Research On Effect Mechanism Of Fe Or Mn On The Grain Refinement Of Mg-3Al Alloy Inoculated By Carbon

Mg-Al base alloys are most widely used in civil field as the promising structural lightmaterials. Grain refinement is an important route to improve the properties of Mg-Al basealloys among which carbon inoculation is considered to be the most effective and promisingway to refine the grains of Mg-Al base alloys.However, the grain refining effect caused bycarbon inoculation are easily to be inhibited by Fe（Mn）.In addition, the relationships betweenpoisoning mechanism of carbonaceous nuclei, the process condition and the inhibition effectare still not clear. The current research results even conflict with each other. In this paper, theeffect of operating sequence of carbon inoculation and Fe and/or Mn addition on grainrefining efficiency was firstly studied and then the effect of holding time on the grain size ofMg-Al base alloys with or without Fe（Mn） inoculated by carbon was explored.Crystallographic calculation was also used to analyze the orientation relationships betweencarbonaceous nuclei and possible phases in order to reveal the possible effect mecha nism ofFe or Mn on the grain refinement of Mg-3Al alloy inoculated by carbon. The main researchresults in the present work are as follows:The effect of Fe and/or Mn on the grain refinement of Mg-3Al alloy inocluated bycarbon was closely associated with the operating sequence of carbon inoculation and Feand/or Mn addition. Fe and/or Mn had no obvious effect on the grain refinement under thecondition that Fe and/or Mn pre-existed in the Mg-3Al melt before carbon inoculation.However, Fe and/or Mn played an inhibiting role under the condition that the Mg-3Al melthad been inoculated by carbon before Fe and/or Mn addition. Regardless of the operatingsequence, There existed four kinds of particles in the Mg-3Al alloy treated by the combinationof carbon inoculation and Fe and/or Mn addition: Al₄C₃particles, the particles with duplexphase structure of Al₄C₃coated on Al-（C）-Fe（Mn）, the particles with duplex phase structure ofAl-（C）-Fe（Mn） coated on Al₄C₃and Al-C-Fe（Mn）-rich metallic compounds. The relativenumber of the four kinds of particles in Mg-3Al melt was the main factor to determine thegrain efficiency. The former two kinds of particles which can act as the potent heterogeneousnucleating substrate for primary-Mg mainly existed in the Mg-3Al alloy firstly treated by Feand/or Mn addition and then carbon inoculation. However, the latter two kinds of particlesmainly existed in the Mg-3Al alloy firstly treated by carbon inoculation and then Fe and/orMn addition. the latter two kinds of particles were not the potent heterogeneous nucleatingsubstrate for primary-Mg so that the nucleating substrate was poisoned and resulted in graincoarsening. With holding time increasing, there was no obvious change in grain size of Mg-3Al alloywithout Fe（Mn） inoculated by carbon. However, the grain size of Mg-3Al alloy containingFe（Mn） inoculated by carbon began to coarsen when the holding time exceeded30min,appearing obvious fading. The tiny Al₄C₃particles in Mg-3Al alloy could grow up or clumptogether to form a mass with holding time increasing, while no evident change was found forthe size and distribution density of the Al₄C₃particles with relatively larger size. When theholding time was within20min, there mainly existed Al₄C₃particles and the particles withduplex phase structure of Al₄C₃coated on Al-（C）-Fe（Mn）. However, Al₄C₃particles and theparticles with duplex phase structure of Al₄C₃coated on Al-（C）-Fe（Mn） would graduallyaggregate and settle so that the proportion of potent heterogeneous nucleating particles wentdown. What’s more, these two kinds of particles would gradually transform intoAl-C-Fe（Mn）-rich metallic compounds with the further extension of holding time andeventually result in obvious fading.The most possible metallic compounds that could be present in Al-C-Fe（Mn） ternarysystem are AlCFe₃（AlCMn₃） phase. The edge-to-edge matching model was used to predictedthe possible orientation relationships（OR） between-Mg, Al₄C₃and AlCFe₃（AlCMn₃） inorder to provide some theoretical support on the poisoning effect caused by Fe（Mn） and theformation of the particles with duplex phase structure of Al₄C₃coated on Al-（C）-Fe（Mn）. Thecalculation results showed that there were no ORs between AlCFe₃（AlCMn₃） and Mg matrix.AlCFe₃（AlCMn₃） was not the potent heterogeneous nucleating substrate for primary-Mg.But there were some ORs between AlCFe₃（AlCMn₃） and Al₄C₃and this played an active rolein the formation of the particles with duplex phase structure of Al₄C₃coated on AlCFe₃（AlCMn₃） which can act as the potent heterogeneous nucleating substrate for primary-Mg.So, to study the effect of Fe（Mn） on the grain refinement of Mg-3Al alloy inoculated bycarbon, the key lies in whether Fe（Mn） favor the formation of the particles with duplex phasestructure of Al₄C₃coated on Al-（C）-Fe（Mn） and if not, the carbonaceous nuclei was morelikely poisoned.