Solidification Modeling of Bulk Amorphous Alloys
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Solidification Modeling of Bulk Amorphous Alloys Sang Bok Lee and Nack J. Kim Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea ABSTRACT Classical heterogeneous nucleation theory coupled with DTA data has been used to closely estimate the crystallization behavior of continuously cooled bulk metallic glass (BMG) alloys. Continuous cooling transformation and time temperature transformation diagrams of three BMG alloys, Zr41.2Ti13.8Cu12.5Ni10Be22.5, Cu47Ti33Zr11Ni6Si1Sn2 and Mg65Cu25Y10 alloys, have been calculated. The critical cooling rates Rc of three alloys were calculated to be 1.7 K/s, 242 K/s and 36 K/s for Zr41.2Ti13.8Cu12.5Ni10Be22.5, Cu47Ti33Zr11Ni6Si1Sn2 and of Mg65Cu25Y10 alloys, respectively, which match well with the experimental values. We conclude that heterogeneous nucleation is more favorable than homogeneous nucleation for the formation of crystals during cooling of BMG alloy liquids. Our approach can be applied to the analyses of crystallization kinetics of BMG alloys with a wide range of critical cooling rates during continuous cooling as well as isothermal annealing. INTRODUCTION In recent years, there are growing interests in materials with amorphous structures since these materials have quite unique properties much different from those of conventional materials [1-3]. In bulk metallic glass (BMG) alloy systems, studies on phase transformation behavior during continuous cooling are of great importance since the actual fabrication process involves solidification which is essentially a continuous cooling process. One of the main difficulties in studying the phase transformation behavior during continuous cooling is that critical cooling rates of most BMG alloys are too fast to obtain quantitative information of phase transformation behavior. Only a few BMG alloy systems, such as Zr-Ti-Cu-Ni-Be [4] and Pd-Cu-Ni-P [5] base alloys, have slow enough critical cooling rates (i.e., enough experimentally accessible time and temperature window) which enable the experimental investigation of phase transformation behavior. In order to generally describe the kinetics of phase transformation of BMG alloys with a range of critical cooling rates, several theoretical approaches have been undertaken [6-8]. When the theoretical models are compared with the experimental results, however, there are large discrepancies between them. Therefore, there clearly is a need for a new approach that can describe the crystallization kinetics of continuously cooled BMG alloy melts. New approach should be applicable to the BMG alloy systems with faster critical cooling rates as compared to those of Zr-Ti-Cu-Ni-Be and Pd-Cu-Ni-P alloys. In this paper, a kinetic approach for analyzing the crystallization behavior of continuously cooled BMG alloys is introduced. Based on the analyses of thermal analysis data coupled with the heterogeneous nucleation theory, the nucleation temperature Tn and the critical cooling rate Rc are calculated. CCT and TTT diagrams are also co
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