Microstructure Analysis of Rapidly Solidified Aluminum-Transition Element Alloys

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Microstructure analysis was made of rapidly solidified Al-transient element systems (Al-Cr, Al-Si, Al-Fe) by means of diffraction techniques (X-ray and electron) and microscopy (optical microscopy, scanning electron microscopy and transmission electron microscopy). Thermal stability of the as-solidified microstructure was also studied. The results are summarized as follows :(1) As-solidified microstructure of rapidly solidified Al-Cr alloys (Cr : 2-6mass%, melt-spun ribbon and atomized powders) is largely dependent on Cr content and cooling rate. Low Cr content and high cooling rate (small powder size) favor the occurrence of supersaturated solid solution, while high Cr content and low cooling rate favor the occurrence of intermetallic compounds.(2) The intermetallic phase identified for the present rapidly solidified Al-Cr alloys is rheta-Al_<13>Cr_2. The rheta-Al_<13>Cr_2 phase occur as "rod" or "spherulites" depending on Cr content and cooling rate. The spherulites often contain icosahedral quasicrystalline phase.(3) For constant Cr content, the microstructure of atomized powders which have less degrees of Cr supersaturation is more thermally stable than that of melt-spun ribbons.(4) Considerable degrees of sulute-supersaturation exists in the airatomized Al-18mass%Si and Al-8mass%Fe alloy powders. The supersaturated solid solutions begin to decompose at below 300^ﾟC for Al-Si and at 350^ﾟC for Al-Fe.(5) Computer-aided X-ray diffractometry is extremely useful technique for structure determination of rapidly solidified Al alloys.