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The Structure of the γ′-Phase in Nickel-Base Superalloys

Published online by Cambridge University Press:  06 March 2019

S. Rosen  and
P. G. Sprang
S. Rosen
Affiliation:
Pratt and Whitney Aircraft, North Haven, Connecticut
P. G. Sprang
Affiliation:
Pratt and Whitney Aircraft, North Haven, Connecticut
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Abstract

Present day nickel-base superalloys are hardened in part by the precipitation of a phase which has variously been identified as Ni3Al, Ni3(Al, Ti) and γ′. X-ray diffraction techniques which include precision lattice parameter measurements, intensity measurements, and phase identification are used to define the structural and chemical relationships upon which this phase is based.

These relationships are developed from the following considerations: crystal chemistry and atomic size factors which relate binary Cu3Au-type T3B phases (e.g., Ni3Al) and ternary Perovskite-type T3BC2 carbide phases (e.g., Y3AlC), the determination of the number and kind of atoms in the unit cell of Ni3Al and certain ternary phases, the crystallographic relationship between the structure of Y3C and Y3AlC, and phase relations in certain quarternary alloys.

From these considerations it is shown that the γ′ phase may best be characterized as a Peiovskite-type carbide phase having the chemical formula TsBCx. A model of the γ′ structure is presented which indicates the position of the various atomic constituents based upon whether they are T or B elements. (An atomic component is considered of the T type if it is capable of substituting for nickel in 3, of the B type if it can replace the aluminum. The essential features of this model are: T and B elements form an ordered T3B lattice of the Cu3Au type ; carbon atoms are located only in octahedral holes in the centers of the Cu3Au-type cells thereby establishing Perovskite-type T3BC3 unit cells; the effective size of T and B atoms in the T3BC3 unit cell is the same: hyperstoichiometric alloys, (ratio of B atoms to T atoms greater than one) will contain B atoms at face-centered positions in addition to a small amount of equilibrium vacant sites; in all alloys aluminum will preferentially occupy the cube corners of the unit cell ; the amount of carbon which is soluble in T3BC3; at any particular temperature is determined both by the distribution of the elements which are carbide-formers and the elements manganese, iron and cobalt. This model accounts for microstructural changes which occur in some nickel-base superalloys as a function of temperature and composition.

Type
Research Article
Information
Advances in X-Ray Analysis , Volume 9: Fourteenth Annual Conference on Applications of X-ray Analysis, August 25-27, 1965 , 1965 , pp. 131 - 141
Copyright
Copyright © International Centre for Diffraction Data 1965

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