Creep Mechanism Transition in Precipitation Strengthened Alloys: Theory and Validation

摘要

Experimental data from a low-alloy ferritic steelhave been analysed to validate one of the predictions of a newtheory of creep in precipitation-strengthened alloys: that atransition in rate-controlling mechanism should be observed incertain commercial alloys under service conditions. The theoryproposes that the climb/glide process of creep in these alloys canbe modelled by a parallel coupling of the climbing dislocationnetwork with two further sequential mechanisms: stochastic"recovery" of dislocations with a rate controlled by theparticulate dispersion parameters; and viscous-glide. There is nothreshold stress in the theory and creep rates are controlled bythe slower of the two sequential mechanisms. Control byviscous-glide is predicted to dominate in practice only when theparticle volume fraction is small and the applied stress low. Thetransition has been confirmed using minimum creep rate data atfive temperature levels for 2?CrlMo steel, a ferritic alloycontaining approximately 1% volume fraction of carbideparticles. Also in agreement with theory, a large database from anickel-base superalloy with a much higher particle volumefraction, has been modelled solely with the equation forrecovery-control. The steel data exhibit certain characteristics ofconventionally defined threshold behaviour and the new theorytherefore offers an alternative (non-threshold) explanation.