THE IMPACT OF COOLING RATES ON THE MICROSTRUCTURE OF AL-U ALLOYS

The impact of cooling rates on the microstructure of Al-U alloys was s tudied by optical, scanning electron, and transmission electron micros copy. A variety of solidification techniques were employed to obtain c ooling rates ranging between 3 X 10(-2) and 10(6) K/s. High-purity ura nium (99.9 pct) and high-purity aluminum (99.99 pct), or ''commerciall y pure'' type Al-1050 aluminum alloys were used to prepare Al-U alloys with U concentration ranging between 3 and 22 wt pct. The U concentra tion at which a coupled eutectic growth was observed depends on the co oling rates imposed during solidification and ranged from 13.8 wt pct for the slower cooling rates to more than 22 wt pct for the fastest co oling rates. The eutectic morphology and its distribution depends on t he type of aluminum used in preparing the alloys and on the cooling ra tes during solidification. The eutectic in alloys prepared from pure a luminum was evenly distributed, while for those prepared from Al-1050, the eutectic was unevenly distributed, with eutectic colonies of up t o 3 mm in diameter. Two lamellar eutectic structures were observed in alloys prepared from pure aluminum containing more than 18 wt pct U, w hich solidified by cooling rates of about 10 K/s. One structure consis ted of the stable eutectic between UAl4 and Al lamella. The other stru cture consisted of a metastable eutectic between UAl4 and Al lamella. At least three different eutectic morphologies were observed in alloys prepared from Al-1050.