TEMPERATURE DEPENDENCE OF YIELD STRENGTH OF Al-Li SINGLE CRYSTALS

Compressive yield strength of single crystals of Al-Li binary alloys were measured as functions of test temperature and aging condition. Dislocation structures of deformed and undeformed crystals were examined by transmission electron microscopy. On the basis of the experimental results, the temperature-dependent mechanisms of plastic deformation were discussed in terms of dislocation-particle interaction. In the underaged crystals, CRSS gradually increased with temperature in the temperature range from 77 to 323K, followed by a rapid increase to a maximum around 423K. TEM observation of slip plane sections revealed super-dislocation pairs in the lower temperature range, with a gradual broadening of the pair distance. Unit dislocations were exclusively observed above 423K. These results suggest that main contribution to the characteristic positive temperature dependence of the strength of underaged Al-Li alloys cornes from the Kear-Wilsdorf locking in the lower temperature range, and from the breakaway of super-dislocation pairs, resulting in the precipitate-shearing by unit dislocations. In the overaged crystals and when the Orowan bypassing is dominant, CRSS decreased with increasing temperature gradually in the lower temperature range and, rapidly in the higher temperature range. Orowan loops surrounding the coherent Al3Li precipitates were observed by TEM at test temperature as high as a half of the melting point of the alloy