Intermetallic compounds in ternary aluminium-rich alloys containing transitional metals

Abstract

Several investigations have now been made of the constitutions of ternary aluminium-rich alloys containing transitional metal solutes. Consideration of the results obtained has led to the conclusion that in aluminium-rich alloys, the transitional metal atoms may accept electrons from the structure as a whole, and that the ternary compounds formed have many of the characteristics of electron compounds. Although the acceptance of electrons may be understood in terms of the completion of the 3 d band of the transitional metal, consideration of the Brillouin zone structures of Co 2 Al 9 , Co 2 Al 5 and NiAl 3 suggests, empirically, that the numbers of electrons accepted per atom are greater than the band theory would indicate, and are more nearly equal to the vacancies in the 'atomic orbitals’ postulated in the Pauling theory of transitional metals. Further information with regard to the alloying characteristics of transitional metals has been sought by investigating ternary alloys of aluminium and silicon with chromium, manganese, iron, cobalt and nickel. The results are discussed in the present paper. Assuming acceptance of electrons by the transitional metals to extents given by the vacancies in the 'atomic orbitals’ postulated by Pauling, it is found that the ternary compound of higher electron: atom ratio in the aluminium-chromium-silicon system has a similar electron concentration to that of the ternary compound of lower electron: atom ratio in the aluminium-manganese-silicon system. This behaviour is repeated for the systems aluminium-manganese-silicon and aluminium-iron-silicon; in this case the compounds involved (α(AlFeSi) and α(AlMnSi)) are isomorphous and form uninterrupted solid solutions with each other. The compound of higher electron: atom ratio in the system aluminium-iron-silicon (β(AlFeSi)) has no counterpart in the aluminium-cobalt-silicon system, but Co 2 Al 9 dissolves silicon until the electron: atom ratio is raised sufficiently to overlap the range of electron: atom ratios characteristic of β(AlFeSi). As the number of vacancies for electrons per atom of transitional metal decreases, the ternary compounds formed tend to move to progressively higher ranges of electron: atom ratio. These results are discussed in relation to previous work, and support the general hypothesis of acceptance of electrons by transitional metals. Of particular significance are the observations that silicon and nickel both dissolve in Co 2 Al 9 until the same maximum electron: atom ratio is reached; that α(AlMnSi) and α(AlFeSi) form uninterrupted solid solutions and, according to the hypothesis, have similar electron:atom ratios; and that structural relationships exist between various compounds whose electron: atom ratios are similar.