Theoretical And Experimental Investigations On Grain Refinement Of Mg-Al Alloys

As a typical metal material with the lightest commercial metal structural, well damping properties, high specific stiffness and high specific strength,magnesium and its alloys are promising structure materials, which are used in numerous areas, like aerospace,3C products and automobiles. Grain refinement can efficiently improve magnesium alloy properties, including strength, malleability and ductibility. Therefore, to find a high efficiency grain refiner for magnesium alloys is the way to expand the application potential of magnesium alloys, and it is not only can enrich the mechanical theories of alloy's grain refinement, and also can direct the application of those materials.In the present work, it focused on finding low-cost, high efficient grain refiner for Al-containing Mg alloys which are in stalemate now. By adding different kind of alloy compound into the pure Mg alloys system, Mg-3Aland Mg-6Al alloys system to reach the purpose of grain refinement. Firstly, established three elements phase diagram through collecting the thermodynamic data together, and designed the Mg-xAl-ySiC ?x=3,6; y=0.2?0.5?0.8?1.5?2.0?and Mg-xAl-ySiO₂?x=3,6; y=0.2?0.5?0.8?1.5? 2.0? alloys system. Q-value calculation and E2EM model are used to predict the potency of grain refining and also to explain the mechanism of grain refinement results. Optical microscope, X-ray diffraction,electron microscope with energy dispersive spectroscope analysis,scanning electron back scattered diffraction, tensile properties test,transmission electron microscopy, are used to analyze the mechanical properties and microstructures of as-cast and as-extruded Mg-Al-X alloys. Obtained the high efficiency grain refiners and also developed the mechanism of grain refining.Showing as follows:Low-cost nonmetallic compounds, SiC, were added into the Mg-Al based alloys, which got significant grain refinement with grain size decreased from 650.34?m to 95.36?m. The tensile strength and compressive strength of as-ast Mg-3Al-0.8SiC alloy are 194 MPa and 298 MPa, respectively, the performerce improved ?15 MPa and-24 MPa, compared to Mg-3Al alloys. Propose a fresh mechanism to explain the grain refinement phenomenon of SiC addition:SiC is not as a nucleus of grain forming and growth. SiC react with Al in the melting and release Si. Because of high Q-value ?Q=9.25?, Si has, it can strongly improve the segregation in magnesium, and therefore to significantly refine the grains of as cast Mg. And then, part of Si combined with Mg to form Mg₂Si. According to the calculation of edge-to-edge matching model, there exist two habit planes between Mg₂Si and Mg₁₇Al₁₂, ???Mg₁₇Al₁₂1.01° from???Mg₂Si,???Mg₂Si // <010>Mg₁₇Al₁₂ and ?044?Mg₁₇Al₁₂0.48°from?202?Mg₂si. That's make sure Mg₂Si can be the nucleation site of the second phase Mg₁₇Al₁₂. Due to the effect of Mg₂Si, Mg₁₇Al₁₂become scattered as particles in the grains, comparing to precipitate at the grain boundary or triangle areas. The Mg₁₇Al₁₂ particle became nucleus of grain forming. The indexing results of diffraction patterns and Kikkuchi patterns illustrated this hypothesis.Adding SiO₂ powder into the Mg-Al based alloys is a totally fresh topic for grain refinement. The pre-experiments found that with 0.5wt.% SiO₂ addition, the alloys grain size sharply decrease from 650.34?m to 86.77?m. The highest tensile strength and compressive strength of as-ast Mg-3Al-SiO₂ and Mg-6Al-SiO₂ alloys are ?298 MPa and-325 MPa, respectively. Under different cooling rate ?1K/sec and 3K/sec?, experiments revealed that high grain refining efficiency in favor of lower cooling rate. Change the sequences of addition elements ?Al and SiO₂? and holding time ?10min and 5min? can also vary the grain refinement results. At cooling rate 3K/sec, with the addition sequence of Mg first, Al and SiO₂ at last, holding for 10ming, we got the best grain refining outcome, the grain size fallen from 1092.45?m to 379.07?m, almost 2.5 times. After extruded, the grain sizes continuly reduced to 15-18 ?m of Mg-6Al-SiO₂ alloy, and the yield strength and tensile strength of as-extruded Mg-6Al-SiO₂ alloys are reached to 219 MPa and 297 MPa,-298 MPa and -325 MPa, respectively, the performerce improved 62 MPa and 57 MPa, compared to Mg-6A1 alloys, and elongation is 13.6%, meanwhile, the compressive strength of it is -447 MPa and the elongation is -12%. The indexing results of diffraction patterns and Kikkuchi patterns illustrated ORs between Mg₂Si and Mg matrix.In the last part, I invested the ORs of Mg₂Sn and Mg₁₇Al₁₂. The experiment results confirm the previous theory hypothesis by others.Mg₂Sn formed at around 450? before the formation of Mg₁₇Al₁₂ ?at around 500??.Proved that there exist two habit planes between Mg₂Si and Mg₁₇Al₁₂, ???Mg17/Al12 1.01°from ???Mg₂si,???Mg₂si// ?010?Mg₁₇Al₁₂ and ?044?Mg₁₇Al₁₂0.48° from ?202?Mg₂si.And make sure Mg₂Sn can be the nucleus of the second phase Mg₁₇Al₁₂.