Creep substructure formation in sodium chloride single crystals in the power law and exponential creep regimes

Abstract

Single crystals of sodium chloride were deformed in compression along the 〈001〉 orientation in the temperature range 373–1023 K. At low temperatures and high stresses, the creep rate decreased monotonically with increasing strain and no steady state was observed even after deformation to large values of strain. True steady state creep behavior was observed only at high temperatures and low stresses where the variation in the normalized creep rate with the normalized stress exhibited a power law relation with a stress exponent of about 5. However, exponential creep was observed when the normalized stress σ/G is greater than 3 × 10 −4, where σ is the applied stress and G is the shear modulus. A microstructural evaluation of specimens crept in the power law and the exponential creep regions showed a gradual transition from large equiaxed primary subgrains at high temperatures and low stresses to a random array of dislocations and poorly formed subboundaries at low temperatures and high stresses. Secondary subboundaries and narrow-walled equiazed cells were observed within the primary subgrains above 873 K and for σ/G ⩽ 10 −4 which generally corresponded to steady state creep conditions. At higher values of normalized stress and lower temperatures, there was a decrease in the number of secondary subboundaries and a corresponding increase in the width of the cell boundaries. No steady state creep was observed under these conditions. The transition from power law to exponential creep was associated with increases in the dislocation density, the cell boundary width and the aspec ratio of the subgrains along the primary slip planes. The relation between dislocation substructure and creep behavior is also discussed.