Amorphization of Zr 3 Al by Hydrogenation and Subsequent Electron Irradiation
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AMORPHIZATION OF ZR3AL BY HYDROGENATION AND SUBSEQUENT ELECTRON IRRADIATION W. J. Meng, J. Koike, P. R. Okamoto, and L. E. Rehn MATERIALS SCIENCE DIVISION, ARGONNE NATIONAL LABORATORY ARGONNE, ILLINOIS 60439 ABSTRACT 1-MeV electron irradiation of hydrogenated Zr3Al (Zr3AIHO.96) at 10K is studied. A more than 20 fold reduction in the critical dose required for complete amorphization is observed for the hydrogenated specimen as compared to the un-hydrogenated Zr3Al under identical irradiation conditions. INTRODUCTION Recent work on the ordered intermetallic compound Zr3AI (L12type superlattice) has demonstrated a correlation between a critical volume expansion of the crystalline lattice and the onset of amorphization during ion bombardment. The volume expansion results in a dramatic softening of the shear modulus, which is remarkably similar to that seen in simple metals during heating to melting[l]. Although the volume expansion preceding amorphization during ion bombardment is associated with the loss of chemical long-range order (CLRO)[2], a lattice dilation of similar magnitude, e.g., 3(Aa/a) - 2.5%, also occurs prior to the onset of amorphization of Zr3Al during hydrogenation[3]. In contrast to irradiation, no significant change in CLRO occurs during hydrogenation[3], indicating that the expansion results from hydrogen occupying interstitial sites in the ordered compound. These observations imply that dilatational strain, rather than chemical disordering per se, plays the key role in the onset of amorphization. Moreover, by also reducing the free energy difference between the crystalline and amorphous phases, the presence of hydrogen can be expected to reduce the amount of displacement damage required to completely amorphize an intermetallic compound. Previous work by Koike et al.[4] on un-hydrogenated Zr3Al has shown that a critical electron dose in excess of 26 displacements per atom (dpa) is needed to completely amorphize the compound when irradiated at 10K with l-MeV electrons. This critical dose is anomalously large compared to the typical 1 dpa or less reported in a recent survey of intermetallic compounds which undergo amorphization during electron irradiation[5]. In contrast to these more easily amorphizable materials, point defect aggregation was observed during irradiation of Zr3AI at 10K, which suggests that the anomalously large critical dose is related to dynamical point-defect recovery processes. As
Mat. Res. Soc. Symp. Proc. Vol. 128. t1989 Materials Research Society
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will be shown, under identical irradiation conditions, the presence of hydrogen causes a more than a 20 fold reduction in the critical dose. EXPERIMENTAL PROCEDURES Zr 3 Al alloys used in this study were prepared by arc-melting, and subsequently homogenizing at 9000C for 12 days. The homogenized alloy consists predominantly of L1 2 -Zr 3 A1, with a few percent of the Zr 2 AI phase. Details of the hydrogenation procedures have been previously described[3]. TEM samples of hydrogenated specimens were prepared by electropolishing
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