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Measurement of Dislocation Creep

Measurement of Dislocation Creep. Based on: Low-Stress High-Temperature Creep in Olivine Single Crystals D.L. Kohlstedt and C. Goetze, 1974. Picture from Couvy et. al, 2004. I. The experiment. II. A closer look at dislocation creep. Designing an experiment to model mantle flow processes.

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Measurement of Dislocation Creep

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  1. Measurement of Dislocation Creep Based on: Low-Stress High-Temperature Creep in Olivine Single Crystals D.L. Kohlstedt and C. Goetze, 1974 Picture from Couvy et. al, 2004

  2. I. The experiment II. A closer look at dislocation creep

  3. Designing an experiment to model mantle flow processes • Goal: produce a steady strain rate at a constant stress

  4. Olivine single crystals • High temperature (1450-1650°C) is needed for strain to occur fast enough to measure readily in the laboratory. • Natural peridotite contains other phases, lowering the solidus below experimental temperatures • Use of single crystal avoids grain boundary issues

  5. San Carlos Peridot

  6. Experimental setup • Furnace • Method of applying precise load • Method of measuring strain

  7. The Apparatus • Molybdenum vs. graphite • Gas inlet for H2, CO2, controls O2 fugacity • Crystals dry rapidly at >1000°C and Atmospheric pressure

  8. Results 101 102 103 104 σ1 – σ3 (bars)

  9. Microstructures

  10. Dislocation Creep: A Mechanism for Plastic Flow

  11. Edge dislocations and glide: the rug analogy

  12. Screw dislocation

  13. Slide on Burgers vectors?Slide on Power law creep equation?

  14. Dislocation tangles & strain hardening

  15. Edge dislocation pile-ups in olivine These sorts of dislocation tangles were commonly observed in crystals deformed at differential stresses above 1 kbar.

  16. Climb andvacancy diffusion

  17. Evidence for climb in olivine In samples deformed under lower stress, dislocation structures appear to have reached an equilibrium concentration, implying the existence of some annealing process such as climb.

  18. Conlusions • Basic laboratory experiments can be used to hypothesize flow laws for the mantle • Dislocation creep is a viable mechanism for plastic flow at high temperature and low differential stress

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