Ductile_Deformation.pptx Engr Dr. Ocheri Cyril.pptx

Give the discription of Ductile deformation

1. DUCTILE DEFORMATION BY ENGR DR. OCHERI CYRIL DEPARTMENT OF METALLURGICAL & MATERIALS ENGINEERING UNIVERSITY OF NIGERIA, NSUKKA [email protected]

2. Ductile Deformation: Change in the Solid State Q: How does matter flow in the solid state? A: Flow occurs at the atomic scale in minerals Changes in shape, composition, and structure of minerals due to crystal imperfections created during crystallization of mineral Deformed quartzite at 800 °C * [email protected]

3. Crystal Imperfections (defects) Point defects a. impurities -bonded atoms not usually part of a mineral structure b. interstitials - unbonded atoms trapped within crystal c. vacancies -holes in crystal lattice (missing atoms) Impurities and interstitials often result in mineral color differences (e.g., ruby and sapphire for corundum) [email protected]

4. Crystal Imperfections (defects) 2. Line defects a. dislocation -edge of an extra half-plane of atoms in crystal lattice Dislocations in olivine (photo width is 200 microns) [email protected]

5. Crystal Imperfections (defects) Vacancies and dislocations move within crystal because they are sites of unsatisfied bonds As defects move, the chemical bonds are broken and reformed one at a time —thus, no loss of cohesion (not brittle deformation) Fracturing involves breaking all bonds simultaneously and permanently [email protected]

6. Ductile Deformation Mechanisms 1. Diffusion -migration of vacancies and atoms in a crystal; occurs within and along edge of crystal -results in change in crystal shape and structure; recrystallization Metamorphism : sillimanite to kyanite ; olivine to spinel structure Diffusive mass transfer : pressure solution; loss of radioactive-decay particles [email protected]

7. Ductile Deformation Mechanisms Diffusion Pressure solution is very important in sedimentary rocks where the temperature is <300 °C; grain-boundary diffusion that involves a film of water attached to grain boundary [email protected]

8. Ductile Deformation Mechanisms 2. Crystal Plasticity -movement of dislocations along a crystal slip plane; primary mechanism for change in crystal shape in solid state where T>300 °C Slip systems in halite [email protected]

9. Ductile Deformation Mechanisms 3. Twinning (mechanical)-one part of a crystal shears past another along a slip plane: different from growth twin; active at T<300 °C (mostly calcite and plagioclase) Calcite twins Mechanical calcite twinning in marble [email protected]

10. Ductile Deformation Mechanisms 4. Superplastic Creep -very rapid diffusion at high temperature Grains slide past each other without friction Active mechanism in fine-grained sedimentary rocks (at high temperature) and mantle rocks [email protected]

11. Ductile Deformation Microstructures Dislocations migrate through crystal and concentrate along grain boundaries—this reduces internal-strain energy of crystal Dislocations in calcite [email protected]

12. Recovery -movement of dislocations to reduce internal-strain energy subgrains microcracks Quartz in quartzite (photo width is 1.8 mm) Pass. & Trouw Pass. & Trouw [email protected]

13. Recrystallization -removal of internal-strain energy after recovery High-angle grain boundaries (often meeting at 120 °) Recrystallized quartz grains (note small size—lower strain energy) Pass. & Trouw [email protected]

14. Recrystallization Dynamic- recrystallization in an anisotropic stress field; grain size reduction—grains smaller than host rock (e.g., mylonite ) Static- recrystallization in an isotropic stress field; large grain size Marble mylonite (photo width is 2 mm) [email protected]

15. Ductile Deformation and Metamorphism If strain rate is >> than recrystallization , a mylonite forms If recrystallization is >> than strain rate, a metamorphic rock forms [email protected]

16. Thank you for your attention [email protected]