Insights into the initiation of plasticity under contact loading of A and M planes of

sapphire were provided by finite element analysis of the resolved shear stress

acting on basal slip and basal twinning systems and by the evaluation of acoustic

emission signals associated with the onset of plasticity. The analysis of acoustic

emission activity utilizes wavelet-based signal representation in the joint timefrequency

domain. The proposed model invoking loading-rate-dependent

competition between basal slip and basal twinning predicted different yield-point

mechanisms for A and M planes. The predicted mechanisms were consistent with

the experimental plasticity initiation patterns. Room-temperature values of critical

shear stress of 12.9–13.9 GPa for basal slip and 12.6–14.4GPa for basal twinning

derived from application of finite element analysis to the results of nanoindentation

experiments were compared with the values expected from the results of theoretical

modeling and extrapolation of high-temperature experimental results.

Role of Competition Between Slip and Twinning in Nanoscale Deformation of

Sapphire. N.Tymiak, D.Chrobak, W.Gerberich, O.Warren, R.Nowak: Physical

Review B, 2009, 79[17], 174116