The Structure of the γ’-Phase in Nickel-Base Superalloys
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 T 3 B phases (e.g., Ni3Al) and ternary Perovskite-type T 3 BC x : carbide phases (e.g., Y3AIC), the determination of the number and kind of atoms in the unit cell of NisAl and certain ternary phases, the crystallographic relationship between the structure of Y3C and Y3A1C, and phase relations in certain quarternary alloys.
From these considerations it is shown that the γ’ phase may best be characterized as a Perovskite-type carbide phase having the chemical formula T 3 BC x . 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 Ni3Al, of the B type if it can replace the aluminum. The essential features of this model are: T and B elements form an ordered T 3 B lattice of the CusAu type; carbon atoms are located only in octahedral holes in the centers of the CusAu-type cells thereby establishing Perovskite-type T 3 BCx unit cells; the effective size of T and B atoms in the T 3 BCx 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 T 3 BCx 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.
KeywordsTitanium Nickel Carbide Chromium Argon
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