Abstract
The effect of low melting point impurities, such as sodium and potassium, on the properties and microstructure of Al-Li alloys has been investigated. These impurities have been shown to occur as discrete lenticular particles along grain boundaries. In underaged and peak-aged conditions, the impurity particles are liquid and the toughness and ductility of alloys in this condition were found to be improved by lowering the test temperature below the freezing point of the impurity. In the overaged condition, the sodium and potassium occur as a solid phase and materials in this condition were found to follow the pattern of most aluminum alloys and show a decrease in Charpy impact toughness as the testing temperature was lowered. The addition of sodium and potassium to Al-Li alloys reduced toughness and ductility at room temperature in the range of compositions investigated (up to 476 ppm Na and up to 23 ppm K). Low melting point impurities were found to have little effect on the toughness or ductility of high purity aluminum. The detrimental effects of sodium and potassium on toughness appear to exist only in alloys containing substantial amounts of lithium or magnesium, which break down the relatively harmless solid NaAlSix phase which exists in conventional alloys. The loss of toughness due to low melting point impurities was found to be about the amount expected from the calculated reduction in surface energy of a grain boundary crack containing a liquid phase.
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Webster, D. The effect of low melting point impurities on the properties of aluminum-lithium alloys. Metall Trans A 18, 2181–2193 (1987). https://doi.org/10.1007/BF02647091
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DOI: https://doi.org/10.1007/BF02647091