Microcrack-enhanced creep in polycrystalline material at elevated temperature

Abstract

A grain size dependent rheological model is presented for high temperature creep that is capable of predicting both primary and tertiary as well as secondary creep. Primary and tertiary creep rate are shown to be strongly influenced by grain size, whereas secondary or minimum creep rate is rather insensitive to it. During the primary creep stage creep rate increases with decreasing grain size, but the reverse is true in the tertiary or accelerating range. An increase in grain size also dictates decrease in the time to reach minimum creep rate concomitant with a decrease in strain. The model is based on intragranular dislocation creep enhanced by grain-facet size cracks produced during deformation by the embrittlement process that is caused by an intergranular sliding mechanism. Incorporation of the kinetics of microcracking activity is the foundation of the theory.