TRANSITION BEHAVIOR OF DEFORMATION MODE FROM SHEARING TO LOOPING IN AL-LI SINGLE-CRYSTALS

Abstract

The transition behaviour of deformation mode from particle shearing to looping has been investigated in the Al-Li alloy single crystals using optical microscopy, transmission electron microscopy and mechanical testing. The observation of slip-line morphology and of dislocation structures indicated the occurrence of a transition of deformation mode from particle shearing to bypassing by Orowan looping. The loop size measurements suggested that the critical loop size is close to the average planar particle size at the onset of the peak-ageing condition. The antiphase boundary energy is calculated to be about 0.100Jm(-2) from the analysis of this transition behaviour. This is smaller than that expected from the observed dislocation pair spacings. The similar analysis of ageing curves of polycrystals containing Zr tends to show a higher antiphase boundary energy. This is because the peak-ageing condition appears relatively rapidly owing to a bimodal size distribution of delta' particles in these crystals. The dislocation pair spacings decrease, during the early stage of ageing, with an increase in particle sizes in agreement with the theory.