Stable and metastable phase equilibria in the Al-Mn system
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INTRODUCTION
DESPITE both technological and scientific interest in A1-Mn alloys, one can reasonably assert that prior to 1970 the only definitively established feature of the A1-Mn phase diagram was the melting point of AI. Although the Al-rich side of the diagram has since been clarified, not all the observed phases of the system have yet been accounted for as stable or metastable phases, nor have all experimentally observed reactions been interpreted in terms of a self-consistent picture of the stable and metastable phase equilibria. Some of the reasons are evident. Mn has been available in high purity only for the last decade or so; it has a high vapor pressure even at relatively high AI content, and it is readily oxidized. But there lurk more subtle traps that have caught many investigators of unquestionable ability. Long-lived metastable phases occur in surprising numbers, and reactions leading to the stable equilibria are sluggish. No experiment in this system can be unambiguously interpreted unless the phases are characterized structurally and the approach to equilibrium is demonstrated. Most of the reactions seen during cooling from the liquid state do not pertain to the stable equilibrium diagram at all. Therefore an analysis of the thermodynamics of the system is needed to construct a self-consistent model of the nature, number, and sequence of the possible reactions. In this paper we present experimental work by differential thermal analysis, transmission electron microscopy, and X-ray diffraction, on alloys containing between 12 and 55 at. pct Mn. Samples were prepared in the form of ribbon by splat-quenching of the liquid. By use of these homogeneous or at worst very finely segregated samples, equilibrium could be approached far more rapidly than by using conventional bulk samples. Our results and other observations critically selected from the literature are interpreted in terms of a calculation of stable and metastable equilibria from thermodynamic functions. We have revised some features of the generally accepted stable diagram, notably in the region near 20 at. pct Mn. We have also reinterJ.L. MURRAY, A.J. McALISTER, R, J. SCHAEFER, L . A . BENDERSKY, E S. BIANCANIELLO, and D.L. MOFFAT are with National Bureau of Standards, Metallurgy Division, Gaithersburg, MD 20899. Manuscript submitted June 12, 1986.
METALLURGICALTRANSACTIONS A
preted thermal analysis cooling data in terms of a sequence of six thermodynamically predicted metastable peritectic reactions. Considerable experimental work remains to be done to verify the details of this composition and temperature region of the diagram. By the present study we have established the feasibility of our experimental approach. The present calculation of the diagram is shown to provide a suitable basis for interpreting nonequilibrium experiments. While the calculation of the diagram will be susceptible to improvement as more accurate thermochemical data are obtained, it places limits, for the first time, on the range of possibilities for the binary
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