| As one kind of important light metallic structural materials, Mg-Al serial alloys have beenwidely paid attention by many researchers. However, wider applications were restricted due totheir poor mechanical properties. It is well known that grain refinement is an effective route toimprove the mechanical properties of this type alloys. Carbon inoculation was widely regardedas one of prosperstive grain refining methods to refine the grains of Mg-Al serial alloys. Thereexisted an issue that the refining efficiency of this refining technique is susceptible to Fe (Mn)elements in the melt. It has been discovered that Fe(Mn) could poison the Al-C-rich nuclei andaccelerate the fading effect of carbon inoculation. In the previous studies performed by thepresent research group, the grain refinement of carbon inoculation combining with Ca additionwas better than that of carbon inoculation. However, the effect of Fe(Mn) on the grain refiningefficiency of this refining route is not studied yet. Specially, the effect of operating sequenceof C-Ca(Sr) combining inoculation and Fe and/or Mn addition on the grain refining efficiencyon the potency of carbonaceous nuclei are still unclear. In the present study, the effect ofoperating sequence of carbon inoculation combining with Ca(Sr) and Fe and/or Mn additionon the grain size of Mg-3Al alloy was studied. The structural characteristics of Al-C-rich nucleiwere observed by EPMA. The formation and evolution mechanisms of Al-C-rich nuclei werealso discussed based on the interfacial adsorbing thermodynamics. The main research resultswere obtained in the present work as follows:1. The operating sequence of C-Ca(Sr) combining inoculation and Fe and/or Mn additionhad no obvious effect on the grain refinement of the Mg-3Al alloy. The morphologies ofcarbonaceous nuclei were mainly assumed as duplex-phased structure of Al-C-rich layercoated on Al-Fe(Mn)-rich particles. Under the condition of Ca(Sr) addition, Fe(Mn) did notpresent inhibition effect on grain refinement by carbon inoculation. In other words, Ca(Sr)could avoid the poison effect of Fe(Mn) on the carbonaceous nuclei. This effect of Ca(Sr) werenamed as poison-free effect in the present study. The key to the poison-free effect was theinteraction between Fe(Mn) and Ca(Sr) could promote the formation of duplex-phasedstructure particles. These duplex-phased particles could act as heterogeneous nuclei andconsequently keep the grain refinement from being poisoned. 2. Wether the existence of Fe(Mn) in the Mg-3Al melt or not, prolongation of holding timehas no obvious effect on the grain size of Mg-3Al alloy treated by C-Ca combining inoculation.Under the C-Ca(Sr) combining inoculation, Fe(Mn) showed no obvious acceleration effect onthe fading.3. The thermodynamic trends of adsorption of Al-C phase layer on the Al-Fe rich particlesand cluster of Al-C-rich particles were evaluated based on the thermodynamic theory.Compared with the cluster of Al-C-rich particles, adsorption contributes lower Gibbs freeenergy of the system, which could promote the formation of duplex phase structure particles.The finer the Al-C particles and the bigger the Al-Fe(Mn) rich particle are, the stronger trendof adsorption has.4. Ca(Sr) are surface active elements and they have spontaneous segregation towardsinterface, which could consequently lower the interfacial energy between two phases.Thermodynamic calculation showed the lower interfacial energy could promote the formationof duplex phase structure. The poison free effect of Ca(Sr) could be reasonably explained bythe theoretical evaluation results.5. Settlement model of particles showed that the Al-Fe particles had a faster settlingvelocity, while the Al-C particles and duplex phase particles could remain suspending in Mg-Al melt in a long holding time. The Al-C-rich particles and duplex-phased particles could actas grain refiner for Mg-3Al alloy. Consequently, the grain refining efficiency could beremained for a long time.The results obtained in the present study have important reference values on widening theapplications of carbon inoculation refining technology applying in the commercial Mg alloyssince all the commercial Mg-Al alloy contain Fe(Mn) elements. And also, the results canprovide important theoretical guidance to the development of higher efficient grain refiner forMg-Al alloy. |
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