Determining Macroscopic Mechanical Properties from Microscopic Calculations
X. Blase, Centre National de la Recherche Scientifique, Lyon, France
K. Lin, A. Canning, S. G. Louie, and D. C. Chrzan, University of California, Berkeley, and Lawrence Berkeley National Laboratory
Research Objectives
The goal of this research is to apply ab initio electronic structure techniques to calculate the minimum stress required to move a dislocation in C in the diamond cubic phase. This value is important to engineering hard materials, as it provides a benchmark for materials performance. In addition, successful completion of this research will allow us to identify the aspects of electronic structure and bonding relevant to the motion of dislocations. This knowledge can then be used to develop an improved physical intuition concerning the strength of materials.
Computational Approach
Calculations to date have been performed using a parallel implementation of the plane-wave ab initio pseudopotential technique. The calculations were run on the T3E. Supercells employed included up to 192 C atoms.
Accomplishments
To date, we have established that the 90° partial dislocation in diamond cubic C is most stable in the period doubled reconstructed in both a dipolar and quadrupolar arrangement. We have run a number of unit cells with differing distances between the dipoles, and have begun to analyze the results within simple elasticity theory to assess if we can use the current calculations to estimate the dislocation core radius, rc, which enters into classical elasticity theory descriptions of the dislocations.
The unit cell used in the calculations. The 90° partial dislocations are clearly visible as the highly distorted regions. The period doubled structure along the core is also clearly visible. The center of the cell reveals the stacking fault separating the two partial dislocations.
Significance
The calculation of the minimal stress required to move a dislocation places a lower limit on the strength of a material. At a minimum, the stress required to observe plastic deformation must be above the limits we will calculate. This information establishes an important benchmark that could be used to gauge the properties of materials engineered to be ultra-hard, and will accelerate the search for hard materials that can be used as protective coatings.