Early stages of Al-nanocrystal formation in Al 92 Sm 8
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Early stages of Al-nanocrystal formation in Al92Sm8 G. Wilde, H. Rösner, N. Boucharat, J. Hamann1, W.S. Tong1, J.H. Perepezko1 Forschungszentrum Karlsruhe, Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany 1 University of Wisconsin-Madison, Department of Materials Science and Engineering, Madison, WI 53706, USA ABSTRACT Several proposals involving solute effects, phase separation or quenched-in nuclei and heterogeneous nucleation have been advanced to account for the high nanocrystal density that evolves during primary crystallization in marginal glass-forming alloys, but recent crystallization measurements and kinetics analyses provide new evidence for the role of the as-quenched structure on nanocrystal synthesis. Here, isothermal microcalorimetry investigations and quantitative electron microscopy measurements including high-resolution imaging and electron spectroscopy analyses were performed on a model system at different temperatures well below the glass transition to monitor the nanocrystallization isothermally as a function of time. From the combined measurements, the size distribution and the transformed fraction can be determined with a high accuracy for extended ranges of temperatures and times. In addition, calorimetric measurements in the glassy, liquid and crystalline states of the model alloy serves to analyze, for the first time, the fragility characteristics of a marginal glass-former that presents an important parameter in the context of the kinetic stability of the material against premature crystallization. INTRODUCTION In-situ composites consisting of high nanocrystal number densities that are embedded in an amorphous matrix can be obtained by partially crystallizing marginal glass-forming alloys [1]. These alloys that are characterized by their transformation characteristics, i.e. by the primary crystallization of high number densities of nanometer-scaled crystallites, present an important class of materials since partial devitrification allows the formation of porosity-free nanoscale composites with exceptional properties and property combinations [2]. Prominent examples are given e.g. by Fe-rich alloys with a high permeability and high saturation polarization such as nanoperm™ or by Al-rich alloys with high mechanical strength and low mass density [3, 4]. Previous work strongly indicates that the formation mechanism that leads to the large nanocrystal number densities is related to heterogeneous nucleation that, in turn, depends strongly on the as-quenched state of the individual samples [5]. In fact, the results obtained upon variation of the vitrification pathway indicate that the formation of so-called quenched-in nuclei plays an important role in nanocrystallization [6]. Yet, important characteristics such as the nature and origin of the nucleation sites and a description of the primary crystallization kinetics are currently not resolved. In this respect, the Al92Sm8 alloy presents a model system since the phase evolution sequence is typical for marginally gla
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