Elastic Instability During Amorphization of Internetallic Compounds by Charged-Particle Beams
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ELASTIC INSTABILITY DURING AM4ORPHIZATION OF INTERNETALLIC COMPOUNDS BY CHARGED-PARTICLE BEAMS* P. R. OKAMOTO, L. E. REHN, J. PEARSON, R. BHADRA, AND M. GRINSDITCH Materials Science Division, Argonne National Laboratory, Argonne, IL
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AB STR ACT Transmission electron microscopy (TEN) and Brillouin scattering methods were used to determine the lattice dilatation and shear elastic constant as a function of the degree of long-range order during room temperature irradiation of Zr 3Al with 1.0-MeV Kr+. The results indicate that the onset of amorphization in Zr 3Al is triggered by an elastic shear instability, and that the instability is directly related to the volume dilatation associated with the destruction of long-range order. It is also shown that the volume dependence of the shear elastic constant associated with radiation-induced disordering and amorphization is virtually identical to that associated with the heating to melting of many metals. I NTR ODUCT ION When certain crystalline solids are subjected to disordering processes at sufficiently low temperatures, the damage to the crystalline lattice can ultimately lead to a transformation to the amorphous state. Well documented observations include, for example, the formation of amorphous phases during low temperature irradiation and ball-milling of intermetallic phases [1]. The transformation to the amorphous state is thought to occur when the free energy of the defect-free compound plus the damage energy stored in the lattice in the form of chemical disorder and/or point-defects and their clusters, exceed the free energy of the amorphous phase. However, the basic mechanism of the transformation, and how it couples to the thermodynamic driving forces, is not well understood. In analogy to the melting process, Johnson [1] has recently suggested that an elastic shear instability may be involved in solid-state amorphizing transformations. In the present paper we report the results of a combined TEN and Brillouin scattering study of radiation-induced amorphization of the intermetallic compound, Zr 3 Al (L1?-Cu 3 Au type), which provides direct evidence in support of a shear instability hiypothesis for amorphization. EXPERIMENTAL RESULTS AND DISCUSSIONS. TEN and Brillouin scattering techniques were employed to measure changes in the degree of long-range order, 5; in the lattice dilatation, Aa/a; and in a shear elastic constant of Zr 3 AI during room temperature bombardment with 1.0-MeV Kr+. Details of the-experimental techniques and preliminary findings have been described previously [2,3]. Bulk specimens of Zr3 Al were individually irradiated to different doses in order to provide specimens for shear constant measurements by the Brillouin technique. Trim [4] calculations assuming a threshold displacement energy of 25 eV were used to convert ion fluence to avera~e number of displacements per atom (i. e., 1 dpa = 3.19 x 1014 Kr+/cm ) in the first 300 nm below the the specimen surface, which is the depth probed by the Brillouin technique. In addition, in situ HVEM
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