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to 295 K by 1-MeV electrons and at 295 K with 1-MeV Kr +. The onset of amorphization is observed when the long-range order parameter decreases substantially with both electron and Kr +. Diffuse streaks are observed in the diffraction pattern prior to amorphization. This is attributed to a softening of the shear elastic constant, C' = (Cn - C12)/2, due to static displacement of atoms. This observation is consistent with a large softening of shear modulus reported in Zr3A1 irradiated with 1-MeV Kr § for which a shear elastic instability has been identified prior to the onset of amorphization. In order to complete amorphization with electrons, a large dose, ~>26 dpa, is required at 10 K, while with Kr +, amorphization is completed by a dose of only 0.8 dpa. After the same dose of 26 dpa with electrons at 57 and 295 K, only partial amorphization and rhombohedral distortion are observed. Defect aggregation is also observed during irradiation at all three temperatures. The anomalously large dose required for complete amorphization with electrons is ascribed to point defect migration, as evidenced by the formation of defect aggregates. The partial amorphization at higher temperatures is explained by two factors, faster defect migration and rhombohedral distortion, which both provide alternative paths to amorphization for reducing the accumulated strain.

I.

INTRODUCTION

THE Zr3A1 was reported to be amorphized over a wide temperature range, from 30 to 700 K, by high energy (0.5- to 2.0-MeV) Ar § irradiation, tlj However, amorphization by electron irradiation has not been clearly established. Mori et al. ~21 reported that Zr3A1 became chemically disordered, but no evidence of amorphization was observed at 162 K after 2-MeV electron irradiation to a dose of 1.3 x 1023 e/cm 2. Instead, they observed defect clusters which are usually observed when amorphization does not occur. On the other hand, Carpenter and Schulson t3] observed a faint diffraction halo by 1-MeV electron irradiation at temperatures between 130 to 320 K, as well as the complete disappearance of superlattice spots, after slightly less than 1 dpa (1 dpa corresponds to 2.3 x 102e e/cm 2 for 1-MeV electrons). At 575 K, steady state of partial disordering was observed and doses of several dpa were required to produce a faint diffraction halo. They suggested that the diffraction halo was due to the amorphous phase produced on a very localized scale. Further study is necessary to determine if complete amorphization occurs in Zr3A1 with electron irradiation. Since the dose required for complete

J. KOIKE, formerly with the Materials Science Division, Argonne National Laboratory and the Department of Materials Science and Engineering, Northwestern University, is a Postdoctoral Fellow at the Center for Materials Science, Los Alamos, NM 87545. P.R. OKAMOTO, Senior Scientist, and L.E. REHN, Group Leader, Irradiation and Kinetic Effects, are with the Materials Science Division, Argonne National Laboratory, Argonne, IL 60439. M. MESHII, Professor, is with the Department o