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National Advisory Committee for Aeronautics, Technical Notes - X-Ray Diffraction Investigation of Minor Phases of 20 High Temperature Alloys

Carbides, nitrides, and other phases distributed throughout the
matrix of alloys play a significant role in determining the char-
acteristics of the alloys. An element dissolved in the matrix may
alter such properties of the matrix as corrosion and oxidation
resistances, hardenability, strength, and crystal structure. When
a minor phase appears in an alloy, stresses within the matrix
result and these stresses may exert a great influence on the high-
temperature properties of the alloy such as creep and stress-
rupture strengths. The identification of these minor phases in an
alloy is important in determining the distribution of the elements
of the alloy among the phases.

Knowledge of the manner in which a specific element is dis-
tributed among the various phases of an alloy would permit evalua-
tion of the function of that element in the alloy. This informa-
tion would also help in selecting the percentage of an element
required in a. new alloy containing the same types, but not the
same quantities, of minor phases as alloys previously studied. For
example, chromium in solution in an iron-base matrix is well known
to produce excellent resistance to oxidation by forming a tightly
adhering oxide film on the surface under oxidizing conditions.
Chromium, however, is a carbide-former; if all the chromium in such
an iron-base alloy forms carbides, this resistance to oxidation is
lost. Therefore, if chromium is to be dissolved in the matrix of
an iron-base alloy in a sufficient amount to give a specified
oxidation resistance, the manner in which chromium distributes
itself among the carbide phases and the matrix must be considered
in determining the percentage of chromium that the alloy should
possess.

Although a large amount of research has been conducted .on the
compositions of the minor phases of steels, light alloys, and
copper-base alloys, little research has been published on the com-
positions of the minor phases present in high-temperature alloys
in current use. The minor phases in Vitallium, a cobalt-base
alloy, have been investigated microscopically by Badger and Sweeny,
who found the phases M2506 (where M is any metallic atcm of
the alloy), M66, and. Cir-703 to be present (reference 1).
An investigation was therefore conducted. at the NADA Cleveland
laboratory to identify the minor phases existing in 20 high-
temperature alloys and to obtain a pattern of the relations between
the composition of these alloys and the fomation of their minor
096hases. The 20 alloys are representative at the materials employed
in current gas -turbine applications. The identification of the
minor phase was based on a determination of their crystal structures
through the use of X-ray diffraction methods.