Study On The Non-classical Nucleation Phenomenon In Hypereutectic Al-15/20Si Alloys

Al-Si alloys can find wide applications in industry,among which hypereutectic alloys show promising potentials for positons in automobile engines.However,the mechanical properties are dependent on the size and morphology of primary silicon particles,optimization of which is thus of significant importance.Controlling of nucleation offers great chance to refine the microstructure and optimize the properties of castings.For a long time,the research on nucleation in metal melts is based on the classical nucleation theory.However,extensive evidence proves the existence and dynamic feasibility of clusters assisted non-classical nucleation passway,which also occurs in metal melts.In this dissertation,Al-10Si-2Fe,Ni-32.5Si and Al-9.2Cr master alloys were employed to trigger the non-classical nucleation of primary silicon in Al-15/20Si alloys.Refinement of primary silicon has been achieved by controlling the nucleation processing.The above master alloys have some features in common.Refinement was not allowed in alloys melted by high purity metals(high purity alloys)while in alloys melted by commercial purity metals(commercial purity alloys)and high purity alloys assisted with some nucleatants,refinement was witnessed.The maim results can be described as follows:(1)AI-3P/Al-5Ti-B were added into high purity Al-15Si alloys,respectively,followed by Al-10Si-2Fe master alloys in both melts.It was found that in high purity Al-15Si alloy,Al-10Si-2Fe master alloy promotes the nucleation of prnmary silicon with the addition of AlP.In contrast,TiB2 addition has no such effect.In commercial purity AI-20Si alloy,Al-10Si-2Fe master alloy promotes the nucleation of primary silicon with the addition of TiB2.There are trace P composition in commercial purity Al-20Si alloy.In such a case,introduction of TiB2 particles leads to the formation of TiB2·AlP coupled particles,making Al-10Si-2Fe master alloys effective again.Therefore,the effects of Al-10Si-2Fe master alloy which works through(AlFeSi)clusters rely on other particles or clusters.P addtion affects the induction of precursors by(AlFeSi)clusters.(2)To explore the common features of master alloys that can trigger clusters assisted nucleation,the structural features of Al-Fe-Si,Ni-Si and Al-Cr melts were investigated with Ab initio molecular dynamics.All of these melts have some stable structure,such as the icosahedronal clusters in Al-Fe-Si system,and the pre-peak in structural factor curves of Ni-Si and Al-Cr systems.This can be criteria for selecting master alloys that can trigger non-classical nucleation.(3)The inoculative effects of Ni-32.5Si master alloy were compared in commercial purity and high purity Al-Si alloys.Ni-32.5Si master alloy can promote the nucleation of primary silicon in commercial purity Al-20Si alloys.Furthermore,some sharp steps on the surface of silicon particles are replaced by swells.In high purity alloys,silicon can not be refined by the master alloy,although the fraction of five-fold branched silicon particles decreases after inoculation.Nanoparticles with 2H and 9R silicon structure were found in silicon matrix within inoculated high purity alloys.The energy of these two structures is close that of diamond silicon.Based on the observation above,clusters assisted multi-step nucleation mechanism was proposed.Ni-32.5Si master alloy envolves into(NiSi)clusters in melts,which would induce nucleation precursors.This processing relies on impurities in melts while the transformation by themselves is slow.The structure of nucleation precursors is 2H,9R and diamond-cubic silicon structure,and aggregation of them develops the nuclei.In the growth stage,aggregation of precusors and silicon atoms makes silicon crystals.These nucleation precursors will transfer to final diamond structure while partial transformation leaves them embedded in the silicon matrix,in the formation of nanoparticles.The aggregation of these precursor clusters submerges the twin plane re-entrant edge,depressing its development.While small clusters aggregrate into large ones,they can leave on the surface of silicon as swells,replacing the sharp steps.(4)Both Al-9.2Cr and Al-17.6Cr alloys consist of Al45Cr7 phase,although in different morphology.Al-9.2Cr master alloy shows higher inoculation efficiency than Al-17.6Cr alloys in commercial Al-Si alloy due to more efficient clusters.AI-9.2Cr master alloy also exhibits different inoculation effects in commercial and high purity Al-15Si alloys,confirming the non-classical nucleation phenomenon.The observation of nanoparticles with 2H silicon structure in silicon matrix further validates the non-classical nucleation passway and the structure of nucleation precursors.