EGU2020-7681
https://doi.org/10.5194/egusphere-egu2020-7681
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Shear coupled grain boundary migration as a deformation mechanism in minerals.

Gill Pennock and Martyn Drury
Gill Pennock and Martyn Drury
  • Faculty of Geosciences, Utrecht University, Utrecht, Netherlands (g.m.pennock@uu.nl)

A grain boundary can move under stress by a mechanism called shear coupled grain boundary migration (SC GBM) and contribute to strain. SC GBM is considered to be a general property of all grain boundaries over a wide range of misorientation angles, although higher deformation temperatures favour grain boundary sliding. Apart from a structured boundary interface, SC also requires a critical shear stress. We examine evidence for SC GBM in ice. An extensive literature study showed that SC GBM of high angle boundaries does occur in ice bicrystals that were probably deformed under conditions close to those found in nature. We conclude that SC GBM is likely to be an important deformation mechanism for geological materials, where extensive GBM occurs and also in nano sized materials, such as fault gauges.

How to cite: Pennock, G. and Drury, M.: Shear coupled grain boundary migration as a deformation mechanism in minerals., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7681, https://doi.org/10.5194/egusphere-egu2020-7681, 2020

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Display material version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-7681, Rüdiger Kilian, 04 May 2020

    Hi,

    very interesting process. You suggest, that SC-GBM might serve as an alternative explanation for observations of gbs rheology but strong CPO development.

    I'm wondering, if CPO development can be attributed to this process, how would one can predict or model a CPO produced through this process?

    Cheers,

    Rüdiger

    • AC1: Reply to CC1, Martyn Drury, 17 May 2020

      Thanks for the very good question.  It is a huge challenge to quantitatively predict the CPO developed by SC-GBM, because little is known about the basics of SC-GBM in mineral systems.  As far as we know there are no models for CPO development in any material that include SC-GBM. Some current CPO models do include twinning as an intra-granular deformation mechanism, but SC-GBM involves “twin-like” behaviour at the grain boundaries. Work is needed to identify which boundaries show SC-GBM behaviour and the influence of deformation conditions and impurity contents on the extent of shear coupling. A model for CPO development would have to describe the evolution of the grain boundary population (including misorientations and grain boundary planes) during deformation and recrystallization.

  • CC2: Comment on EGU2020-7681, Jianye Chen, 06 May 2020

    Hi Gill,

    Very interesting model and result. My simple shear experiments of calcite at 550-600C at 1 nm/s showed that the mechanism you proposed here (SC-GBM) must have occured. My question is: do you have an idea how to get the constitutive flow law for this mechanism? (Can we simply convert from the 'compactional' creep law from literature? ) If further experiments could be done to quantify the law, any suggetion? 

    Jianye Chen (Utrecht)    

    • AC2: Reply to CC2, Martyn Drury, 29 May 2020

      Thanks for the comment and the important question. As far as we know, there are no creep laws for deformation specially involving dominant shear coupled grain boundary migration (SC-GBM). Some creep models include the effect of dynamic recrystallization on high temperature creep but the models only include conventional GBM driven by strain energy. Stress is an additional driving force for migration in the case of SC-GBM.

      It may be possible, as you suggest, to include SC-GBM in existing creep laws for grain size sensitive deformation as an additional process to grain boundary sliding, pressure solution and dislocation glide in the grains. Similar to the case for grain boundary sliding (e.g. Goldsby and Kohlstedt 2001), SC-GBM may occur sequentially with dislocation glide and either process could be rate controlling.

      It would be very interesting to look at the results of your experiments and do some further shear experiments on calcite at high temperatures where grain boundary migration is rapid to quantify the role of SC-GBM in deformation.  Experiments on samples with grain strain markers, like the examples shown in our presentation, would enable the strain accommodated by different mechanisms to be quantified.