Deformation-induced crystallization and amorphization of Al-based metallic glasses
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Deformation-induced crystallization and amorphization of Al-based metallic glasses Rainer J. Hebert and John H. Perepezko Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, U.S.A. ABSTRACT A main requirement for the application of nanostructured materials for structural applications is their thermal stability. Structural materials are often exposed to mechanically-induced stress states. Nanomaterials for structural applications should therefore retain their microstructure not only within a defined temperature range but also under applied load. Cold-rolling experiments with melt-spun Al87Ni10Ce3 ribbons containing a dispersion of nanocrystallites in an amorphous matrix demonstrate that during the continued deformation through repeated rolling and folding crystallization as well as amorphization reactions could be induced. The results indicate that in addition to the microstructure control through annealing of precursor materials, deformation processing represents an effective approach to the synthesis of amorphous and nanophase composite materials.
INTRODUCTION Strengthening mechanisms based on the dispersion of second-phase particles or fibers are fundamental to many structural materials. Among the latest examples for composite materials are amorphous, Al-based alloys containing nanocrystallites. The high density (approximately 1021 to 1022 m-3) of primary Al-nanoparticles or quasicrystals dispersed in the amorphous matrix contributes to an exceptional specific strength. For example, a tensile strength of 1560MPa has been measured for Al88Ni9Ce2Fe1 melt-spun ribbons [1,2]. The nanocrystallites are induced through annealing of amorphous precursor materials. Of particular interest are therefore marginal glass-formers, i.e. amorphous systems that reveal a primary crystallization reaction of Al. The temperature range for primary crystallization reactions is between 170ºC and 280ºC for most Albased systems. Due to the high cooling rates of 105-106Ks-1 that are necessary to avoid growth of nuclei or nucleation reactions during the melt-spinning process [3] bulk samples can not be processed directly for the alloys used so far. Current efforts to produce bulk amorphous Al alloys focus mainly on the compaction and extrusion of amorphous, atomized powders [4,5]. In many cases the deformation of amorphous phases was observed to yield a crystallization reaction, for example upon cold-rolling of Al-Y-Fe and Al-Sm based amorphous ribbons [6], upon bending of different Al-based amorphous ribbons [7], warm-extrusion of gas-atomized amorphous powders [4,5] if the extrusion ratio exceeds a critical value or nanoindentation of a Zr-based glass [8]. A different approach towards bulk amorphous nanocomposites is the continued rolling and folding of crystalline, elemental multilayer arrays [9,10]. After several rolling and folding cycles of Al-Sm multilayer samples, for example, a glass-transition could be detected [11]. The observation of cold-rolling induced crystallization rea
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