Anelasticity and Anisotropy in Relation to Creep and Fatigue Resistance

EPSRC research grant GR/J31544

Summary of final report

Equipment has been constructed for strain measurement down to 10^-7 and the variation of strain with time under changing stresses at elevated temperatures in vacuum, without significant errors due to temperature fluctuations. New information has been gained on the mechanical response to transient stress conditions at low stress levels for diverse metallic materials, including Al, Ti, Cu, Zn, Mg, Ni and its alloys, austenitic steels and oxide dispersion strengthened materials. In particular, the phenomenon of anelastic recovery and relationships governing its occurrence after stress removal have been examined. Across this diversity of materials, relationships of similar form have become apparent, pointing to the significance of dislocation movements involving the time dependent straightening of dislocation lines that have previously been bowed under small applied stress. The importance of dislocation climb was examined with regard to diffusional processes and the likelihood of a dominance of dislocation pipe diffusion becoming rate controlling at the lower end of the temperature range. The experiments showed a logarithmic time dependence of anelastic strain recovery, but at initial rates that could not be fully accounted for solely by pipe diffusion, even at the highest dislocation densities, thus implying that some other mechanisms were also operating. A definitive link was found between anelastic relaxation and the kinetics of primary creep. The latter was shown to follow an Andrade type power law even at lowest stresses, giving strains two orders of magnitude below than those previously investigated. The link established was between the initial rate of primary creep under a specific stress and the initial rate of anelastic strain recovery when that stress was removed. At low strains, the ratio between these strain rates was dependent on strain level, as would be expected from a contribution from dislocation bowing but, at somewhat higher strains exceeding about 10^-5 the two rates were closely similar. The effects of microstructure and of anisotropy were examined. Under given conditions, the extent and rates of this form of creep and anelastic relaxation were much greater for materials in a heavily worked state, with high yield stress, than when they were in an annealed, softer, condition. From a scientific point of view the results clearly point to the significance of small scale dislocation motion. Discussions with industrialists suggest the work can be related to a wide range of practical problems, covering the fields of (1) nanoscale engineering involving the performance of lightly stressed components; (2) analysis of thermal ratchetting leading to fatigue in composite structures; (3) behaviour of long wire products; (4) stress relaxation in bolting materials, (5) dimensional stability of strong heavily cold worked materials under low stresses at only slightly elevated temperatures; (6) control of distortion after straightening, and (7) uncoiling of coiled sheet after finishing.

Contact:

Prof GW Greenwood,
Department of Engineering Materials,
University of Sheffield,
Mappin Street, Sheffield S1 3JD
Telephone: 0114 222 5517
Fax: 0114 222 5943