Generalized Melting Criterion for Amorphization
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GENERALIZED MELTING CRITERION FOR AMORPHIZATION 2
1 1 1 2 R. DEVANATHAN , , N. Q. LAM , P. R. OKAMOTO and M. MESHII 1 Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 2 Dept. of Materials Science and Engineering, Northwestern University, Evanston, IL 60208
ABSTRACT We present a thermodynamic model of solid-state amorphization based on a generalization of the well-known Lindemann criterion. The original Lindemann criterion proposes that melting occurs when the root-mean-square amplitude of thermal displacement exceeds a critical value. This criterion can be generalized to include solid-state amorphization by taking into account the static displacements. In an effort to verify the generalized melting criterion, we have performed molecular dynamics simulations of radiation-induced amorphization in NiZr, NiZr 2 , NiTi and FeTi using embedded-atom potentials. The average shear elastic constant G was calculated as a function of the total mean-square atomic displacement following random atom-exchanges and introduction of Frenkel pairs. Our results provide strong support for the generalized melting criterion.
INTRODUCTION Over the past three decades, extensive studies have been performed in an effort to understand solid-state amorphizing transformations. Many of these studies have focussed on radiationinduced amorphization [1], which represents a destabilization of the crystalline lattice caused by particle bombardment at temperatures well below the melting point. In particular, electron irradiation-induced amorphization has attracted considerable interest because it results in simple damage, and can be studied in situ in a high voltage electron microscope (HVEM). In addition to HVEM experiments, molecular dynamics simulations have been employed to understand the mechanism of amorphization [2-6]. These simulations have shed new light on the atomistic details of the amorphization process. They have also brought forth evidence indicating the existence of parallels between melting and solid-state amorphization. In this report, we present a generalized melting criterion that offers a unified picture of melting and solid-state amorphization.
CHARACTERISTICS
OF AMORPHIZATION
Irradiation by MeV electrons produces Frenkel pairs and chemical disorder. The relative importance of these two defects to the amorphization process has been a contentious issue [7-9]. In order to understand the mechanism of amorphization, we have performed molecular dynamics simulations using embedded-atom potentials in NiZr, NiZr 2 , FeTi and NiTi. Details of the simulations and the interatomic potentials can be found elsewhere [3-5]. The behavior of the compounds that undergo amorphization fall into two distinct classes, which are represented by NiZr 2 and FeTi. In this section, we will discuss the changes in properties of NiZr 2 and FeTi with Frenkel pair introductions and random atom exchanges. The doses for these two types of damage are in displacements per atom (dpa) and exchanges per atom (epa), respectively. Figure 1
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