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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
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I. The experiment
II. A closer look at dislocation creep
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Designing an experiment to model mantle flow processes
• Goal: produce a steady strain rate at a constant stress
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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
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San Carlos Peridot
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Experimental setup
• Furnace
• Method of applying precise load
• Method of measuring strain
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The Apparatus
• Molybdenum vs. graphite
• Gas inlet for H2, CO2, controls O2 fugacity
• Crystals dry rapidly at >1000°C and Atmospheric pressure
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Results
101 102 103 104
σ1 – σ3 (bars)
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Microstructures
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Dislocation Creep: A Mechanism for Plastic Flow
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Edge dislocations and glide: the rug analogy
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Screw dislocation
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Slide on Burgers vectors?
Slide on Power law creep equation?
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Dislocation tangles & strain hardening
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Edge dislocation pile-ups in olivineThese sorts of
dislocation tangles were commonly
observed in crystals deformed at differential stresses above 1 kbar.
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Climb and
vacancy diffusion
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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.
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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