A Unified Approach to Solid-State Amorphization and Melting
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A Unified Approach to Solid-State Amorphization and Melting Nghi Q. Lam and Paul R. Okamoto Introduction The crystalline-to-amorphous (c-a) phase transformation can be induced by a variety of solid-state processes ranging from energetic particle irradiation, interface interdiffusion reactions, hydrogen charging, and mechanical deformation to the application of high pressures.1 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.1"11 A common feature underlies all solidstate amorphization processes: The atomic disorder created in the crystalline lattice in the form of static atomic displacements can induce volume change and elastic softening of the lattice.12'13 A particularly striking example of the softening effect is shown in Figure 1 for the case of radiation-induced amorphization of the intermetallic compound Zx3Al.u The compound, which has the Ll2 (Cu3Au)-type superlattice structure, was irradiated with energetic ions 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.14 The volume depen-
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MRS BULLETIN/JULY 1994
dence of this disorder-induced elastic softening is also similar to that associated with heating.8"12 In both cases, the shear modulus is a linearly decreasing function of volume expansion.21214 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. The far stronger effect of static atomic displacements in inducing lattice softening relative to thermal vibrations suggests that solid-state amorphization can be regarded as a disorder-induced melting process. Indeed, the melting temperatures of materials generally scale with the shear elastic constants so that one can expect the melting temperature of a defective crystal to decrease with increasing amount of static atomic disorder. This connection between static disorder, lattice softening, and melting implies the existence of a critical degree of disorder beyond which the melting temperature of a defective crystal becomes smaller than the glass transition temperature. Hence, melting to a glass of metastable crystal, or solid-state amorphization, can be viewed as a natural consequence of
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