The Effect Of Rare Earth Oxide Nd₂O₃on The Microstructure And Properties Of Hypoeutectic Al-Si-Cu Alloy

Hypoeutectic Al-Si-Cu alloys are being widely used in the aerospace and automobileindustry due to their fine characteristics such as low density, good castability and highspecific strength. But the the coarse plate Si phase of the alloys will cause serious stressconcentration inside the matrix and thus have some negative effect on the mechanicalproperties. So, it is meaningful to change the morphology of the eutectic Si phase to improvethe mechanical propoties of hypoeutectic Al-Si-Cu alloys.Nowadays, it is common to fine the coarse plate Si phase by modification method, Srand Na are the most widely used modification agent. But there are several defects remainingto be solved about these agents, for example, high cost and short modification time etc. andit is necessary to go on developing new kind of modification agent. Rare earth to modifyeutectic Si are now under research and the research shows that rare earth can also modify thealloys well and by testing different kind of rare earth oxide, Nd₂O₃has been found to be aqualified modification agent.By analyzing the as-cast Al-Si-Cu alloy with different content of Nd₂O₃, it has beenfound that accompanying the increasing content of Nd₂O₃, the modification effect and themechanical properties are also been improved. When the content reaches to0.4wt.%, thecoase plate Si phase transformes into fine fiber morphology and the modification effect ishomogeneously distributed and as a result, the mechanical properties reach the best. Whenthe content is over0.4wt.%, the modification effect is unobvious and so is the mechanicalproperties.The most popular theory about the modification mechanism of the eutectic Si is theimpurity-induced twinning（IIT） theory which believes that the atoms of the modificationagent will absorb onto the steps on the Si surface changing the growth pattern of Si phaseand thus causing huge amounts of twin crystals and the Si phase begins to grow by the twinplane re-entrant edge（TPRE） mechanism. This article tries to explain the mechanism of theeutectic Si modification to some extent depending on the experimental phenomenon. Todetect the distribution of Nd₂O₃, XRD has been carried out and a left migration of the Si（111）crystal facet has been found when the Nd₂O₃content is0.4wt.%and there is no migration ofother phase in the alloy. The atoms of the Nd are believed to absorb into the eutectic Si phaseand thus changes the way Si nucleates and grows. The DSC has also been carried out and a3℃decrease has been found at the arrest point of not only the α-Al phase but also theeutectic Al-Si phase of the0.4wt.%modified alloy and this indicates that Nd₂O₃changes thenucleation and growth pattern of both α-Al phase and the eutectic Al-Si phase. Steps arefound on the0.4wt.%modified Si phase. The conclusions drawn through a series of experiment are as follows:1) Rare earth oxide Nd₂O₃can modifiy hypoeutectic Al-Si-Cu alloys well and whenthe content reaches0.4wt.%, the coarse plate Si phase turns into fine fibermorphology and the fiber Si phase is homogeneously distributed and when thecontent is over0.4wt.%, the alloys are beyond modification with tiny plate-like Siphase.2) A3℃decrease has been found at the arrest point of not only the α-Al phase butalso the eutectic Al-Si phase of the0.4wt.%modified alloy and the increasedundercooling causes more independent eutectic nucleation and the eutectic Si phasetends to grow around the eutectic Al phase. Steps are found on the suface of the Siphase modified by0.2wt.%and0.4wt.%Nd₂O₃which indicates that the Si phasestill grows by step mechanism to some extent despite of the IIT theory. Moreindependent nucleation of Si phase with the trend of surrounding eutectic Al phase,which will blend together while growing and more twin crystals make theanisotropic plate Si phase grow into fine fiber morphology.3) The modification effect of Nd₂O₃improves the tensile properties of thehypoeutectic Al-Si-Cu alloy at room temperature and high temperature to someextent.