Theoretical prediction and direct observation of dislocation-free zone formation near a grain boundary in austenitic stainless steel under electron irradiation

Abstract

We have investigated the radiation-induced segregation (RIS) near a grain boundary in an Fe± Cr± Ni alloy under electron irradiation taking account of the evolution of faulted dislocation loops and network dislocations. First, we predicted, on the basis of the rate theory, the formation dislocation-free zone (DLFZ) in the vicinity (about 30 nm) of a grain boundary and then carried out direct observation of the DLFZ by in-situ irradiation with high-voltage electron microscopy. The inter-relationship between RIS and heterogeneous defect clustering near a grain boundary has been substantially clari® ed. § 1. INTRODUCTION A defect-free zone (DFZ) is known to occur near a grain boundary in metals. Some examples are the formation of a precipitation-free zone (PFZ) of a so-called denuded zone under thermal ageing (Nicholson 1968), of a void-free zone (Anthony 1972) and a helium-bubble-free zone (Ryazanov et al. 1985) under irradiation. Such heterogeneous microstructural formation of defect clusters near a grain boundary is commonly understood in terms of the interrelation between the di usional process of supersaturated impurities or point defects, that is vacancies and interstitials, towards the grain-boundary sink and the microstructural evolution process there. An exceptional case is DFZ formation by deformation, which has been observed not near a grain boundary but in a matrix of metals during deformation followed by quenching or irradiation (Suzuki et al. (1991) and reference therein).In spite of a large body of experimental evidence on DFZ relatively few extensive theoretical studies can be found in the literature (for example Ryazanov et al. (1985)). The reason for this is that a heterogeneous microstructural treatment in a theoretical model such as the di usional rate equation model, becomes fairly complex to solve dynamically, compared with a homogeneous microstructural study.In this letter we predict dislocation-free zone (DFL Z) formation near a grain boundary in an austenitic stainless steel under electron irradiation by means of computer simulation, directly solving the coupled rate equations of the alloy elements and of point defects which take dislocation evolution into account. The present model is a modi® cation of a previous model developed for radiation-induced segregation (RIS) near a grain boundary under irradiation (Watanabe, Sakaguchi, Hashimoto andTakahashi 1994, 1995). We also present a con® rmative observation of DLFZ formation near a grain boundary during in situ observation under 1 MeV electron irradiation with a high-voltage electron microscope.