Microstructural Aspects of Irradiation Damage in A508 Gr 4N Forging Steel: Composition and Flux Effects

Abstract

Neutron irradiation can promote significant changes in the microstructure and associated mechanical properties of low alloy steels. In particular, irradiation can induce the formation of non-equilibrium phases and segregation, which may lead to a degradation in toughness. In this study, the microstructural changes caused by neutron irradiation have been characterized in A508 Grade (Gr) 4N-type steels (~3.5% Ni) using a variety of state-of-the-art analytical techniques including 3D-Atom Probe Field-Ion Microscopy and Small Angle Neutron Scattering, along with post-irradiation annealing studies combining Positron Annihilation Lineshape Analysis and hardness measurements. Important differences between conventional and "superclean" A508 Gr 4N steel have been identified in this investigation. The data indicate that Ni is not the controlling factor in the irradiation damage behavior of these materials; rather, the Mn content of the steel is a dominant factor in the irradiation-induced microstructural development of soluterelated hardening features.

The Dominating Mechanisms for the Formation of Solute-Rich Clusters in Steels under Irradiation

“…1i3), is possible, so these types of clusters are thermally less stable. This reminds of the so-called "unstable matrix damage" sometimes invoked to explain neutron dose rate effects in RPV steel embrittlement [69,70]. In summary, the conceptual model depicted in Fig.…”

“…Evidence of the formation of vacancy-Mn-Ni solute clusters from PAS has indeed been found [73][74][75][76]. It is unclear, however, whether these clusters can be associated with the so-called "unstable matrix damage" postulated as an additional hardening contribution at high neutron dose rate [69,70], because it is unclear whether they will actually measurably contribute to hardening and ensuing embrittlement. MD simulations suggest that the presence of vacancies in the cluster reduces, rather than increasing, its strength [95], when interacting with edge dislocations.…”

Current understanding of radiation-induced degradation in light water reactor structural materials

“…Mn is an alloying element, typical content of 0.5-1.5% in RPV steels, and is known from recent data to enhance radiation hardening and embrittlement [13,[17][18][19]. A clear enhancement was observed in high-Cu model steels with increasing Mn from 0.01 to 1.63% [17].…”

RPV-1: A Virtual Test Reactor to simulate irradiation effects in light water reactor pressure vessel steels

“…• Nickel has a strong deleterious impact on irradiationinduced embrittlement of RPV steels (e.g. [30][31][32][33][34][35][36]). This impact increases synergistically with the copper content and may become very high for nickel contents over about 1.1%.…”