Deformation Micromechanics DUCTILE DEFORMATION AND BRITTLE-DUCTILE TRANSITION.
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Transcript of Deformation Micromechanics DUCTILE DEFORMATION AND BRITTLE-DUCTILE TRANSITION.
Diffusion Creep
Motion of point defects Diffusion of through the matrix
Diffusion along grain boundaries
Reactions at grain surface
Observations Lattice-preferred orientations (LPOs)
Strain rate related to grain size:
Affected by the chemistry of point defects
m
D
d
General Questions
Definitions of terms What are glide, climb, creep, burgers vector, and
cross-slip?
Brittle-ductile vs. Brittle-plastic What’s the difference? Spatial, temporal
relationships, etc.?
Stress and strain relationships (modern form of constitutive law) Which stresses, which strains?
Definitions
What are glide, climb, creep, burgers vector, and cross-slip?
Still don’t understand, the difference between glide, climb, cross slip and what’s burger’s vector.
Burger’s vector – what is it?
Glide, Climb, Cross-slip, Burgers vector
Glide is the movement of edge dislocations in 1-D Single plane, in a single direction
Burgers vector (blue arrow) is magnitude and direction of lattice distortion resulting from dislocation
Propagation of an edge dislocation through a crystal lattice (neon.materials.cmu.edu)
Glide, Climb, Cross-slip, Burgers vector Climb occurs when a dislocation moves up, perpendicularly,
relative to glide Activates at higher temperature
Climb + glide = creep
www.geosci.usyd.edu.au
[1] http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_5/backbone/r5_1_2.html
Motion of a mixed dislocation
[1]
We are looking at the plane of the cut (sort of a semicircle centered in the lower left corner). Blue circles denote atoms just below, red circles atoms just above the cut. Up on the right the dislocation is a pure edge dislocation
on the lower left it is pure screw. In between it is mixed. In the link this dislocation is shown moving in ananimated illustration.
Glide, Climb, Cross-slip, Burgers vector Cross-slip is similar to climb, but applies to screw-
dislocations Screw dislocations operate similar to a zipper
chemistry.tutorvista.com www.matter.org.uk
Brittle-Ductile vs. Brittle-Plastic
On page 11, they describe the transition from brittle to plastic deformation to occur in two stages. Are these two spatially or temporally distinct? Why two stages? I’m just not quite sure how ductile and plastic are different.
Does the Semibrittle stage between brittle and plastic always occur, or are there sharp transitions?
To what extent can we observe both isolated and combined effects of the various factors on Semibrittle deformation in the lab?
Why is semi-brittle failure difficult to describe? No constitutive law.
Brittle-Ductile vs. Brittle-Plastic
The brittle-ductile transition is a change from localized to distributed failure. Brittle-plastic is a change from brittle cracking to plastic flow alone.
– The Authors
16
Stress/strain relationships
For the creep models, a relation between strain rate and stress is established. But, which strain or stress is used?
What does “stress” mean in the constitutive equations?
How would equation (15) be restated if we considered the six independent components of the stress tensor? Could true triaxial experiments be performed to estimate parameters for the resulting set of constitutive equations?
Stress, strain relationships
Stress (σ) is always differential stress, confining pressure is expressed in an exponential term as (P)
Strain is often expressed in terms of creep rate ( = s-1; percent change per second)
General from of most commonly used flow law:
(Hirth & Kohlstedt, 2003)
Stress, strain relationships
How would equation (15) be restated considering components of the stress tensor? Could true triaxial experiments be performed to estimate parameters constitutive equations?
Which Law?
How do people choose the evolution laws for the dislocation creep? It seems completely dependent on the rock type…
Given Tables 1, 2 and 3, how does a scientist make an informed decision on which model to use? How do they account for the assumptions and limitations?
Once the deformation mechanism of a rock sample is identified, how can the various flow-law parameters be estimated for that particular sample, given the somewhat wide range of experimentally derived values?
Which Law?
Once the deformation mechanism of a rock sample is identified, how can the various flow-law parameters be estimated? Typically, parameters are estimated prior to
determination of a dominant mechanism
Whichever is fastest
Which Law?
Whichever is fastest Experimentally determined parameters for diffusion,
dislocation, and low-T plasticity are plugged into flow law
Fastest mechanism is dominant
Field observations
What are the features a geologist would look for to infer the dominant mechanism? What techniques are used?
How would we recognize that one specific mechanism dominated, judging only from field samples?
What does the pressure solution deformation look like in thin section/hand sample?
Features
Most common indication is LPO Dislocations are
crystallographically controlled, and will give rise to LPO
Diffusion is not, and will typically manifest as random lattice orientation