Bubble Formation in Zr Alloys Under Heavy Ion Implantation
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ABSTRACT We report here the results of a study conducted to examine the effect of Kr ion irradiation on bubble formation in Zr alloys. We used the HVEM/Tandem facility at Argonne National Laboratory to irradiate several Zr alloys, including Zircaloy-2 and Zircaloy-4, at temperatures from 300 to 800 C and to doses up to 2 x 1016 ion.cm-2. Both in-situ irradiation of thin foils as well as irradiation of bulk samples with an ion implanter were used in this study. For the thin foil irradiations, a distribution of small bubbles in the range of 30-100 A was found, at all temperatures with the exception of the Cr-rich Valloy where bubbles of 130 A were found. The irradiation of bulk samples at high temperature (700-800 C) produced large faceted bubbles (up to 300 A) after irradiation to 2 x 1016 ion.cm-2. The results are examined in the context of existing models for bubble formation and growth in other metals. INTRODUCTION Zirconium alloys have been used for nuclear reactor fuel cladding for the last 40 years because of their combination of low neutron absorption, excellent resistance to high temperature corrosion, and good mechanical properties [1]. Their response to radiation damage has been extensively studied [1,2]. In particular they undergo radiation creep and growth but in contrast to steels do not exhibit void swelling. The absence of voids in neutron irradiated zirconium alloys has been explained by the fact that both vacancy and interstitial loops form under irradiation, that is, the void embryos collapse to loops which are stable [3]. Small voids have been occasionally reported in zirconium alloys, such as Zircaloys [4], specially after implantation with ions [5,6 ]. This suggests that bubble formation could be induced in zirconium alloys by ion implantation under suitable conditions. We have in this work conducted a systematic study of the conditions of bubble formation in zirconium alloys under Kr ion irradiation. The objective was to determine the boundaries of the different regimes of bubble behavior, specially the regions of gas and bubble mobility that enable the formation of larger bubbles by coalescence. A second goal was to derive mechanistic understanding of the process of bubble formation and growth in zirconium alloys under irradiation.
EXPERIMENTAL METHODS Five different materials were tested: four commercial alloys, (Zircaloy-2, Zircaloy-4, Valloy, NSF) and nominally pure zirconium. The commercial alloys were furnished by GE, and the Zr (99.8%) was obtained from Goodfellow. The composition of these alloys (in wt. %) is given in table 1. 201
Mat. Res. Soc. Symp. Proc. Vol. 398 0 1996 Materials Research Society
Table 1 - Chemical Composition ( wt %) of the Studied Materials
*
Alloy Sn Fe Cr Ni Nb Zircaloy 4 1.5 0.21 0.1 , * ............................. -............................. ".......................... i........................... S............................................. i........................ ............................ Zcao .2...................... ... .......... 15......
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