Atom-probe microanalysis of a nickel-base superalloy
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I.
INTRODUCTION
A number of papers have shown that the FIM atom-probe technique is a powerful tool for physical metallurgy even for complex materials such as superalloys 1 or steels. 2 Because of its high spatial resolution the instrument is particularly suitable for quantitative microanalysis of fine precipitates,3 grain boundaries,4 or two phase interfaces. 5 The spatial resolution is about 1 to 5 nm at the specimen surface and can reach a single atomic layer in depth. Under some conditions, a quantitative compositional analysis can be performed on an atomic plane-by-plane basis. This technique has been recently used by M. Yamamoto and D. N. Seidman for the study of an ordered Ni4Mo alloy.6 In the present paper, the atom-probe is used for analyzing a Nb-Mo containing superalloy fabricated from prealloyed powders. The material under study is a recently developed nickel base superalloy for use in turbines in aero-engines. The microstructure of such nickel base alloys consists essentially of two phases: a fcc solid solution (3, matrix) and finely dispersed 3" precipitates. Some carbides (M23C6,MC) may also be present. The 3" phase has an ordered structure (L12) based on the formula (Ni, X) 3(A1, Y) and is coherent with the 3' matrix. It is well known that the mechanical properties of these alloys are strongly dependent upon the volume fraction, particle size, and composition of the precipitates. Moreover, partitioning of Nb and Mo between phases is of prime interest because of their respective influence on the 3" precipitation and the hardening of the 3' solid solution. It is therefore important to control these parameters. The first aim of this study is to determine 3' and 3" compositions in fully heat treated conditions. The atom-probe data can be correlated with the compositions obtained by chemical analysis of extracted precipitates. 7 Hence a more reliable volume fraction of 3" particles can be derived. It has been sometimes suggested that some large atomic diameter elements (W, Ta, Nb, Mo) may segregate at 3'-3" interfaces and therefore modify the creep behavior of superD. BLAVETTE, Research Scientist, A. BOSTEL, Research Engineer, and J. M. SARRAU, Assistant Professor, are with the Unite Associe6 au CNRS 808, Facult6 des Sciences de Rouen B. P. 67, 76130 Mont Saint Aignan, France. Manuscript submitted September 11, 1984. METALLURGICAL TRANSACTIONS A
alloys. Atom-probe analyses of such interfaces were performed to clarify the situation. Even though the partitioning of alloying elements between the two phases can be determined accurately in superalloys, it is still not well established what kind of lattice site (Ni or AI type) elements such as Cr, Ta, W, Mo, Nb preferentially occupy in the 3" sublattice. In this study, a plane-by-plane analysis of 3" phase has been carried out and a tentative derivation of occupation probabilities is achieved.
II.
EXPERIMENTAL
The technique has been described in detail elsewhere. 8 The instrument consists essentially of two parts: a field ion microscope (FIM) and a time o
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