Effect of Temperature on the Formation of Creep Substructure in Sodium Chloride Single Crystals

Authors


  • D. S. Wilkinson–contributing editor

Abstract

The effect of temperature on the formation of creep substructure in sodium chloride single crystals was studied in the temperature range 573 to 873 K. Microstructural observations conducted on polished specimens deformed under a constant value of normalized stress, σ/G∼ 4 × 10−4, where σ is the applied stress and G is the shear modulus, showed different types of substructural features such as a high density of dislocation etch pits, cells, and subgrains. Detailed measurements of the sizes of the cells and subgrains revealed that the cell size was essentially independent of temperature while the subgrain size increased only by about a factor of 2 for a 300 K increase in temperature. Although the cell and subgrain sizes were not significantly dependent on temperature in comparison to the effect of stress, there was a qualitative change in the substructure morphology with a variation in temperature; i.e., the cells and subgrains were better defined at higher temperatures. The volume fraction of the cell boundaries decreased with increasing temperature, thereby indicating a refinement of the microstructure at the higher temperatures. These observations suggest that while temperature does not change the final dimensions of the substructure significantly, it appears to influence the kinetics of substructure formation by influencing the recovery rate. As a result, the stresses acting on the cell boundaries increase considerably with increasing temperature to values several times the applied stress.

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