Effect of Eshelby twist on core structure of screw dislocations in molybdenum: atomic structure and electron microscope image simulations

Abstract

This paper addresses the question as to whether the core structure of screw dislocations in Mo in the bulk can be obtained from high-resolution electron microscopy (HREM) images of such dislocations viewed end-on in a thin foil. Atomistic simulations of the core structure of screw dislocations in elastically anisotropic Mo were carried out using bond order potentials. These simulations take account automatically of the effects of the surface relaxation displacements (anisotropic Eshelby twist). They show that the differential displacements of the atoms at the surface are different with components perpendicular to the Burgers vector about five times larger than those in the middle of the foil, the latter being characteristic of the bulk. Nye tensor plots show that the surface relaxation stresses strongly affect the incompatible distortions. HREM simulations of the computed structure reflect the displacements at the exit surface, modified by interband scattering and the microscope transfer function. Nye tensor plots obtained from the HREM images show that interband scattering also affects the incompatible distortions. It is concluded that it would be very difficult to obtain information on the core structure of screw dislocations in the bulk Mo from HREM images, even under ideal experimental conditions, and that quantitative comparisons between experimental and simulated images from assumed model structures would be essential.

Keywords:

• screw dislocation
• thin foil
• Eshelby twist
• free surface
• HREM

Acknowledgements

We thank Yuri Mishin for sharing his code for plotting the Nye tensor maps. RG acknowledges support from the Czech Science Foundation (Grant No. P204/10/0255) and from the Academy of Sciences of the Czech Republic (Research Project No. AV0Z20410507). VV acknowledges support from the US Department of Energy, BES, Grant No. DE-PG02-98ER45702. PDN and PBH gratefully acknowledge the provision of facilities by the Department of Materials, University of Oxford. The access to the METACentrum supercomputing facilities provided under the research intent MSM6383917201 is highly appreciated.