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Role of competition between slip and twinning in nanoscale deformation of sapphire
Role of competition between slip and twinning in nanoscale deformation of sapphire
N. Tymiak,1,* D. Chrobak,1,? W. Gerberich,2 O. Warren,3 and R. Nowak1,?
1Nordic Hysitron Laboratory, Helsinki University of Technology, 02015 HUT, Helsinki, Finland
2CEMS, University of Minnesota, Minneapolis, Minnesota 55455, USA
3Hysitron, Inc., Minneapolis, Minnesota 55344, USA
Received 6 January 2009; revised manuscript received 26 February 2009; published 26 May 2009
Insights into the initiation of plasticity under contact loading of A and M planes of sapphire are 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 emis-
sion activity utilizes wavelet-based signal representation in the joint time-frequency domain. The proposed
model invoking loading-rate-dependent competition between basal slip and basal twinning predicts different
yield-point mechanisms for A and M planes. The predicted mechanisms are consistent with the experimental
plasticity initiation patterns. Room-temperature values of critical shear stress of 12.9–13.9 GPa for basal slip
and 12.6–14.4 GPa for basal twinning derived from application of finite element analysis to the results of
nanoindentation experiments are compared with the values expected from the results of theoretical modeling
and extrapolation of high-temperature experimental results.
DOI: 10.1103/PhysRevB.79.174116 PACS numbers: 46.80.j, 62.25.g, 43.60.d, 62.20.F
I. INTRODUCTION
The use of sapphire in advanced technologies1,2 stimulates
interest in the mechanical response of Al2O3. Whether an
ultimate goal is preserving component integrity or a deliber-
ate defect generation, understanding the mechanisms of plas-
ticity initiation is crucial. Recognized for its ability to pro-
vide valuable information about plasticity onset mecha-
nisms,3–6 nanoindentation becomes e
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