An experimental technique for the rapid determination of binary phase diagrams: The Al-Mg system
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INTRODUCTION
THE importance of phase diagrams is immense in all aspects of material development. The conventional method of casting, heat treatment, and subsequent analysis of literally hundreds of samples of varying composition can take years of tedious work to determine completely the phase diagram for even simple systems. The technique of using solid state diffusion couples comprising two pure metals for the determination of binary phase diagrams has been used (j~ but has found limited applicability since solid state diffusion is relatively slow and the appearance of all equilibrium phases is not guaranteed because of differences in diffusion rates in the various phases, l~l A new method of binary phase diagram determination involving liquid diffusion couples which allows rapid determination of phase diagrams and any number of isothermal sections has been developed and reported in this investigation. In its simplest form, this method consists of producing a diffusion couple with the concentration of elements varying continuously across the portion of the diagram being investigated, heat-treating, and then performing microscopic and microchemical analysis on the couple to identify phases. The couple contains all phases and all phase fields in the composition range and, therefore, there is little ambiguity in identification of phases. In this communication we describe the new experimental technique used to determine binary phase diagrams and report some of the reSults obtained to clarify the confusion concerning the occurrence of various phases in the A1-Mg binary system in the composition range 35 wt pet to 50 wt pct Mg. The presence of R phase at approximately 42 at. pct Mg has been reported by several researchers. [2-81 However, the temperature range for the existence of this phase remains in doubt. For example, Murray 16] has reported that the R phase forms by the peritectoid reaction of A13Mg2 (/3) and Alt2Mg~7(~/) phases at 370 ---5 ~ and that it transforms back to (/3 + "y) below approximately 320 ~ Conversely, SchOrmann and Fischer'" have reported this N.C. GOEL is Metallurgical Engineer with Bristol Aerospace Ltd., P.O. Box 874, 660 Berry Street, Winnipeg, MB, Canada R3C 2S4. J. R. CAHOON is Professor with Metallurgical Sciences Laboratory, Department of Mechanical Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 2N2. B. MIKKELSEN is President of Dan-Bo Energy Ltd., Calgary, AB, Canada. Manuscript submitted May 12, 1987. METALLURGICAL TRANSACTIONS A
phase (designated e by them) to be formed via the following two reactions: Liquid (40.8 at. pet Mg) ~_~c/3 (39.5 at. pct Mg) + e (42.5 at. pct Mg) and Liquid (42.5 at. pct Mg) + ~ (46.4 at. pct Mg) 45-'~0~ • (44.6 at. pet Mg) where the ~ phase is AIMg. Schiirmann eta/. 17"slreported that the ~ phase forms between the composition range of 46.4 at. pct Mg and 48.1 at. pct Mg and a temperature range of 410 to 450 ~ via the following peritectic reaction: Liquid (44.5 at. pct Mg) + y (50.6 at. pct Mg) 45-~"~2~~ (48.1 at. pet Mg) The phase ~ (AIMg
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