High‐temperature deformation of diopside single crystal: 2. Transmission electron microscopy investigation of the defect microstructures

Abstract

The defect microstructures induced by high‐temperature creep in diopside single crystals are reported, based upon investigation by transmission electron microscopy (TEM). The creep experiments as well as the steady state creep laws are reported in a companion paper (Raterron and Jaoul, this issue); they clearly show that two deformation regimes occur, one at temperatures above a critical temperature T c of 1130°–1140°C and one at temperatures below T c . At high temperature, diopside is stronger than would be expected by extrapolation of the creep law at lower temperature. TEM observations indicate that the same glide systems {110}1/2〈110〉 are activated above and below this critical temperature; in both cases, evidence for dislocation climb has not been detected. The only noticeable difference between the two deformation regimes is the occurrence in the samples deformed in the higher temperature range of a large and homogeneous density of tiny precipitates of a glassy phase; these precipitates interact with the mobile dislocations. The size and the density of the precipitates increase dramatically with temperature. Just below 1130°–1140°C, a very limited and heterogeneous precipitation seems to occur along the dislocation cores in the form of very tiny precipitates less than 10 nm size. At 1250°C the precipitates reach 0.1 μm diameter. It is suggested that pinning of mobile dislocations by these precipitates is responsible for the hardening effect observed above 1130°–1140°C. Although the volume proportion of glass may be extremely low, its occurrence clearly means that partial melting occurs at temperatures well below the widely accepted melting temperature of this clinopyroxene (1350°C at 1 atm). Such a phenomenon which can be detected directly only by TEM (and indirectly by its consequences on the mechanical properties of the material) might also occur in a number of other silicate minerals.