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Some Aspects of Plastic Flow in Silicon Nitride

  • T. Rouxel (a1), J. Rabier (a2), S. Testu (a3) and X. Milhet (a2)

Abstract

The different scales of plastic flow in silicon nitride were investigated either by indentation experiments and compression under hydrostatic pressure in the 20-850°C temperature range, and by stress relaxation and creep above 1350°C. [0001], 1/3<11-20> and 1/3<11-23> dislocations were evidenced by Transmission Electron Microscopy (TEM) in the low temperature range. Cross-slip events in {10-10} prismatic planes were observed at temperature as low as 20°C by Atomic Force Microscopy (AFM) on micro-hardness indents. By increasing the temperature, the deviation plane becomes {11-20} prismatic planes. The easiest slip system is by far the [0001]{10-10} system. Above 1350°C, the creep strain could be fitted by the sum of a transient component, εt[1-exp-(t/τc)bc], where τc reflects the duration of the transient creep stage, and bc is between 0 and 1, and a stationary component, εsst =Aσnt, where σ is the stress and n is the stress exponent. The increase of ε with temperature is interpreted on the basis of the formation of liquid intergranulary phases above 1400°C by progressive melting of some of the grains. A creep exponent of 1.8 was determined. A single value could hardly be given to the activation energy since an S-shape curve was observed in the In εs versus l/T plot, as for most glasses over large temperature ranges. The stress relaxation kinetics was found to follow the Kohlrausch-Williams-Watt expression: σ/σo=exp [-(t/τr)br], where br ranges between 0 (solid state) and I (liquid state) and τr is a characteristic relaxation time constant. As in the case of glasses, τr decreases rapidly whereas br increases from about 0.2 to 0.7 as the temperature increases from 1400 to 1650°C. But again, it is very difficult to get a single value for the activation energy from the In τr versus 1/T plot.

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Some Aspects of Plastic Flow in Silicon Nitride

  • T. Rouxel (a1), J. Rabier (a2), S. Testu (a3) and X. Milhet (a2)

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