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First principles based multiscale modeling of single crystal plasticity : application to BCC tantalum

Citation

Wang, Guofeng (2002) First principles based multiscale modeling of single crystal plasticity : application to BCC tantalum. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:11132009-112545862

Abstract

In principle, the macroscopic plasticity properties of crystalline materials are derivable from the physical processes involving dislocations and interactions between dislocations with other defects. However, a quantitative theory of plasticity based on the dislocation mechanism requires crossing multiple length and time scales. To accommodate these requirements, we developed a multiscale approach for modeling crystalline solids. In this thesis, to establish the connections between simulations in different length and time scales, I mainly focus on identifying and determining the importance and influence of various unit processes involving the dislocations through atomic level simulations. These unit processes in turn play a major role in modeling the single crystal plasticity. Key Results from Atomistic Simulations Dislocation core structure and core energy: Using the first-principles qEAM force field (FF), we determine the core energy for 1/2a<111> screw dislocation and 1/2a<111> edge dislocation in bcc Ta. We find that the core energy of edge dislocation is 1.77 times higher than that of screw dislocation. This ratio (1.77) is a fundamental material property used as input to the macroscopic model. Furthermore, we find that the central 12 atoms closest to the 1/2a<111> screw dislocation line have distinguishably higher atomistic strain energy than the other atoms. Thus, we arrive at a physical definition of dislocation core. Screw dislocation mobility: In this thesis, we proposed a new method to investigate dislocation mobility by analyzing the process of migration of a screw dislocation dipole. The new method is based on the energy distribution at the atomistic scale and is used to calculate the Peierls potential barrier and Peierls stress for dislocation continuous motion. The calculated Peierls stress is in good agreement with results obtained using other method. Simulating dislocation motion at finite temperatures (from 20 K to 300 K), we find that the activation energy for dislocation motion is about 6 times lower than computed at 0.001 K. Our results suggest that the decrease in the correlation between neighboring segments in the dislocation line accounts for the decrease of activation energy. We observe that the formation of kink pair along the dislocation line enhances the dislocation mobility. This verifies the traditional belief that the screw dislocation in bcc metals moves by first kink pair nucleation and subsequently lateral movements of kinks along the dislocation. Kinks in screw dislocations: To bridge the atomistic process of dislocation motion with continuum model, we accurately calculate the material properties, such as kink pair formation energy and effective kink pair length, using atomic level simulations. In detailed structural analysis, we discover the substructures of different kinks when the screw dislocation core is asymmetric. There are only two kinds of elementary kinks in the dislocation and the others are the composite kinks consisting of an elementary kink and one or two flips. Based on these findings, we further explain the observed trend of the formation energy and mobility of different classes of kinks. (Note: Similar trend and conclusion could have been found in earlier studies but not mentioned by the authors of those papers.) In summary, we have used quantum mechanics based interaction potentials to investigate the unit processes that play important role in single crystal plasticity and verified the findings using the quantitative results obtained from the atomic level simulation in a macroscopic model for single crystal plasticity.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Materials Science
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Goddard, William A., III
Thesis Committee:
  • Johnson, William Lewis
  • Fultz, Brent T.
  • Wang, Zhen-Gang
  • Haile, Sossina M.
Defense Date:30 April 2002
Record Number:CaltechTHESIS:11132009-112545862
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:11132009-112545862
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:5374
Collection:CaltechTHESIS
Deposited By: Tony Diaz
Deposited On:17 Nov 2009 21:56
Last Modified:01 Aug 2014 17:28

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