Study Of The Phosphides In Al-X (Si,Zr,Ca)-P Alloys

The phosphides in Al-Si-P, Al-Zr-P and Al-Si-Ca-P alloys were systematically studied using high scope video microscope (HSVM), electron probe micro-analyzer (EPMA), X-ray diffraction (XRD), differential scanning calorimeter (DSC), transmission electron microscope (TEM) and field-emission scanning electron microscope (FESEM), etc. The AlP cluster structures in Al-Si-P melt were analyzed. Furthermore, it was discovered that AlP grows with the twin plane reentrant edge (TPRE). A novel Al-Zr-P master alloy with ZrP particles was successfully synthesized and the phase transformation between ZrP and AlP was discussed. Meanwhile, the inhibitory effect of Ca on the phosphorus modification treatment was studied in Al-Si-Ca-P alloy. In addition, the extensive applications of Al-Si-P and Al-Zr-P alloy in the refinement of hypereutectic Al-Si alloys were also researched.The major research efforts of the present study are as follows:(1) The cluster structure and growth mechanism of AlP in Al-Si-P meltThe structure of liquid Al80Si15P5 alloy was investigated by ab initio molecular dynamics simulation. It was found that the overwhelming chemical short-range order around P atoms is the characteristic structure of liquid Al18Si15P5 alloy. The P-centered Al6P structural units exist at 1100℃, however, the structure units are slightly distorted into (6Al+Si)P at 2600℃.The typical microstructure of Al-Si-P master alloy is composed of a-Al, Si phases and AlP particles. Based on the thermodynamic data derived, an AlP crystal is typically faceted on the close-packed {111} faces and non-faceted on {100} and {110} faces. In the fracture plane of Al-Si-P alloy, AlP hexagonal platelets with twin reentrant and ridges are observed. Meanwhile, the twin-related growth model is applied to explain the growth process of AlP crystals. Nucleation events are allowed to take place at the reentrant corners. As the AlP crystals continue to grow, some high-index planes would disappear, AlP crystals would be surrounded by close-packed {111} planes.(2) The phase transformation and growth mechanism of phosphide in Al-Zr-P alloyThe structure of liquid Al90Zr5P5 alloy was investigated by ab initio molecular dynamics simulation. It was found that the affinity between Zr and P atoms is much stronger than that between other atoms. That is to say the Zr atoms will be found most frequently at the first shell rather than other coordination shell of P atoms. The (6Al+Zr)P structural units exist at 800℃. However, the structural units are distorted into (7Al+Zr)P at 1600℃. With the addition of Zr into Al-P alloy under certain conditions, the transformation from AlP to ZrP can occur and a novel Al-Zr-P alloy without AlP is successfully synthesized, which is composed of a-Al, ZrAl3 and ZrP. With further studies, it is found that Si in the melt can promote the transformation from ZrP to AlP.Under the present conditions in fabrication process, the extracted ZrP particles with a NaCl structure present the cubic three-dimensional morphologies clearly. In the crystal growth process, the difference of the growth rate between each plane will lead to crystals precipitated with different morphologies. Furthermore, due to the atoms diffusion in the melt, Zr and P atoms are enriched more easily on the corners or edges of ZrP particles, and the growth rate along <111> crystal orientation is promoted. Meanwhile, Al atoms are enriched at the center of {100} crystal planes, which leads to the {100} planes as hopper structure, and consequently this structure could supply necessary growth steps for ZrAl3 phase.(3) The inhibition effect of Ca on AlP modification in Al-Si-Ca-P alloyAs a common impurity, Ca has serious influence on the phosphorus modification of Al-Si alloys. It is found that Ca takes the form of Al4Ca and Ca3P2 phases. Furthermore, Ca3P2 compounds exhibit flocculent morphology with rough surface. In addition, in Al-xSi-2Ca-1P alloys in which the concentrations of Si are 6.0%,12.0% and 18% respectively, all of calcium compounds take the form of Al2Si2Ca and Ca3P2 phases. It is found that these two compounds exhibit thick plate-like shape in the fracture plane of Al-12Si-0.4Ca-0.2P alloy.The phosphorus modification effect on Al-12Si alloy can not be obtained with the addition of 400 ppm Ca and 200 ppm P. Due to the formation of Ca3P2 compounds, there is no AlP in the melt which can act as the heterogeneous nucleation sites for primary Si. Meanwhile, the residual Ca can make the microstructure of Al-12Si alloy as that modified by Na. Furthermore, it can react with Al and Si atoms to form the coarse plate-like Al2Si2Ca phase.(4) Application of Al-Si-P and Al-Zr-P master alloys on the refinement of primary Si in hypereutectic Al-Si alloysBased on the cluster structure, dissolution and precipitation characteristic of AlP in the melt, the behavior of AlP in Al-Si melt can be deduced as follows:larger crystal→dissolution→clustering of A6P unit→AlP microcrystal. According to different phosphorus modifier, the refinement mechanism is the same, which is the heterogeneous nucleation mechanism of AlP for the primary Si. Meanwhile, the transformation of AlP listed above is necessary for the modifier to display excellent refinement performance.Firstly, it was found that the microstructure of Al-17Si-2.5P master alloys has a certain influence on the refinement performance. For example, the fine nodular-like AlP particles with uniform distribution can be obtained with rapid solidification processing. After addition of the master alloy into Al-Si melt, these AlP particles are easier to dissolve into the melt due to the higher energy and dislocation defect, which improves the P recovery and nucleation rate and thus leads to an improved refining performance.Secondly, an orthogonal test was designed to investigate the integrated effects of refining factors and subsequently to optimize the processing parameters. It is found that under the optimized conditions, i.e., phosphorus addition of 375 ppm, melting temperature of 800℃, and holding time of 30 min, the average sizes of primary Si can be most remarkably decreased from 116.3μm to 14.0μm with sphere-like morphology. Meanwhile, the Brinell hardness and tensile strength can be significantly increased by 14.1% and 27.8%, respectively.Al-6Zr-2P master alloy has excellent refinement effect on the primary Si in hypereutectic A390 alloys. The refinement mechanism is that the stronger affinity between Zr and Si atoms and the chemical kinetic factors promote the transformation from ZrP to AlP and subsequently AlP particles act as the heterogeneous nucleation sites of primary Si. The transformation behavior of ZrP in the Al-Si melt can be expressed as follows:ZrP crystal→dissolution→local structural reorganization→clustering of A6P unit→AlP microcrystal.