Solidified Microstructural Evolution Of High Purity Al-3Fe Alloy During Heat Treatment

This paper studied high purity Al-3Fe alloy casted by iron mould, the effects of temperature and holding time on solidification behavior as well as solidification structures of Al-3Fe alloy were studied by means of optical microscope, scanning electron microscope, transmission electron microscope and phases analysis of X-ray diffraction. What is more, the evolution regular of solidification structures under different conditions during heat treatment processing was also discussed. The main conclusions as followed:1. The edge of microstructures in as-cast Al-3Fe alloy consisted of well-developed a-Al dendrites and eutectic structures such as Al-Al6Fe in dentritical clearance. At half radius, the rich-iron phases existed in the form of dot-like, short rod-like and radial-like that grew parallel to each other as well as had a certain orientation in the matrix. In the center, almost dot-like rich-iron phases dispersed in the matrix. It studied that the rich-iron phases in the form of short rod-like and radial-like were Al6Fe and dot-like were Al6Fe, Al5Fe2and Al3Fe. Additionally the average size was about130nm.2. The experiments at550℃、605℃、625℃and640℃for different holding time in terms of Al-3Fe alloy were done. Studies had shown that α-Al dendritical interface became ambiguous even disappeared with the extension of holding time under these four kinds of heat treatment systems. At half radius, short rod-like and radial-like rich-iron phases fused and granulated at550℃and605℃during heat treatment processing. In the center, the size of almost dot-like rich-iron phases increased slightly with the extension of holding time. Moreover, the coarsening mechanism was studied and proved to be according with the classicall LSW theory. No matter at half radius or in the center, the evolution of rich-iron phases experienced from growing to fusing then to granulation. The needle-like rich-iron phases were testified to be Al3Fe.3. At625℃and640℃, At the growth stage of rich-iron phases, the higher heating temperature was, the larger the size of rich-iron phases were under the condition of the same holding time. In the course of fusing, the rich-iron phases fused more thoroughly at640℃ than those at625℃.4. Fusing of the rich-iron phases attributed to Al6Fe metastable phases at550℃and605℃for different holding time. However, we can owned the fusing to stable phases called Al3Fe at625℃and640℃for different holding time.5. The results indicated that the metastable phases transformed into stable phases during heat treatment. It can be seen that the Al6Fe/Al5Fe2→Al3Fe transformation was a relatively slow process at550℃and605℃, but Al2Fe2transformed faster than Al6Fe. Al6Fe at625℃and640℃transformed faster than those at550℃and605℃.6. Under the condition of different holding time at relatively low temperature, it formed the Precipitation Free Zone (PFZ). Width of PFZ was more sensitive on heating temperature than holding time. At relatively high temperature, we could not see the PFZ clearly.