Transmission electron microscopy investigation of lattice defects in Al2SiO5 polymorphs and plasticity induced polymorphic transformations

Abstract

Dislocations induced by both natural and experimental deformation in the three Al2SiO5 polymorphs (kyanite, andalusite and sillimanite) have been studied by transmission electron microscopy. For the three materials the dislocation core structures (dissociations) are strongly affected by the temperature (T) and pressure (P) of deformation. For T, P values near the phase boundaries certain dislocation types become widely dissociated and, in deformation tests at constant strain rate, the flow stress increases dramatically. It is suggested that some fault energies decrease for T, P values near the phase boundaries because the faulted ribbons are layers (with unit thickness) of the more stable neighbouring polymorph. This relationship is established in the case of the sillimanite-kyanite transformation which is induced by experimental deformation. Small domains of the new phase (kyanite) are observed to be nucleated on dissociated dislocations. A crystallographic model is proposed which accounts for the great change in flow stress during polymorphic transformation.