Study Of The Heterogeneous Nucleation And Growth Mechanisms For Si-Rich Phases In Several Si-Containing Alloys

Al-Si alloys are widely used for power-transmission parts such as the pistons in engines. In order to improve the performance, it is necessary to use modification treatment, and P modification is widely used in Al-Si alloys in which AlP particles act as the nuclei of primary Si. In piston production, P is mainly added in the form of red phosphorus, phosphate salt or Cu-P master alloys, but there are certain disadvantages for these additives such as unstable modification efficiency and environment pollution. Solving the modification problems is a meaningful work.The nucleation and growth mechanisms for rich-silicon phases in Cu-50Si, Ni-38Si, Mn-30Si and near eutectic Al-12Si alloys were systematically studied using optical micrograph (OM), electron-probe micro-analyzer (EPMA), transmission electron microscope (TEM), differential scanning calorimeter (DSC) and X-ray diffraction (XRD) etc. in this dissertation. By discussing the heterogeneous nucleation of several Si-rich phases, a new modifier Cu-P-Si master alloy has been prepared, and two new methods different from traditional P modification have been put forward in this dissertation. The major research efforts of the present study are as follows:(1) Refinement of the primary Si in Cu-50Si alloy and the preparation of Cu-P-Si master alloyThe primary phase in hypereutectic Cu-50Si alloy is primary Si, the size of which is over 1mm, and it is important to use refinement treatment. The addition of Al can refine the primary Si in Cu-50Si alloy by suppressant growth mechanism. Al and Cu form halos which surround primary Si and suppress the growth of primary Si. Since Al and P can form AlP particles which can act as the heterogeneous nuclei of primary Si in Cu-50Si alloy, the primary Si can be greatly refined from more than lmm to less than 50 urn with addition of Al and P. To combined the suppressant growth mechanism and heterogeneous nucleation, Al and P should be added with Al:P>1 (atomic ratio), and so that part of Al react with P forming AlP and the surplus form Al-rich halos suppressing the growth of primary Si. In this case, the primary Si can be refined to less than 30 urn in Cu-50Si alloy.Since primary Cu(PSi)₃ with 38.7wt.% P containing can be formed in Cu-Si alloy with addition of P, a new type of Cu-P-Si master alloy with P containing up to 30wt.% can be prepared, and it has advantages of lower density and higher P containing than Cu-P modifier. Moreover, addition of certain trace elements such as Ni and Bi can reduce the melting temperature of the Cu-P-Si master alloy by more than 60℃, so low melting point Cu-P-Si master alloy which has good modification efficiency on hypereutectic Al-Si alloys was invented in this work.(2) The heterogeneous nucleation of rich-Si phases in Ni-38Si and Mn-30Si alloysNi-38Si alloy is composed of NiSi and dendritic NiSi₂, and it is found that Al is mainly solutionizing in NiSi₂ phase in Ni-38Si. AlP can act as the heterogeneous nuclei for both NiSi and NiSi₂ phases, so that the morphology of NiSi₂ can be changed from dendrites to block-like, which makes the structure of Ni-38Si more compact. Moreover, the addition of AlP can decrease the solidification supercooling degree of Ni-38Si alloy.Mn-30Si alloy with typical eutectic microstructure is composed of MnSi and Mn₅Si₃ phases. Al and P which solutionize in neither MnSi nor Mn₅Si₃ phases react and form AlP particles in Mn-30Si alloy, then part of Al aggregates around AlP. Al and AlP can both act as the nuclei of MnSi, and Mn-30Si alloy tends to grow with abnormal eutectic structure by addition of Al, but it grows with abnormal eutectic structure by addition of Al and AlP absolutely, i.e. the eutectic constitution MnSi and Mn₅Si₃ precipitate separately: MnSi precipitates firstly, and then the Mn₅Si₃ phase precipitates.(3) Structure heredity of Ni-38Si in Al-12Si alloyThe addition of Ni-38Si master alloy can promote the precipitation of primary Si in near eutectic Al-12Si alloy, but the addition ways of Ni and Si have great influence on the microstructures, i.e. the primary Si can precipitate when Ni and Si were added in the form of Ni-38Si alloy but not separately. Moreover, the viscosity, correlation radius, number of atoms in clusters and coordination number of the Al-12Si melt with Ni-38Si addition are larger than that with Ni and Si added separately. The results show that the structure heredity of Ni-38Si is responsible for the precipitation of primary Si in Al-12Si alloy. Besides Ni-38Si, there is also structure heredity of Cu-50Si in Al-12Si alloy. Parameters for the modification of ZL109 alloy with addition of Ni-38Si by using structure heredity are as follows: the melt treatment temperature is 740-760℃, the addition level is 1-2%, holding time is 60-90 min. Theholding time can be elongated to more than 10h, so it is feasible to modify Al-Si alloys by the structure heredity.(4) Induced nucleation of the primary Si in near eutectic Al-Si alloysAn induced nucleation mechanism can be used to control the precipitation of primary Si in Al-12Si alloy. The induced particles can not nucleate primary Si directly, but result in the precipitation of AlP which can nucleate primary Si with low level of P (<10ppm), so that the industry near eutectic Al-Si alloys can be modified by the induced nucleation without extra P addition. The orientation relationships between AlP and AlB₂, TiB₂, TiAl₃ (TiAl_xSi_y) as well as TiC have been calculated by the edge-to-edge matching model. The crystallographic orientation relationships between AlP and AlB₂ as well as TiB₂ are as follows:The crystallographic orientation relationships between AlP and TiAl₃ are as follows:The crystallographic orientation relationships between AlP and TiC are as follows: Four-branched compounds coupled Si and iron-rich intermetallics can be obtained by addition of Mn into Al-12Si alloy with P modification, and there is also induced nucleation mechanism for the iron-rich intermetallics. In this case, AlP precipitates firstly, and then nucleates primary Si, the subsequent twinning within primary silicon provides four-fold coordination sites on their surface, which is important for the continuously branched growth of theα-Al(Fe, Mn)-Si particles.In a word, the research of the present study lies not only in the further investigation and broadening on the present modifiers, but also in the new techniques different from the traditional modification. At the same time, it provides the referencable ideas for the studies of refinement of metals and alloys which are more prevailed internationally. The new type of Cu-P-Si modifier and modification technique has independent knowledge authority, and will provide the more important theoretical basis and a new way for the modification of Al-Si alloys.