A Unified Approach to Solid-State Amorphization and Melting

Abstract

The crystalline-to-amorphous (c-a) phase transformation can be induced by a variet of solid-state processes ranging from energetic particle irradiation, interface inter diffusion reactions, hydrogen charging and mechanical deformation to the application of high pressures. During the past decade, such transformations have become the focus of considerable research not only because of their potential technological applications, but also because of strong scientific interest in the relationship between the c-a transition and the melting process.A common feature underlies all solid state amorphization processes: The atomic disorder created in the crystalline lattice in the form of static atomic displacement can induce volume change and elastic softening of the lattice. A particularly striking example of the softening effect is shown in Figure 1 for the case of radiation-induced amorphization of the intermetallic compound Zr3Al. The compound, which has the Ll2 (Cu3Au)-type superlattice structure, was irradiated with energetic ion at room temperature in a high-voltage electron microscope interfaced to a tandem ion accelerator. The rapid decrease in the intensities of both fundamental and superlattice reflections show that irradiation introduces antisite defects (chemical disorder) as well as static atomic displacements. The disordering of the long-range ordered structure, which occurs prior to the onset of amorphization, is accompanied by a volume expansion of about 2.5% and a ~25% decrease in the average velocity of sound. This decrease in sound velocity corresponds to a ~50% decrease in the average shear modulus, which is comparable to that observed for many metals during heating to melting. The volume dependence of this disorder-induced elastic softening is also similar to that associated with heating. In both cases, the shear modulus is a linearly decreasing function of volume expansion. However, for a given amount of expansion, the softening associated with static atomic displacements is nearly twice as large as that associated with increasing anharmonic lattice vibrations.