The effect of low melting point impurities on the properties of aluminum-lithium alloys
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1.
INTRODUCTION
AT the present stage of their development, A1-Li alloys generally compare unfavorably with conventional aluminum alloys in terms of toughness and ductility at equivalent strength levels. For example, AI 7075 and AI 2024 extrusions made from commercial ingots have significantly greater toughness than identical AI 8090 and A1 2090 extrusions ~ (Figure 1). There is evidence in the literature that this difference in toughness may be related to low melting point impurities such as sodium and potassium. Payne and Eynon, in 1957 British Patent 787665, suggested that embrittlement of A1-Li alloys occurred if the sodium content was over 50 ppm. Vaynblat et al. 2 in 1976 found a 16 pct drop in toughness of AI-2 Li-0.13 Zr alloys when the sodium content was increased from 31 to 76 ppm. More recently, Vasuddvan et al. in 19823 observed linear decreases in toughness in AI-Li alloys containing up to 480 ppm sodium, and Webster 4 observed toughness decreases in A1-Li alloys containing up to 434 ppm sodium and 23 ppm potassium. The addition of sodium and potassium promotes the grain boundary fracture mode in AI-Li alloys3 and these elements have been found by various surface analysis techniques to segregate to the grain boundaries. 3-6 The aim of the present work is to identify the nature of the grain boundary segregation and the changes it undergoes during aging. II.
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