Modeling dislocation evolution in irradiated alloys

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I.

INTRODUCTION

D I S L O C A T I O N S are responsible for much of the observed mechanical behavior of structural materials. The generation and motion of dislocations permit materials to deform plastically without fracturing, and the detailed stress-strain behavior of materials can be correlated with the evolution of the dislocation structure. At high temperatures, dislocation motion and the ability of dislocations to act as vacancy sources and sinks have a major influence on the creep behavior of materials. Dislocations also play a major role in determining the response of materials to displacive irradiation. When structural materials are irradiated with high-energy neutrons or charged particles, higher-than-equilibrium concentrations of both vacancies and interstitials are obtained. Most of these radiation-produced point defects recombine with an antidefect; but a very small fraction of the defects survives. Their survival leads to such observable effects of radiation as void swelling and irradiation creep. The surviving defect fraction is directly related to the density and type of extended defects that act as point defect sinks. In particular, differential partitioning of the two types of point defects is required in order to obtain a net change in the microstracture of irradiated materials. This, in turn, requires that more than one type of sink exist, and that at least one of the sinks has a capture efficiency for either vacancies or interstitials that is different from that of the other sink(s). One example of this differential partitioning is that which arises due to the interaction of the longrange strain fields of dislocations and interstitials. Because

R.E. STOLLER, Research Staff Member, is with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376. This paper is based on a presentation made in the symposium "Irradiation-Enhanced Materials Science and Engineering" presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD. METALLURGICAL TRANSACTIONS A

of this interaction, dislocations have a larger capture efficiency for interstitials than they do for vacancies, and the dislocation capture efficiency for interstitials exceeds that of cavities for interstitials. These two capture efficiency differences provide the interstitial ~bias" that drives void nucleation and growth in irradiated materials. The ratio of the dislocation to cavity sink strength has a significant impact on void nucleation, and this ratio largely determines the swelling rate at steady state. This concept is discussed in detail by Lee and Mansur. t~j Since these sink strengths change during irradiation, an explicit model of their evolution is required to simulate swelling or creep. A model of dislocation evolution based on the reaction rate theory has been developed and coupled with a model of cavity evolution that has been shown to

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