The constitution of the aluminium-rich aluminium-cobalt-iron alloys, with reference to the role of transitional elements in alloy formation

Abstract

The behaviour of transitional metals in aluminium-rich alloys is of theoretical interest. From a knowledge of the system aluminium-iron-nickel (Raynor & Pfeil 1946-7 a ) it has proved possible to predict the general form of the equilibrium diagram for the aluminium-rich aluminium-cobalt-nickel alloys (Raynor & Pfeil 1946-7 b ). Similar predictions may be made for the aluminium-iron-cobalt alloys. The present paper describes the results of an examination of these alloys undertaken to test the predictions. Using micrographic and X-ray methods, isothermal sections have been established at 640, 600 and 550°C. Only the two intermetallic phases Co 2 Al 9 and FeAl 3 have been recognized. Vertical sections across the aluminium-rich corner of the ternary model have been established by thermal analysis, and the fields of primary separation have been determined. Analysis of extracted primary crystals has shown that Co 2 Al 9 dissolves appreciable amounts of iron, while FeAl 3 dissolves a similar amount of cobalt. In each case the solution proceeds by atomic replacement. The results are discussed in their relation to the theoretical considerations involved, and it is shown that close analogies exist between the alloys of aluminium with iron and nickel, with iron and cobalt, and with cobalt and nickel.