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Dislocation Dynamics Simulations of Plasticity at Small Scales Caizhi Zhou Iowa State University

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Dislocation Dynamics Simulations of Plasticity at Small Scales Caizhi Zhou Iowa State University Iowa State University Capstones, Theses and Graduate Theses and Dissertations Dissertations 2010 Dislocation dynamics simulations of plasticity at small scales Caizhi Zhou Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/etd Part of the Materials Science and Engineering Commons Recommended Citation Zhou, Caizhi, "Dislocation dynamics simulations of plasticity at small scales" (2010). Graduate Theses and Dissertations. 11677. https://lib.dr.iastate.edu/etd/11677 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Dislocation dynamics simulations of plasticity at small scales by Caizhi Zhou A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Major: Materials Science and Engineering Program of Study Committee: Richard LeSar, Major professor Alan M. Russell Scott Beckman Ashraf Bastawros Wei Hong Iowa State University Ames, Iowa 2010 Copyright © Caizhi Zhou, 2010. All rights reserved. ii TABLE OF CONTENTS LIST OF FIGURES ...............................................................................................................v ACKNOWLEDGMENTS ................................................................................................... xi ABSTRACT ....................................................................................................................... xii CHAPTER 1 INTRODUCTION .................................................................................................................1 1.1 EXPERIMENTAL OBSERVATIONS OF PLASTICITY IN SINGLE CRYSTAL......................................... 1 1.2 PLASTICITY IN POLYCRYSTALLINE THIN FILMS ......................................................................... 5 1.3 DISLOCATION DYNAMICS SIMULATIONS OF PLASTICITY IN SINGLE CRYSTALS ........................... 6 1.4 DISLOCATION DYNAMICS SIMULATIONS OF PLASTICITY IN THIN FILMS ..................................... 9 1.5 THESIS OBJECTIVES ................................................................................................................ 11 REFERENCES ............................................................................................................................... 12 CHAPTER 2 DISLOCATION DYNAMICS SIMULATIONS ................................................................. 18 2.1 THE DISPLACEMENT FIELD OF DISLOCATIONS IN ISOTROPIC CRYSTALS ................................... 18 2.2 STRAIN AND STRESS FIELDS ................................................................................................... 22 2.3 SELF FORCE OF DISLOCATIONS ............................................................................................... 24 2.4 PARAMETRIC DISLOCATIONS .................................................................................................. 25 2.5 EQUATIONS OF MOTION.......................................................................................................... 27 2.6 SIMULATION PROCEDURE ....................................................................................................... 28 REFERENCES ............................................................................................................................... 31 CHAPTER 3 IMAGE STRESSES IN DISLOCATION DYNAMICS SIMULATIONS ............................ 33 3.1 BOUNDARY ELEMENT METHOD .............................................................................................. 35 3.2 NUMERICAL RESULTS............................................................................................................. 43 3.2.1 Stress fields associated the edge and screw dislocations ................................................................. 43 3.2.2 Eshelby twist by a coaxial screw dislocation ................................................................................. 46 3.2.3 Image stress of a straight edge dislocation in a cylinder ................................................................. 47 3.2.4 Image force on a screw dislocation in thin film .............................................................................. 49 iii 3.2.5 Effect of image stresses on the flow stress ..................................................................................... 50 REFERENCES ............................................................................................................................... 53 CHAPTER 4 SIZE EFFECTS ON PLASTICITY OF FCC SINGLE CRYSTALS .................................... 55 4.1 SIMULATION PROCEDURES ..................................................................................................... 55 4.2 EFFECT OF LOADING DIRECTION ............................................................................................. 60 4.3 CROSS-SLIP ............................................................................................................................ 63 4.4 EXHAUSTION HARDENING ...................................................................................................... 69 4.5 SIZE EFFECTS ......................................................................................................................... 73 4.6 CONCLUDING REMARKS ......................................................................................................... 77 REFERENCES ............................................................................................................................... 79 CHAPTER 5 PLASTIC DEFORMATION MECHANISMS OF FCC SINGLE CRYSTALS AT SMALL SCALES ............................................................................................................................. 83 5.1 SIMULATION PROCEDURES ..................................................................................................... 85 5.2 STABILITY OF INTERNAL DISLOCATION SOURCES.................................................................... 86 5.3 DISLOCATION STARVATION (DS) MODEL................................................................................ 91 5.4 SINGLE-ARM DISLOCATION (SAD) MODEL ............................................................................. 94 5.5 DISLOCATION INTERACTIONS CAUSING HARDENING AT SMALL SCALES .................................. 97 5.6 IMPLICATIONS FOR PLASTICITY AT SMALL SCALES ............................................................... 100 5.7 CONCLUDING REMARKS ....................................................................................................... 102 REFERENCES ............................................................................................................................. 104 CHAPTER 6 SIMULATIONS OF THE EFFECT OF SURFACE COATINGS ON PLASTICITY AT SMALL SCALES ............................................................................................................. 108 6.1 SIMULATION PROCEDURES ................................................................................................... 109 6.2 EFFECT OF TRAPPING DISLOCATIONS .................................................................................... 110 6.3 BANDED STRUCTURES FORMED BY CROSS-SLIP .................................................................... 113 6.4 CONCLUDING REMARKS ....................................................................................................... 116 REFERENCES ............................................................................................................................. 118 CHAPTER 7 iv DISLOCATION DYNAMICS SIMULATIONS OF PLASTICITY IN POLYCRYSTALLINE THIN FILMS ............................................................................... 119 7.1 SIMULATION PROCEDURES ................................................................................................... 119 7.2 VALIDATION OF SIMULATION RESULTS ................................................................................. 123 7.3 GRAIN SIZE DEPENDENT STRENGTH ...................................................................................... 125 7.4 FILM THICKNESS DEPENDENT STRENGTH .............................................................................. 131 7.5 SPIRAL SOURCE MODEL ........................................................................................................ 137 7.6 CONCLUSIONS...................................................................................................................... 143 REFERENCES ............................................................................................................................. 146 CHAPTER 8 DISLOCATION DYNAMICS SIMULATIONS OF BAUSCHINGER EFFECTS IN METALLIC THIN FILMS ................................................................................................ 150 8.1 SIMULATION PROCEDURES ................................................................................................... 151 8.2 EFFECT OF PASSIVATION LAYERS ON THE FILM STRENGTH .................................................... 152 8.3 EFFECT OF PASSIVATION LAYERS ON REVERSE PLASTICITY OF THIN FILMS ............................ 154 8.4 BAUSCHINGER EFFECT IN PASSIVATED THIN FILMS ............................................................... 158 8.5 CONCLUSIONS.....................................................................................................................
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