Synthesis and Stability of Amorphous Al Alloys
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Synthesis and Stability of Amorphous Al Alloys J. H. Perepezko, R.I. Wu, R. Hebert, and G. Wilde1 Univ. of Wisconsin-Madison, Dept. Mat. Sci. and Eng. 1509 Univ. Ave., Madison WI 53706, 1 Forschungszentrum Karlsruhe, INT, P.O. Box 3640, D-76021 Karlsruhe, Germany ABSTRACT The recent innovations in metallic glasses have led to new alloy classes that may be vitrified and a re-examination of the commonly used criteria for glass formation and stability. The new alloy classes are usually at least ternary systems and often higher order that can be grouped into two general categories. In one case large, bulk volumes may be slowly cooled to the glassy state which signifies a nucleation controlled synthesis. The other important class is represented by Al and Fe based glasses that can be synthesized only by rapid solidification processes such as melt spinning. These glasses are often called marginal glass formers that are synthesized under growth controlled kinetic conditions. With marginal glass forming alloys the termination of the amorphous state upon heating is often not characterized by a clear glass transition signal, Tg , but instead by the rapid onset of a primary crystallization reaction that represents the partial crystallization into a high number density of nanocrystals of the major component (i.e. Al or Fe) dispersed within a residual amorphous matrix. However, a closer examination by modulated or dynamic differential scanning calorimetry (DDSC) has identified a true reversible Tg signal that confirms the amorphous state and has revealed additional low temperature annealing behavior that impacts the overall thermal stability and microstructure evolution. At the same time alternate synthesis strategies involving deformation alloying by intense cold rolling reveals that the primary crystallization reaction can be bypassed. Alternatively other approaches have demonstrated that by suitable alloying it is possible to innoculate the primary crystallization reaction and increase the density of primary nanocrystals by about an order of magnitude. These developments represent a major level of microstructure control that have an impact on the structural performance and stability. INTRODUCTION A common theme in the development of new microstructural options by advanced solidification and other non-equilibrium processing methods is the occurrence of metastable structural states. Along with the development of new microstructural options , materials processing technologies and the attainment of new levels of performance, it has become apparent that a number of the existing models for the analysis of microstructure evolution must be modified and extended in order to treat the new domains of processing and kinetic conditions that are now accessible. For example, with the development of amorphous Al alloys that can be partially devitrified to yield a high density of Al nanocrystals in an amorphous matrix, tensile strengths approaching 1300 MPa have been reported [1,2]. At this strength level the nanocrystal dispersed Al alloys ar
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