| An attempt io explain the low temperature fracture mechanism was made by investigating fracture processes involued in microcrack formation of an 8090 Al-li alloy. A specially designed chilling equipment was attatched to a wedge loading stage in an SEM for the low temperature in-situ observation. In the S-L oriented specimen, microcracking occurred along grain boundaries at the low stress level, and the crack propagated readily along the grain boundary, while the cracked grain boundary did not appear to have any significant effect on the crack propagation process in the L-T oriented specimen. It is interesting to note that slip bands were hardly observed at the low temperature case, although a number of slip bands were found at room temperature because of the well developed planar slip lines. Thus, the direct observation of the crack initiation and propagation strongly suggests that the most important factor affecting the cryogenic properties of the 8090 Al-Li alloy is a slip mode since homogeneous slip was predominant at cryogenic temperatures. |
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