A Model for Irradiation-Induced Amorphization
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S. X. Wang, L. M. Wang and R. C. Ewing, Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109 ABSTRACT A model based on cascade melting and recrystallization is derived to describe ion irradiation-induced amorphization. The accumulation of amorphous volume fraction during irradiation is represented in a single equation. Depending on the extent of recrystallization of a subcascade, the amorphous volume accumulation can be described by a set of curves that change from exponential to sigmoidal functions. The parameters (including temperature, cascade size, crystallization rate, glass transition temperature, dose rate) that affect the extent of recrystallization are included in the model. The model also describes the temperature dependence of critical dose for amorphization. 1. INTRODUCTION There are two general approaches used to describe radiation-induced amorphization: the direct impact model [1,2] and the defect accumulation model [1,3,4]. The direct impact model assumes that the amorphous domain forms directly in the core of a displacement cascade in a manner similar to liquid quenching [1,5-8]. The defect accumulation model assumes that the incoming particle produces defects, and the defects accumulate with continued irradiation until an amorphous phase forms when the (local) defect density reaches a threshold level [1,3,9,10]. Both the direct impact and the defect accumulation models were developed by Gibbons [1]. According to Gibbons, the change in amorphous fraction with ion dose is simply an exponential function for the direct impact model; while for the defect accumulation model (or overlap model), the function is sigmoidal. The amorphization in-growth curve given by the model is generally adopted to distinguish between direct impact or defect accumulation as amorphization mechanisms [4]. For heavy ion irradiations, especially at low temperatures, the amorphization process is assumed to be caused by direct amorphization within the cascade. While for light ions (or electrons and neutrons) the dominant mechanism is assumed to be defect accumulation [1,4,6]. Other models have been developed to interpret and to model amorphization induced by ion-bombardment [9,11-16]. Most of the recent models are based on defect accumulation or are combined with direct impact model [3,13,14]. In this paper, we present a new model. We assume that amorphization is only due to the direct impact of energetic particles (including heavy and light ions). Although direct impact amorphization is difficult to visualize in the case of light ions, they are included in the same mathematical model. 2. THE MODEL Each incident ion is assumed to create one or several subcascades. Inside each subcascade, all atoms are mobile. Initially, the subeascade size increases (high-density energy redistribution) and shrinks (energy dissipation) after reaching its maximum size. The damage evolution is as follows: (1) When a subcascade forms, all of the structural "memory" of that volume is lost. The subcas
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