Amorphization in Zr₃Al irradiated with 1-MeV e^- and Kr⁺

In situ electron microscopy is used to study irradiation-induced amorphization in Zr₃Al from 10 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′ = (C₁₁-C₁₂)/2, due to static displacement of atoms. This observation is consistent with a large softening of shear modulus reported in Zr₃Al 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.