Grain Refinement Of Mg-Al Based Alloys By Al-B(Mn)-C Master Alloy And Y

Mg-Al based alloys, which can meet the demands of lightweight and low energy consumption, are receiving increased attention in automobile industry; however, their low strength, plastic deformation capability and creep resistance restrict their application seriously. Grain refinement is an important method for Mg-Al based alloys to improve strength and plastic deformation capability simultaneously, however, there still has no a reliable, eco-friendly and commercial grain refiner up to the present. Addition of alloying elements can improve the creep resistance of Mg-Al based alloys at elevated temperature. Therefore, it is vital to develop an effective and practical grain refiner and study the effect of alloying elements on the microstructures of Mg-Al based alloys.High scope video microscope (HSVM), X-ray diffractometer (XRD), differential scanning calorimeter (DSC), electron probe microanalyzer (EPMA) and field emission scanning electron microscope (FESEM) were employed to analyze the phase identification, microstructure and grain refining efficiency of C-containing master alloy grain refiners which were fabricated by melt reaction and powder metallurgy, and the possible grain refining mechanisms were also studied; the influence of Y on the grain size and microstructure, especially the characterization of the new-formed intermetallic particles in Mg-Al alloys, were studied in detail. The main research results in the present study are as follows:(1) Fabrication and grain refining efficiency of C-containing master alloy grain refiners for Mg-Al based alloysAl-B-C mastert alloy has been fabricated by melt reaction method, in which A13BC particles with the sizes from 2μm to 8μm distribute uniformly in Al matrix. The formation mechanism of Al3BC particle is that most of C firstly reacts with Al melt to form Al4C3 particles, and then the solute B reacts with Al4C3 to generate Al3BC phase. The instruction of B decreases the size and improves the segregation of Al4C3 particles in as-cast Al-C binary alloys. The Al-B-C master alloy has good grain refining efficiency on AZ63 alloys, with addition of 2wt.% Al-B-C master alloy at 760℃, the grain size of AZ63 can be reduced from 710μm to 70μm. It is calculated out that there exist good crystallographic match relationships between a-Mg and Al3BC, so the Al3BC particles can act as effective nucleating substrates of a-Mg which leads to the grain refinement of AZ63 alloy.Al-Mn-C master alloy has also been fabricated by melt reaction method. Due to that the C exists in the form of solid-solution atoms and carbides in Mn-C, Al4C3 particles in Al-Mn-C master alloy have evident differences on size. Mn-C alloy in which Mn is the carrier of C solves the problem of introduction of C into Al melt. The master alloy can refine AZ63 alloy efficiently, after adding 1wt.%Al-Mn-C at 760℃, the grain size is reduced from 690μm to 110μm.In Mn-10C powders after 10h high energy ball milling, C absolutely solve into Mn particles forming Mn(C) solid solution. The Mn-10C powders can refine Mg-3Al alloy effectively; adding 0.5wt.%Mn-10C powders into Mg-3Al melt reduce the grain size from 480μm to 180μm. Al-Mn-C master alloy is also fabricated by powder metallurgy, Al6Mn and Al4C3 particles are generated from the reaction between Mn(C) solid solution and Al powders during the sintering process. The master alloy shows preferable grain refining efficiency on AZ63 alloy; addition of 1wt.% Al-Mn-C master alloy into AZ63 melt at 750℃can refine the grain size to one sixths of the original size.(2) Effect of Y on the grain size and microstructure of Mg-Al alloyThe introduction of Y can influence the grain size and microstructure of Mg-6Al alloy evidently. With increasing Y contents, the grain sizes of Mg-6Al alloys increase remarkably from 120μm to 980μm first with addition of 0.2wt.%Y due to that the surface active nature of Y restrains the growth restriction effect of Al; however, for Mg-6Al-0.8Y and Mg-6Al-1.6Y alloys, the grain sizes begin to decrease from 980μm to 400μm and 180μm, respectively, due to the in situ formation of Al2Y particles which can be effective nucleating substrates of primaryα-Mg.With increasing Y contents, the number and average sizes of Al2Y particles increase, and these Al2Y particles mainly distribute in a-Mg matrix and grain boundaries, which contributes to the high temperature creep resistance for Mg-Al based alloys. Three-dimensional morphologies of Al2Y particles possess regular polyhedron morphologies, especially octahedron, and it is believed that the Al2Y particles with octahedron morphologies can be nucleating substrates of primary a-Mg more easily than that with other morphologies.