The Effect of As-quenched Structure on Primary Phase Crystallization in Amorphous Aluminum Alloys
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0903-Z12-03.1
The Effect of As-quenched Structure on Primary Phase Crystallization in Amorphous Aluminum Alloys Joseph A. Hamann and John H. Perepezko University of Wisconsin-Madison, Department of Materials Science and Engineering, Madison, WI 53706, USA ABSTRACT From the reported experience, primary nanocrystallization in amorphous Al alloys involves transient kinetic behavior that is sensitive to the initial as-quenched structural arrangements. In order to examine the precursor effects, systematic studies outlining the evolving growth kinetics of primary aluminum nanocrystals (nc) during pre-crystallization annealing treatments, and investigations on the microstructure refining effect of specific alloying substitutions represent new approaches for an analysis of primary phase crystallization kinetics. In Al88Ni8Sm4 melt-spun ribbon (MSR), incremental substitutions of Cu for Ni (0 – 1at%) affect the thermal stability of the material (crystallization onset shifts to lower temperature) and refine the size of the primary phase nanocrystals. However, continuous heating calorimetry measurements indicate that the primary crystallization enthalpy remains approximately constant with increased Cu substitution. From a structural analysis standpoint, quantitative microstructure examinations applied in parallel with calorimetry measurements have been employed to characterize the as-quenched volume of MSR samples. The increased primary Al nanocrystal particle density with Cu substitution reflects the modification of local structural arrangements, necessitating an understanding of the local structure of the as-quenched sample in order to develop a description of primary phase nanocrystal growth kinetics.
INTRODUCTION As a result of investigations on novel structure synthesis in amorphous Al alloys, it has become clear that the processing conditions play a major role in determining the selection of specific product phases and microstructural morphologies. In-situ composites consisting of high nanocrystal number densities that are embedded in an amorphous matrix have been obtained by partially devitrifying marginally glass-forming alloys [1]. Numerous studies indicate that partial substitution of small amounts of Cu or Ag for the transition metal component in some Al-TMRE (TM = transition metal; RE = rare earth metal) systems effectively refines the size and increases the density of primary phase nanocrystals precipitated from the amorphous precursor material [2-4]. Increasing the amount of Cu substitution has also been shown to systematically decrease the thermal stability of the material. During heterogeneous nucleation, nucleation catalysts are often considered to originate from impurity clusters with the most effective catalysts having sizes on the order of r*, the critical cluster size for nucleation. Previous work indicates that the formation mechanism that leads to the large nanocrystal number densities is related to a heterogeneous nucleation mechanism that is driven by the as-quenched state of rapidly quenched alloy sa
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