate, solute content, and the number density and size of nucleant particles were analyzed. Furthermore, additions of trace elements (Ca, Sr, Ti and Zr) were found to further improve the grain refining efficiency of Al2Gd nucleants. In addition, the heat treatment process for the Al refined Mg-(10,12)Gd-3Y alloys were optimized, and the tensile properties of the Al refined alloys were comparable to that of the Zr refined counterparts. In addition, the microstructural evolution during heat treatment was identified. Details are as follows:a) the addition of Al led to substantial grain refinement of Mg-lOGd and Mg-10Gd-3Y alloys. The grain size of the above Al refined alloys is comparable to that of the Zr refined counterparts. Al2Gd and Al2(Gd0.5Y0.5) particles were reproducibly observed at grain centers, and an orientation relationship, i.e.<112>Al2RE‖<2110>α-Mg,{110}Al2RE‖{0110}α-Mg,{111}Al2RE‖{0001}α-Mg, was reproducibly observed between the above Al2RE particles and Mg matrix, indicating that these particles were potent heterogeneous nucleants for the Mg matrix and led to grain refinement.b) In the Al refined Mg-Gd-Y alloys, Al2(GdxY1-x) nucleant particles were formed after the samples were taken from the crucible, and the number density and size distribution of these particles were affected by both solute content and cooling rate. A high cooling rate and high solute contents were needed to obtain a good grain refining efficiency.c) Grain sizes of Mg-Gd-Y base alloys were found to be related to solute content (defined by the growth restriction factor, Q), cooling rate (T), and it was found that Based upon previous reports, grain sizes were related to number density (ρns) and size (dp) of nucleant particles, Q and and data of Al refined Mg-Gd-Y alloys were in agreement with the deduced formula. In the above equations, a, b, a’ and b’ are constants.d) The combined addition of trace elements (Ca, Sr, Ti and Zr) improved the grain refining efficiency of the1%Al refined Mg-lOGd alloy, which was attributed to the increase in the number density of active Al2Gd nuclei.e) Many plate-shaped particles precipitated in the0.8%Al refined Mg-(10,12)Gd- 3Y alloy. Compared to Mg–(10,12)Gd–3Y(–Zr) alloys, a higher solution treatmenttemperature was needed to dissolve the majority of the plate-shaped particles. In theoptimized heat-treated condition, the0.8%Al refined Mg–(10,12)Gd–3Y alloys had atensile strength comparable to that of the Zr refined counterparts, indicating that Al can beused to replace Zr to refine grains of Mg–(10,12)Gd–3Y alloys.f) the survived plate-shaped particles were with the18R-type long period stackingordered (LPSO) structure with the chemical composition of Mg–7.9at.%Al–7.8at.%Gd–3.1at.%Y. During isothermal ageing at250°C, the precipitation sequence in the Algrain-refined Mg–10Gd–3Y alloy is the same as that in the Zr refined counterpart, i.e.supersaturated solid solution precipitation (S.S.S.S.)'β″(D019)'β′(bco)'β1(fcc)'β (fcc). |
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