Masafumi Akamatsu

APFIM investigation of clustering in neutron-irradiated FeCu alloys and pressure vessel steels

Abstract Pressure vessel steels used in PWRs are known to be prone to hardening and embrittlement under neutron irradiation. The changes in mechanical properties are commonly supposed to result from the formation of point defects, dislocation loops, voids and copper-rich precipitates. However, the real nature of the irradiation induced damage, in these particularly low copper steels ( A new experimental work has been carried out thanks to atom probe and field ion microscopy (APFIM) facilities and, more particularly with a new generation of atom probe recently developed, namely the tomographic atom probe (TAP), in order to improve: - the understanding of the complex behavior of copper precipitation which occurs when low-alloyed Fe Cu model alloys are irradiated with neutrons; - the microstructural characterization of the pressure vessel steel of the CHOOZ A reactor under various fluences (French Surveillance Programme). The investigations clearly reveal the precipitation of copper-rich clusters in irradiated Fe Cu alloys while more complicated Si, Ni, Mn and Cu-solute ‘clouds’ were observed to develop in the low-copper ferritic solid solution of the pressure vessel steel.

Microstructural characterization of atom clusters in irradiated pressure vessel steels and model alloys

Abstract In order to characterize the microstructural evolution of the iron solid solution under irradiation, two pressure vessel steels irradiated in service conditions and, for comparison, low copper model alloys irradiated with neutrons and electrons have been studied. The characterization has been carried out mainly thanks to small angle neutron scattering and atom probe experiments. Both techniques lead to the conclusion that clusters develop with irradiations. In Fe-Cu model alloys, copper clusters are formed. These clusters do not appear to be pure copper. Nevertheless, their actual shape and composition are still a matter of investigation. In the low copper industrial steels, the feature is more complex. Solute atoms like Ni, Mn and Si, sometimes associated with Cu, segregate as clusters. These Si, Ni, Mn associations, may facilitate the copper segregation although the initial iron matrix contains a low copper concentration.

Experimental evidence of several contributions to the radiation damage in ferritic alloys

Abstract In order to identify the various contributions to the radiation damage of RPV steels, we performed a comparative analysis of a set of our experiments. They were carried out on three ferritic Fe Cu alloys irradiated at 290°C with electrons or neutrons. The evolution of the mechanical properties has been characterized by Vickers (4.9 N) hardness and the microstructure has been studied by small angle neutron scattering (SANS), Atom Probe (AP) and Positron Annihilation. We have identified three irradiation-induced features which can harden low copper ferritic model alloys: point-defect clusters and copper-rich clusters, produced by displacement cascades, as well as copper-vacancy(ies) complexes, induced by isolated vacancies.

Implicit Stress Integration for High-Temperature Inelastic Constitutive Models Using Linearization

In this study, a linearization approach is used to develop an implicit integration scheme for high-temperature inelastic constitutive models based on non-linear kinematic hardening. A non-unified model is considered in which inelastic strain rate is divided into the transient and steady parts driven, respectively, by effective stress and applied stress. By discretizing the constitutive relations using the backward Euler method, and by linearizing the resulting discretized relations, a tensor equation is derived to iteratively achieve the implicit integration of constitutive variables. The integration scheme is then programmed as a subroutine in a finite element code and applied to a lead-free solder joint analysis. It is thus demonstrated that the integration scheme affords the quadratic convergence of iteration even for considerably large increments.

Comparison of Cyclic Inelastic Features of a Lead-Free Solder Alloy and a Nickel-Base Superalloy

In this study, a time-dependent constitutive model with nonlinear kinematic hardening is applied to simulating the cyclic inelastic behavior of a lead-free solder alloy Sn-3.0Ag-0.5Cu at room temperature and a nickel-base superalloy IN738LC at 850 ̊C. The development of kinematic hardening is assumed to consist of strain hardening and power-law dynamic recovery. Then, by simulating the hysteresis loops of stress and strain under cyclic loading, it is shown that the nonlinearity in the dynamic recovery of kinematic hardening strongly influences the Bauschinger effect in simulated cyclic inelastic behavior, and that the Bauschinger effect of Sn-3.0Ag-0.5Cu is much weaker than that of IN738LC. It is thus demonstrated that quicker evanescence of anisotropic hardening renders the Bauschinger effect less significant.