Nonequilibrium behavior in the Al-Ge alloy system: Insights into the metastable phase diagram
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INTRODUCTION
THE
development and production of novel microstructures via nonequilibrium processing methods have received a great deal of attention lately. Alloys produced by such methods as rapid solidification processing (RSP) have been shown to exhibit metallurgical characteristics unattainable by conventional, near-equilibrium methods. Typical modifications resulting from such departures from equilibrium include increases in the maximum solid solubility limits of solute elements in the parent phase, grain size refinement, reduction or elimination of microsegregation, suppression of undesired phases, and production of new nonequilibrium phases, such as metastable crystalline and noncrystalline (amorphous) phases. ]1-7] Clearly, the ability to produce metastable crystalline phases in a controlled and reproducible manner is important for developing the structure-property-processing relations necessary for tailoring the properties of alloys containing them. However, our understanding of the factors controlling their formation is still incomplete, and therefore, it is difficult to produce controlled microstructures containing these phases. The difficulties arise from the complex microstructures that develop during nonequilibrium transformations due to the competitive TAHAR LAOUI, formerly with the Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, is Postdoctoral Research Fellow, Department of Materials Science and Engineering, Katholieke Universiteit Leuven, Heverlee, Belgium. MICHAEL J. KAUFMAN, Associate Professor, is with the Depamnent of Materials Science and Engineering, University of Florida, Gainesville, FL 32611. Manuscript submitted October 24, 1990. METALLURGICAL TRANSACTIONS A
nucleation and growth kinetics of the different phases (both stable and metastable). As a result, this area remains fertile and in need of considerable research before such materials are developed for technological applications. Although the phenomenon of metastability in metallurgical alloy systems has been the subject of a fairly large number of investigations, it is by no means fully understood how the metastable phases nucleate and grow and relate to each other as well as to the stable phases. Prior knowledge of such information is required to construct metastable phase fields on equilibrium phase diagrams needed to provide insight concerning the possible reaction sequences that might occur during nonequilibrium processing. The A1-Ge alloy system appears to be a simple binary eutectic system, with the equilibrium eutectic point at about 420 ~ and 28.4 at. pct Ge. Both elements have limited solid solubility of the other with maximum values of only --2 at. pct Ge in a-A1 and --1.1 at. pct A1 in /3-Ge at 420 ~ The nonequilibrium behavior of A1-Ge alloys has been investigated quite extensively using a variety of processing techniques, including rapid solidification (splat quenching and melt spinning), amorphous-film crystallization, submicron powder atomization, and droplet undercool
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