Analysis of Primary Crystallization in Amorphous Aluminum Alloys
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Analysis of Primary Crystallization in Amorphous Aluminum Alloys John H. Perepezko1, William S. Tong1, Joe Hamann1, Rainer J. Hebert1, Harald R. Rösner and Gerhard Wilde Forschungszentrum Karlsruhe, INT, P.O. 3640, D-76021, Karlsruhe, Germany 1 University of Wisconsin-Madison, Dept. Mat. Sci. and Eng., 1509 Univ. Ave. Madison WI 53706 USA ABSTRACT The annealing response of amorphous Al based alloy samples were investigated to assess the role of the as-quenched state on primary crystallization of Al nanocrystals(nc). Continuous heating differential scanning calorimetry (DSC) traces of amorphous Al87Ni10Ce3 powders were compared to those from melt spun ribbon (MSR) to examine the effect of sample subdivision on primary crystallization. While the powders exhibited the same onset temperature as MSR, thermal cycling experiments show fine powder sizes reacting at the onset temperature and coarse powder sizes with the lowest melt quench rate transforming at the highest primary reaction temperature. In Al92Sm8 MSR, a kinetics analysis of Al nc distributions indicates a notable effect of the as-quenched state on primary crystallization during isothermal annealing. With Al88Y7Fe5 MSR intense deformation can induce the development of an Al nc distribution without thermal annealing. In each case examined, the results support the inclusion of quenched in clusters in the analysis of primary crystallization reactions. INTRODUCTION Amorphous aluminum alloys containing surprisingly high Al nc densities (1021m-3) have been found to exhibit unprecedented high strengths [1] that make them potential materials for structural applications. This has provided an impetus to investigate the factors that control the nucleation and growth of ncs in the amorphous matrix. Although there have been a number of examinations of primary crystallization behavior in Al-based metallic glasses, the main characteristics that have been demonstrated in these studies include the early development of a high Al nc nucleation density that increases modestly during continued reaction, and an Al nc size that exhibits sluggish growth. At the highest reaction temperatures, growth may be inhibited by diffusion field impingement between neighboring Al ncs, and throughout the reaction the partitioning of the low diffusivity rare earth component acts to limit growth. Experimental studies based only upon the standard Kolmogorov-Johnson-Mehl-Avrami [2-4] type of analysis of calorimetric measurements cannot be applied directly to primary crystallization reactions [5]. Moreover, this standard approach neglects key information on the evolution of the nc size distribution during a reaction that is essential in the kinetics analysis. At the same time, a complete examination that correlates the calorimetric and related quantitative microstructural data that can be used to analyze the crystallization kinetics requires a clear description of the initial (i.e. as-quenched) state of the amorphous samples. In the current work, three separate investigations are examined in
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