Over the past 15 years important advances have been made in the experimental study of the microstructural changes occurring during the non-linear steady-state creep of single phase crystalline matter at elevated temperatures. Curiously, although the results of these painstaking studies have gone a long way toward elucidating the mechanism of this phenomenon, they have been largely ignored in favour of some simple dislocation mechanisms that are not only inconsistent with these observations, but are also unable to describe correctly the known phenomenology. This review concentrates primarily on the recent experiments on microstructural alterations occurring during creep; however, it also surveys the many mechanistic models that attempt to describe this phenomenon, and finds them all deficient.

Steady-state creep of single-phase crystalline matter at high temperature

Recovery and Recrystallization during High Temperature Deformation

Five-power-law creep in single phase metals and alloys

Mechanical properties of alloys consisting of two ductile phases

Substructure of type 316 stainless steel deformed in slow tension at temperatures between 21° and 816°C

Specimens of 304 stainless steel subjected to different thermomechanical histories develop different internal stresses, σi, and different substructures. Creep rate is uniquely related not to the applied stress, σA, but to the effective stress, σ*=(σA−σi). Values of σ* are determined from experimental results and σi calculated from σi=(σA−σ*). Results show σi increases with the applied stress according to σi∝σA1.7. Transmission electron microscopic observations show that the density of dislocations within subgrains, ϱD, and the subgrain diameter,D, vary with applied stress according to: ϱD∝σAK,D ∝ σA−0.8, whereK=1.4 to 2.0. Subgrain misorientation is independent of creep stress, strain, or temperature. The contributions of these structural variables to the internal stress are discussed.

Internal stresses and structures developed during creep

Previous studies of solid-solution strengthening of alloys of iron revealed that titanium-bearing alloys exhibit unusual strength between 300 and 600°C. The nature and the stability of this strengthening effect of titanium were investigated because of its possible value in creep-resistant steels. Repeated hot-hardness surveys indicated that the hardening remained even after annealing to temperatures as high as 900°C. Creep studies at 500°C revealed that the strengthening was stable for at least 200 h and that it was associated with a high creep-activated energy. On the basis of these results and the results obtained by other workers, we suggest that the high-temperature strengthening results from the presence in the lattice of stable clusters containing titanium, carbon, and/or nitrogen atoms which reorient in the stress field of mobile dislocations and thereby impede their motion.

L. J. Cuddy

Papers

Internal stresses and structures developed during creep

Elevated-temperature strengthening of iron alloys by titanium