Microstructures of Glassy and Metastable Crystalline Phases Formed from FE-C- Ternary Alloys
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B.H. Kear,
B.C. Giessen,
and M. Cohen,
199
editors
MICROSTRUCTURES OF GLASSY AND METASTABLE CRYSTALLINE PHASES FORMED FROM FE-C- TERNARY ALLOYS
ERHARD HORNBOGEN AND INGO SCHMIDT Institut ffir Werkstoffe, Ruhr-Universitat Bochum,
463o Bochum,
FRG
ABSTRACT Fe-C-base alloys were rapidly cooled from the liquid and then reheated from ambient temperature. In addition to the transitional elements (V,Cr,Mo,W), B was added as third element to substitute C. A systematic approach to the understanding of phases and microstructures, which form under these conditions is the purpose of this work. This includes the considerations of forming ability and thermal stability of metallic glasses. Three aspects require attention: a) absolute stability of the glass phase, b) relative stability i.e. the difference of Free Energy between the amorphous and particular crystalline phases, c) kinetic stability based on the rate and mechanisms of crystallization reactionsincluding the rate determing diffusion process. Light and electron microscopy and electron diffraction were used for the analysis of the structure of the alloys.
INTRODUCTION New metastable phases and ultrafine dispersions obtained during rapid cooling of liquids have attracted a renewed interest in addition to the formation of metallic glasses. Three aspects are required for a complete understanding of these phenomena [i]: 1. the (absolute) stability of the liquid or amorphous phase expressed for example by the concentration dependence of Free Energy F(x)(Fig. 1), 2. the relative stability of the amorphous phase as compared to crystalline phases, which can be expressed by the difference in Free Energy AF = Fa-Fc (Fig. 1). This provides evidence for phases, which may form in stable or metastable equilibrium, 3. the kinetic stability is determined by the mechanism of crystallization i.e. nucleation and growth (Fig. ). The three principle reactions are shown in Fig. 3. In addition to the type of phases, which are formed, these reactions also determine the nature of microstructures to which the crystalline phases are arranged. There exists a large amount of work on crystallization reactions of quenched Fe-B-alloys [7] because of their good glass forming ability (gfa). Binary Fe-C-alloys are known for a very bad gfa; however, much information is avialable on the structure of Fe-C-base liquids as well as on Fe-C-based solid solutions and carbides in connection with the structure of steels. Characteristic for the behaviour of rapidly cooled Fe-C-alloys is the extension of ranges, in which solid solutions form: In addition to the a- and2-Fe, the hexagonal E-phase is known to occur at high carbon concentrations [2I. In this work results are reported on the effect of transitional elements replacing Fe on the gfa of Fe-C-alloys, the concentration range, in which metastable crystalline phases occur and the resulting microstructures. Based on this information the different behaviour of C and B on the formation of crystalline phases and glasses is discussed.
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