Study On The Grain Refinement Behavior And Its Mechanism Of Mg Alloys By Melt Oxidation Inoculation

The oxide inclusion can be used to effectively enhance the properties of casting,and it has been successfully utilized to realize the grain refinement in the field of iron and steel.Since oxide inclusion is easily formed in Mg melt due to the strong oxygen affinity of Mg,it is more necessary to study the effect of oxide inclusion on the microstructure of Mg and its alloys.Howerver,such work is yet to be deeply studied.In the present study,we systematically investigated the grain refinement behavior of Mg and its alloys inoculated by oxide inclusions.The influence of solutes on the grain refinement was revealed and the mechanism was uncovered.The main conclusions were obtained as follows.Oxides were prepared by combusting the chips of pure Mg,Mg-12.7Al and AZ31 alloys in the ambient atmosphere.The main component of the oxide was Mg O.Small amount of Mg Al₂O₄ was produced from Mg chips containing Al element.The resultant oxides were added in the Mg melt to realize grain refinement.It was found that the pure Mg was not refined by the addition of oxides.However,significant grain refinement was achieved in both the Mg-3Al and AZ31 alloys.Some Mg O particles were found in the central region ofα-Mg grains and acted as the effective nucleating sites.Transmission electron microscope（TEM）was used to investigate the interfacial structure ofα-Mg/Mg O.There was a transition region existed between the crystalline structure of Mg O and that of Mg.Mg O particles behaved as the effective heterogeneous nucleating sites forα-Mg grains and induced the grain refinement.The first-principles calculation showed that the adsorption energy of Mg on the Mg O surface was increased when Al was adsorbed on the Mg O surface,leading to the accelerated heterogeneous nucleation ofα-Mg grain.However,the adsorption energy of Mg was reduced while either Zn or Mn was adsorbed on the Mg O surface.As a result,the refining efficiency of Mg O nuclei was degraded.In addition,the grain refinement of Mg-3Al and AZ31 alloys was achieved by the inoculation of either commercial Mg O or commercial Mg Al₂O₄ powder.Both Mg O and Mg Al₂O₄ were confirmed as the effective nucleation phase forα-Mg grains.An in-situ oxidation process（in-situ OP）was successfully developed by blowing a mixed gas（O₂+Ar）into the Mg melt.In-situ OP was highly effective in the grain refinement for the AZ31 alloy melt at different temperatures.The average grain size of AZ31 alloy was significantly reduced from 1762±213mm to 442±35mm.Both the ultimate tensile strength and elongation were greatly improved.Mg O and Mg Al₂O₄oxide inclusions were generated in the AZ31 melt by in-situ OP.According to the E2EM model,the minimum atomic misfit was7.63%between Mg O and Mg,and was 2.34%between Mg Al₂O₄ and Mg.Both Mg O and Mg Al₂O₄ were suggested to possess a suitable crystalline structure for the potential nucleating sites ofα-Mg grain.The refining efficiency of in-situ OP was closely related to the type of solutes in Mg melt.Although both Mg-Al and Mg-Zn alloys were effectively refined,no grain refining was observed in the Mg-Ca alloy.After the treatment of in-situ OP,Mg O was produced in both Mg-Al and Mg-Zn alloys.The heterogeneous nucleation was consequently facilitated.When the Al content in Mg-Al alloy melt reached 3 wt.%or beyond,Mg Al₂O₄ spinel oxide inclusions were generated,leading to more efficient grain refining.As for Mg-Ca alloy,since the electronegativity of Ca is lower than that of Mg,it was easier to form Ca O instead of Mg O in the melt inoculated by in-situ OP.Mg O inclusion was absent and so was the the grain refining.No grain refining was observed if the solute in the melt is chemically more active than Mg.Si and Fe are common impurity and widely exist in Mg-Al alloys.It was interesting to find out that Mg₂Si can act as the heterogeneous nucleating sites for Mg₁₇Al₁₂ phase.Minor Si in AZ31 alloy was beneficial to the formation of fine and well-dispersed Mg₂Si particles,which generally existed in the central region of Mg₁₇Al₁₂ phase.Fe also induced the grain refinement of Mg-Al alloys.Based on the JMat Pro software calculation and TEM observation,a new mechanism was uncoevered for the grain refinement induced by Fe in the Mg-Al based alloy.Al₂Fe phase precipitates beforeα-Mg phase and provides the heterogeneous nucleation sites forα-Mg grains.An orientation relationship of<01（?）1>_Mg{（?）011}_Mg||<011>_Al2Fe{11（?）}_Al2Feexisted between Al₂Fe andα-Mg.The first-principles calculation suggested that when Fe was adsorbed on the O terminal surface of Mg O,the adsorption energy of subsequent Mg was reduced.Consequently,the refining efficiency of in-situ OP was degraded by Fe impurity.In this work,the oxidation inoculation refinement behavior of Mg alloy was systematically studied.The grain refinement of Mg alloy was successfully achieved by utilizing oxide inclusions as heterogeneous nuclei.The refining efficiency of oxidation inoculation was highly dependent on the solutes in Mg melt.It was found that the Mg₂Si phase produced from Si impurity acted as the heterogeneous nucleation site of Mg₁₇Al₁₂.A new grain refinement mechanism was unveiled for Al₂Fe in the heterogeneous nucleation ofα-Mg.This study presented a novel approach for the utilization of impurities and inclusion phases,which is important to guide the design and development of Mg alloys.