| Mg-Al based alloys, as lightweight structural materials, are receiving increased attention on replacing other metals in automobile industry, but their relatively low strength and plastic property restrict their development. Grain refinement of cast Mg-Al based alloys is an important method to improve their strength and plastic property simultaneously, however, to date, there is no effective grain refiner for industrial application. What is more, understanding the grain refining mechanism of carbon inoculation, addition of alloying elements and addition of FeCl3 clearly is of great help to controlling the grain sizes of Mg-Al based alloys, but the mechanism is not clear. Therefore, it is vital to fabricate an effective grain refiner and explore the grain refining mechanism of various methods of grain refinement.High scope video microscope (HSVM), X-ray diffraction (XRD), differential scanning calorimeter (DSC), field emission scanning electron microscope (FESEM), electron probe microanalyzer (EPMA) and transmission electron microscope (TEM) were employed for the analysis of phase identification, microstructure and grain refining efficiency of C-containing grain refiners, fabricated by melt reaction, powder metallurgy and high-energy ball milling, and for the clarification of grain refining mechanism of carbon inoculation as well as the influence of Mn, Ce and Fe on the grain size and Mn-rich phase transformation in Mg-Al based alloys. The main research results in the present study are as follows:(1) Fabrication and grain refining efficiency of C-containing grain refiners for Mg-Al based alloysAl4C3-containing Al-Ti-C master alloys (Al-0.6Ti-1C and Al-1Ti-1C) have been fabricated through introducing Ti element into Al-C melt by melt reaction, in which Al4C3 particles are surrounded by TiC particles. The formation mechanism of particulate phases are most of C firstly reacts with Al melt and forms Al4C3 particles, and then solute Ti reacts with Al4C3 particles and forms TiC particles when Ti is introduced, leading to the decreased sizes and improved distribution of Al4C3 particles. Al-1Ti-1C master alloy with 4-10μm Al4C3 particles can refine AZ31 efficiently, while Al-0.6Ti-1C master alloy with 15-30μm Al4C3 particles shows less efficient grain refinement. Hence, appropriate addition of Ti is believed to increase the grain refining efficiency of Al4C3-containing Al-Ti-C master alloys.Al-C master alloys (Al-1C and Al-2.5C) have been fabricated by powder metallurgy, in which the mixture of Al4C3 and Al(C) solid solution is located at the boundaries of aluminum particles, showing net-like distribution. Al4C3 is formed by reaction between part of Al(C) solid solution and aluminum during fabrication, and ball milling and briquetting ensure the formation of Al4C3 phase. Al-1C master alloy in which Al4C3 particle has a size of 2-3μm shows preferable grain refining efficiency on AZ31. After addition of 2wt.% Al-1C master alloy, the grain size of AZ31 can be reduced from 580μm to 140μm.High-energy ball milled Al-25C powder can bring remarkable grain refinement to Mg-3Al, AZ31 (Mg-3Al-1Zn-0.45Mn) and AZ63 (Mg-6Al-3Zn-0.35Mn). With addition of 0.4wt.% Al-25C, the grain sizes of Mg-3A1, AZ31 and AZ63 can be refined from 400μm to 120μm, from 850μm to 180μm and from 650μm to 170μm, respectively. Al-25C can react and form Al4C3 at 710.4℃; therefore, when plunged into the Mg alloy melt, it can form Al4C3 phase. In C-treated Mg-3A1, fine Al4C3 particles act as the heterogeneous nucleating substrates for primaryα-Mg.(2) Mn-rich phase transformation and its influence onα-Mg grain size of Mg-Al based alloysMn alloying using Mg-0.72Mn alloy can not evidently refine the grain size of Mg-3Al alloy. Al0.89Mn1.11 compound is the dominant Mn-rich phase in Mg-3Al-0.3Mn, Mg-3Al-0.4Mn and Mg-3Al-0.5Mn alloys, and distributes in primaryα-Mg matrix and interdendritic regions. The number of Al0.89Mn1.11 particles increases gradually as Mn level increases while the grain sizes fluctuate at 390μm in Mg-3Al, Mg-3Al-0.3Mn, Mg-3Al-0.4Mn and Mg-3Al-0.5Mn, indicating Al0.89Mn1.11 is not a potent heterogeneous nucleating substrate for primaryα-Mg.Mn-rich phases play an important role in the formation of nucleating substrates in C-treated Mg-Al-Zn-Mn alloys, and the grain refinement is due to duplex nucleation, that is, Al4C3-coated Al0.89Mn1.11 and Al4C3-coated Al8Mn5 act as the duplex nucleating substrates in C-treated AZ31 and AZ63, respectively, and Al4C3, the interfacial phase between primaryα-Mg and duplex nucleating substrates, is the direct potent heterogeneous nucleating substrate for primary a-Mg.Ce shows relatively good grain refining efficiency on AZ31. Specifically, with addition of 0.4wt.% Ce, the grain size of AZ31 decreases from approximately 850μm to 450μm. The reason for grain refinement effect of Ce is that during solidification of Ce-alloyed AZ31, Ce is impeded into the diffusion layer ahead of the advancing solid/liquid interface leading to constitutional undercooling, thus restricting the grain growth by reducing the diffusion rate of solute. Combined addition of 0.4wt.% Ce and C does not lead to further grain refinement, and instead leads to the lost of part of grain refinement effect of carbon inoculation. It is due to that Ce addition hinders the formation, increases the size and aggravates the aggregation of Al0.89Mn1.11, thus hindering the formation of effective Al4C3-coated Al0.89Mn1.11 duplex nucleating particles, which are potent heterogeneous nucleating substrates for primaryα-Mg.Addition of FeCl3 into AZ31 alloy melt can bring slight grain coarsening to AZ31, and promote the transformation of Mn-rich phases from Al0.89Mn1.11 to Al8(Mn,Fe)5, as well as the precipitation of Al8(Mn,Fe)5 prior to that of primaryα-Mg. Al8(Mn,Fe)5 is not potent heterogeneous nucleating substrate for primaryα-Mg, and its precipitation in advance reduces the Al content in the melt and constitutional undercooling during the solidification of the alloy leading to the weakened grain growth restriction, therefore, the grain size of AZ31 is increased.
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