Elastic softening versus amorphization in a simple model of ion-induced radiation damage

Recent experimental work on irradiated films of Nb₃Ir have shown that under radiation-induced damage the films show a large ({}40%) reduction in an average shear modulus while remaining essentially crystalline. The shear modulus saturated at larger radiation doses. This paper presents a simple 2D central-force model that displays the same qualitative behavior as it is progressively disordered by random displacements of its lattice sites. Bragg peaks are always present at every level of ``damage,'' although modified by an envelope function. The effect of disorder on the Bragg peaks of the model is solved analytically in closed form; the effect on the elastic moduli is found numerically, via computer simulation.