Irradiation damage in 304 and 316 stainless steels: experimental investigation and modeling. Part II: Irradiation induced hardening

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Abstract

The consequences of irradiation damage in austenitic stainless steels on their mechanical properties, namely the yield stress, are investigated both experimentally and theoretically. The observed hardening is correlated with the quantitative characteristics of irradiation defects population. A simple model allowing for the defaulting of Frank loops under stress predicts the hardening and its saturation at large doses.

Introduction

In the companion paper [1], a systematic quantitative characterization of irradiation induced microstructures in 304L, 316 and 316 Ti austenitic stainless steels has been performed on samples irradiated under different conditions: flux, dose, energy spectra (fast and mixed spectrum) and irradiation temperature (330 and 375 °C). These results have been modeled using a cluster dynamics approach specially adapted to account for the evolution of the point defects created by irradiation and for the formation of a Frank loop dislocation substructure.

As a result of this microstructural evolution under irradiation, these materials undergo a substantial increase in yield stress and reduction in ductility. In the present contribution, we will focus our attention only on the consequences of this microstructural evolution on the yield stress. We will assume that the hardening is mainly due to the population of Frank loops, as irradiation induced precipitation (e.g. γ), which could contribute to irradiation hardening, was not observed in our specimens. This hardening depends both on the initial metallurgical state of the alloy and on the irradiation conditions. Of special interest will also be the question of the possible saturation of microstructural evolution and related hardening for long term irradiation.

The aim of this paper is to provide a quantitative measurement of the evolution of the mechanical properties (yield stress) by post-irradiation tensile tests at constant strain rate. The modeling of the evolution of yield stress for a given microstructure will be proposed using classical dislocation theory. The parameters entering this model will be identified from the experimental tensile curves, and the prediction of the model will be compared with experiments for larger doses.

The materials investigated and the irradiation conditions have already been reported in companion paper I; they will not be recalled in this paper. In Section 1, we outline the proposed mechanism for the evolution of yield stress, and give a brief summary of the findings of paper I. Section 2 presents the experimental conditions for tensile testing and the measured yield stresses for the different alloys and irradiation conditions. In Section 3 we propose a model for the yield stress evolution coupling the hardening by dislocation loops and the question of their stability. The predictions of this model are finally compared with the experimental results.

Section snippets

Tensile testing method

Small cylindrical specimens with a gauge length of 12 mm and a diameter of 2 mm were machined from the three investigated materials: solution annealed SA 304L, CW 316 and CW Ti-modified 316 stainless steels (Fig. 1).

For the SA 304L plate, specimens were cut and machined perpendicularly to the rolling direction at mid-thickness of the plate. For the CW 316 bars, specimens were sampled along the drawing direction, at about mid-radius of the rod.

These specimens were irradiated in different

Modeling the yield stress

In this section we propose a model for the yield stress evolution coupling the hardening by dislocation loops and the question of their stability, and we compare the predictions of this model with the experimental results.

Conclusions

A systematic experimental quantitative characterization (mainly microstructural investigation in a companion paper [1] and yield stress determination in this contribution) has been performed on irradiated SA 304, CW 316 and CW 316Ti austenitic stainless steels for different irradiation conditions in terms of temperatures, fluxes, doses and energy spectra. The evolution of the microstructure in terms of dislocation loops has been simulated with a ‘cluster dynamic’ model. The description of the

Acknowledgements

Enlightening discussions with Dr D. Rodney are gratefully acknowledged. Authors are grateful to Dr G.R. Imel at ANL, US, for performing neutron irradiation in EBR-II reactor, Dr V. Golovanov and Professor V. Shamardin at RIAR, Russia for performing neutron irradiations in BOR-60 reactor and to Professor V. Prokhorov for post-irradiation tensile tests. This work was performed in the frame of the French R&D Project ‘PWR Internals’ sponsored by Electric Power Research Institute (Joint Owner Baffle

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