Micro-Inhomogeneous Structure Of Liquid Al-Fe System Alloys

Using the θ-θ X-ray diffractometer, the structure of liquid Al-Fe system alloys at different composition was investigated at a constant temperature 1550℃. The relationship between parameters of the pre-peak in the structure factor and composition was studied. In addition, the structural change of the liquid hypoeutectic Al-Fe alloy and pure Al etc. with temperature was also studied. Based on the experiments and theoretical analyses, the following main results were obtained. Analysis of the X-ray diffraction data shows that the cluster size i.e. the correlation radius rc is sensitive to the change of the medium-range order. According to the relationships of rc -T(temperature), coordination number N -T , and the nearest neighbor distance r1 -T, the structural transformation in the pure Al melt occurs in the region of 1075¹250℃. Using X-ray diffraction it is for the first time found that the structure of the liquid Al-Fe system alloys correlates closely with the Al-Fe phase diagram. The compound forming ability i.e. the height of the pre-peak in the structure factor of liquid Al-Fe alloys is higher in the region on the phase diagram where nearly all the intermetallic compound with higher melting temperature gather, and the highest at Al₅Fe₂. For the first time, theoretical analysis reveals the complicated origin of the pre-peak. The origin of the pre-peaks is the Fe-Fe correlation in the Al rich alloys, or the Al-Al correlation in the Fe rich alloys, or their mixture around Al50Fe50 alloy. Based on the above analysis, a crystalline structural model for the clusters in the molten alloys was constructed. This model can not only describe well the pre-peak position in the wide composition range 12.50 84.40 at. % Fe, but also reveals that the disappearance of the pre-peak at about 8.00 at. % Fe and 84.40 at. % Fe composition results from the destroy of the micro-superstructure in the melts. One point of view of this dissertation is that the similarity between the liquid and solid structure of Al-Fe alloys lies in that unlike atoms preferentially coordinates, the solid structure in the medium-range is probably kept into the liquid structure; the characteristic bonding length dFeAl (0.248 nm) varies little with the change of composition. The "micro-inhomogeneous model" of the melts is for the first time applied in the high temperature and complicated Al-Fe alloy system. It is shown that the entire concentration region can be divided into four intervals with the border alloys as Al, Al7Fe, Al5Fe2, Fe3Al and Fe. Alloys in the intervals of Al^Al7Fe, Al7Fe^Al5Fe2, Al5Fe2^Fe3Al and Fe3Al^Fe consist of two types of cluster whose compositions represent their border alloys. Alloys situated at the borders of the intervals contain only one type cluster with border alloy composition. It is for the first time found that the liquid hypoeutectic Al-Fe alloy has a critical temperature of 920℃ in the curve of the cluster size to temperature. Below 920℃the structural change is predominantly in the medium-range order, while in the short-range order above 920℃. Experimental data shows that the pre-peak position of liquid Al5Fe2 alloy and its superstructure line of the solid nearly overlap. The coordination number of Al5Fe2 after melting is 9.61. Theoretical analysis implicates that there is covalent bond in the Al5Fe2 melt, and that the liquid structure of Al5Fe2 alloy is similar to that of its solid .