Effect Of The Al₂X?X=Ca,Y,Ce? Compounds On Grain Refinement, Microstructure And Mechanical Properties Of Mg Alloys

As the lightest structure metal, the applications of magnesium(Mg) alloys have attracted in many fields widely due to its abundant resources. However, the poor mechanical properties and formability, limited its development. Grain refinement has been considered as one of the most effective methods which can increase both strength and ductility. Among the current available grain refinement approaches for cast Mg alloys, inoculation is regarded as an effective and practical method. So far, zirconium inoculation is the most efficient grain refiner for cast Mg free Al. But, there are some issues in industry applications, for example, high price, high alloying temperature and low utilization rate duo to setting. Therefore, cheaper and effective grain refiners are still under investigation. Recently, it has been reported that Al2 Ca particles are effective inoculants for heterogeneous nucleation of thin strips of Mg produced by twin-roll-cast. Besides, it was found that Al additions in the Mg-10 Y alloy led to significant grain refinement, and the gram refinement mechanism was attributed to the in situ formation of Al2 Y particles which were potent nucleants.In present work, the effect of the Al2 Ca and Al2 Y compounds on grain refinement mechanism and mechanical properties of as-cast Mg alloys have been investigated by in situ synthesis and addition. In addition, the addition of A1 led to substantial grain refinement of Mg-Ce alloys, indicating that Al2 Ce particles were potent heterogeneous nucleants for the Mg matrix and led to grain refinement.Finally, it is summarized the physical and chemical conditions of Al2 X as the heterogeneous nucleation particles for magnesium alloys need to meet. Details are as follows:As-cast Mg-Al-Ca alloys containing the single Al2 Ca second phase has been successfully in-situ synthesized by controlled the Ca/Al atom ratio about 0.7. The optimal content of Al2 Ca phase for microstructure refinement is 4 wt.%. This are mainly attributed to the effect of Ca and Al solutes. After hot extrusion, the Al2 Ca is sharply broken into granule. These particles were heterogeneous nucleants for the Mg matrix during the process of dynamic crystallization. The extruded Mg-4Al2 Ca alloy exhibits superior mechanical properties with the UTS of 324 MPa, the YS of 275 MPa and the elongation of 10.2 %. When the addition of the Al2 Ca is 1.1 wt.%, as-cast AZ31 magnesium alloy can be efficiently refined. The grain refinement mechanism is attributed that Al2 Ca can act as the efficiently heterogeneous nucleation agent of Mg matrix.The the Mg-10Al2 Y master alloy containing the Al2 Y particles was prepared. The addition of Al2 Y master alloy led to substantial grain refinement of Mg-3Y alloy and the optimum adding amount of Al2 Y is 2 wt.%. In addition, grain refining performance would not failure within 5~60 min holding time. Meanwile, the setting of Al2 Y particles in Mg alloy melt is not obvious at long setting time. The tensile properties of as-cast Mg-3Y alloy can be improved significantly by addition 2 wt.% Al2 Y. It is comparable to that of the Zr refined counterparts.Compared with the effect of Al addition on grain refinement of Mg-3Y alloy, the Mg-10Al2 Y master alloy has evident advantages. In the Al refined Mg-Y alloys, Al2 Y nucleant particles were in situ formed only when the Y content is over 6 wt.%. However, Al2 Y master alloy can significantly refine the Mg alloys with different content of Y. And with the increasing of Y content, the grain refining efficiency was aslo increased. The grain refining efficieny of Mg-7Y alloy by addition of 2 wt.% Al2 Y reached to 3%.It examines entirely the grain refining of Al2 Y master alloy in Mg alloys containing other RE elements. The results shown that excellent grain refinement effect is achieved in Mg-3RE-1Y(RE=Ce,La,Nd,Gd) alloys by addition 2 wt.% Al2 Y. The grain refining efficiency was up to more than 88%. The Al2 Y particles were reproducibly observed at the centres of many refined grains. As the content of Y increases, the stability of Al2 Y particles increases, result to the grain refining efficiency of Al2 Y master alloy also increases.A novel particle of heterogeneous nucleation using grain refinement of Mg alloys is obtained. The addition of Al led to substantial grain refinement of Mg-6Ce and Mg-3Ce alloys. Acording to the microstructure analysis, the Al2 Ce particles with potent nucleant capabilities were in situ formed before the solidification of the ?-Mg. These particles were reproducibly observed at grain centers and have a good orientation relationship with Mg matrix.The addition of Al led to significantly improved of tensile property of Mg-6Ce alloy. The reasons were fine grain size and secondary phase of fiber Al11Ce3 strengthening. The thermal stability of the grains refined by Al2 Ce or Zr are all much higher up to a temperature of 500? for 12 h. The mechanisms of the superior thermal stability of the refined grains due to Al and Zr addition in the Mg-3Ce alloy are the effect of the pinning of Al11Ce3 and Mg12 Ce phase distribution along grain boundaries. However, the compression properties at room and high temperature of Al refined Mg-3Ce alloy is superior to the alloy with Zr addition.Finally, we consider that if the Al2 X particles as a potent nucleant in Mg alloys, it is need to meet three conditions at the same time: Firstly, a good crystal orientation between Al2 X with Mg matrix is necessary. Secondly, the Al2 X particles exist in Mg alloys melt steadily. More important, the Al2 X particles are dispersion distribution within the grain, the number density reaches a certain value and the size distribution range is relatively narrow.