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National Advisory Committee for Aeronautics, Technical Notes - Survey of Portions of the Chromium Cobalt Nickel Molybdenum Quaternary System at 1200°C

A survey was made of portions of the chromium,cobalt—nickel-molybdenum
quaternary system at 12000 C by means of microscopic and X-ray diffraction
studies. Since the face-centered cubic (alpha) solid solutions form the
matrix of almost all practically useful high-temperature alloys, the solid
solubility limits of the quaternary alpha phase were determined up to
20 percent molybdenum. The component cobalt-nickel—molybdenum, chromium-
cobalt-molybdenum, and chromiumenickel-molybdenum ternary systems were
also studied. The survey of these systems was confined to the determina-
tion of the boundaries of the face-centered cubic (alpha) solid solutions
and of the phases coexisting with alpha at 12000 C.

In the development of technologically useful alloys it is usually of
considerable help if the phase relationships and solid solubility limits
are known. At the Metallurgy Department of the University of Notre Dame,
a project has been in progress for some years to determine the phase
relationships in alloy systems involving chromium, cobalt, nickel, iron,
and molybdenum, the transition elements of greatest importance in highs
temperature alloys.

The determination of phase diagrams for systems of four or more come
ponents is an extremely laborious task. The problem.must be approached
in a systematic manner in order to avoid becoming hopelessly lost. The
best method of attack is to begin by establishing the phase relationships
in systems of two or three components and then continue by adding one new
element at a time. The problem of presenting quantitative phase relation—
ships diagrammatically for systems of three or more components necessitates
holding one or more thermodynamic variables constant. For example, a
ternary phase diagram may be presented as a series of isothermal sections
or as a series of sections in each of which the amount of one component
is held constant. For a'quaternary system, it is necessary to hold both
temperature‘and the amount of one component constant in order to obtain
two-dimensional diagrams. The temperature 12000 C was chosen as that at
which an initial isothermal survey could be most—profitably made.

This temperature is of immediate interest because it lies within the range
of solution treatment for most high-temperature alloys now in use-and
also because here diffusion rates are fast enough to allow equilibrium
conditions to be approached in reasonably short annealing periods. At—
lower temperatures, such as 8000 C, the determination of"these phase
diagrams within extensive composition ranges would be too time-consuming
and therefore expensive. werk of this kind is planned only for limited
important composition ranges.