A new continuum model of the incoherent interface compared with growth of a spinel rim on an olivine grain
In a polymorphic change in which the phases differ only by a reversible difference in specific volume, kinematics requires a unit mass to suffer deviatoric strain in the instant it is transformed. Unlike the Eshelby stress–free strain, this strain is a property of the motion. Its existence must be c...
Saved in:
| Published in: | Physics of the earth and planetary interiors 2019-07, Vol.292, p.29-41 |
|---|---|
| Main Author: | |
| Format: | Article |
| Language: | eng |
| Online Access: | Get full text |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|
| Summary: | In a polymorphic change in which the phases differ only by a reversible difference in specific volume, kinematics requires a unit mass to suffer deviatoric strain in the instant it is transformed. Unlike the Eshelby stress–free strain, this strain is a property of the motion. Its existence must be considered when formulating the constitutive relation for the product of an incoherent transformation. To show this, two models are compared: in both, the (Nabarro) condition of vanishing shear stress is imposed at the incoherent interface; they differ only in the treatment of the deviatoric strain at issue. In the existing model, deviatoric stress within a unit mass of product is determined by total deviatoric strain from its initial state as parent phase. In the new model, lattice reconstruction is assumed to erase all memory within the unit mass of deviatoric strain suffered before, or during, its transformation. The existing model is not consistent with experiments on the olivine spinel–phase change in single crystals. It predicts that when the pressure applied exceeds a critical value, samples should transform completely at almost constant rate; instead, growth is seen to slow, and may even cease. The new model predicts this. Without adjustable constants, fair agreement is obtained with experiments on samples having 75–200 ppmw of water. Because elastic deformation by itself can explain those observations, the very thin rims seen on even drier samples suggest that water may be essential to lattice reconstruction in this phase change.
•We pose a new constitutive relation including the effect of lattice recon-struction on the product phase. of the lattice.•Though the new model contains no adjustable constants, adequate agree-ment is obtained with published experiments on olivine samples containing 75–200 ppmw of water.•For nominally anhydrous samples, much thicker product rims are predicted than are observed.•Thin rims can be explained if interface kinetics are sensitive to water.•With the constitutive behaviour of single crystals now seemingly under-stood, numerical modelling of polycrystals is feasible. |
|---|---|
| ISSN: | 0031-9201 1872-7395 |