The Evolution Of Al₃BC And Its Strengthening On Mg-Al Based Alloys

The use of Magnesium-Aluminum based alloys in automotive, aerospace industries is receiving increased attention due to their low density, high mechanical property and low cost. However, their application is still limited due to their bad plastic deformation capacity and relatively low absolute intensity. Grain refining is one of the most important methods to improve both of the intensity and plastic property according Hall-Petch equation. Thus, many grain refining methods have been applied. Among these methods, adding master alloy is known to be an important way for its easy to operating and stable effect. Based on the previous research, Applying Al-B-C master alloy is considered as a promising grain refining method for Mg-Al based alloy. As the major second phase in Al-B-C master alloy, studying on the evolution of Al3BC in aluminum alloy and magnesium melt has significant influence in optimizing the microstructure of Al-B-C master alloy; improving grain refining efficiency and understanding the grain refining mechanism.Field emission scanning electron microscope (FESEM), electron probe micro-analyzer (EPMA), X-ray diffractometer (XRD) and high scope video microscope (HSVM) were employed to analyze the phase identification, microstructure and grain refining efficiency of Al-B-C master alloy. And the evolution of Al3BC in Magnesium melt was studied. The main research results in the resent study are as follows:(1) Fabrication of Al-B-C master alloy and formation mechanism of Al3BC.Al-B-C master alloys were fabricated by liquid-solid reaction method, it is found B and C contents have significant influence on the microstructure of Al-B-C master alloy. At lower contents, When B/C ratio approaching to1, only Al3BC and α-Al were found, when B/C ratio higher than1, Al4C3emerged in the master alloys. Increasing the content of B and C contents, B and C cannot react compeletely. Al3BC, AlB2and Al4C3was found in Al-5B-5C master alloy frabricated by ball milling, and a-Al, AlB12、AIB2 and Carbon was found in the Al-5B-5C master alloy frabricated without ball milling,The process of ball milling increased the wettability of the graphite with aluminum matrix by changing its morphology and thus improved the formation of Al4C3. It is beneficial to the formation of Al3BC. And ball milling process has no influence on the boron-contained plastid.Ball milling process has improved the formation and refined the morphology of Al3BC in the Al-B-C master alloy.(2) Evolution behaviors of Al3BC in the magnesium melt.Al3BC can evolve into fibrous Al4C3phase in Mg-Al melt, A13BC transformed into fibrous Al4C3phase in Mg melt without Mn, and B element can dissolve in the melt with low level. However, when Mn is added in the melt, the A13BC will transformed into Al8Mn5Bx and Al4C3phase completley by doping behavior of B to Al-Mn phase.(3) Strengthening of Al3BC on magnesium alloyGrain refinement experiment shows that the Al-2.5B-2.5C master alloy exhibits excellent grain refinement efficiency on AZ91(Mg-9Al-1Zn). At720℃, when adding0.5%of the master alloy, the average grain size was reduced sharply from704μm to80μm. It is supposed that the in-situ formation of Al4C3transformed from Al3BC is the direct heterogeneous nucleant substrate for primary a-Mg. Increasing the holding time is beneficial to the evolution of Al3BC to Al4C3. With addition of1%Al-2.5B-2.5C master alloy, prolonging the holding time from10min to30min, the average grain size of AZ91decreased from9μm to83μm.Al3BC particles were added into the AZ91melts by Al-B-C master alloy and the particulate reinforced AZ91composites were fabricated. Comparing with the AZ91alloy (119.2MPa), the tensile strength of the composites were improved to173.7MPa. The wear loss of the composites decreased from34mg to29mg in50mins with the loading of20N.