Theory of intergranular creep cavity nucleation, growth and interaction

Kostas Davanas, Purdue University

Abstract

The evolution of creep cavitation is examined with continuous cavity nucleation and intercavity diffusion and coalesence. The elastic interactions between equilibrium pores is investigated. A repulsive force is found to exist between the pores at small separations, when the applied tensile stress is perpendicular to the center-to-center axis of the pores. Cavity nucleation theories are critically examined and found unable to give realistic nucleation rates, so empirical rates are used. Cavity nucleation is assumed random and dependent only on the steady state creep strain. Using a Monte-Carlo technique, long cylindrical cavities are assumed to nucleate and grow in grain boundaries perpendicular to the applied stress until a critical damage fraction causes rupture. The product of steady state strain rates and rupture times is found insensitive to stress and temperature, consistent with the Monkman-Grant relation. Other power-law dependencies of the rupture time on material parameters are obtained as well. The cavity number density is found dominated by cavity nucleation and also found to increase substantially while the cavity size distribution retains a stable bell-shape. The calculations are extended to include pre-existing cavities on grain boundaries. In this case, the evolution of cavitation is strongly affected by initial cavities when the cavity-to-cavity diffusion distances are small and coarsening is important. The cavity number density is dominated by coarsening and remains relatively constant leading to a rupture time that varies inversely with stress. The effect of 2-D diffusion on cavity growth is also investigated. Assuming lenticular cavities it is found that 2-D diffusion kinetics predict higher rupture times than 1-D diffusion models. Finally, the competitive growth of face and edge/corner cavities is also examined. It is found that previous simplified models underpredict the actual cavitation growth rates. Also, the growth rates of solely edge and solely corner cavities is investigated. When all cavities are on grain faces, the rupture times are longer than when all cavities are at the grain edges or at the grain corners. Edge cavities are found to grow slightly faster than corner cavities.

Degree

Ph.D.

Advisors

Solomon, Purdue University.

Subject Area

Nuclear physics

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