A Study of the Phase Decomposition of Fe-Ni-Al-Mo Alloys
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A STUDY OF THE PHASE DECOMPOSITION OF Fe-Ni-Al-Mo ALLOYS M.K. Miller*, M.G. Hetherington**, J.R. Weertman*** and H.A. Calderon*** *Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376. "**Department of Materials, University of Oxford, Oxford, OX I 3PH, Great Britain ***Department of Materials Science and Engineering, Northwestern University, Evanston, I1 60208. ABSTRACT The aging of 03'NiAl precipitates in ferritic Fe-Ni-Al alloys has been studied by transmission electron microscopy (TEM) and atom-probe field-ion microscopy (APFIM). The addition of Mo alters the lattice parameter of the phases and segregation of Mo to the interface between the matrix and the particles may alter the interfacial energy. The compositions of the matrix, precipitates and interfaces have been measured by TEM and APFIM. The results are compared. INTRODUCTION Superalloys derive their mechanical stability at elevated temperatures from the presence of small ordered coherent precipitates. The Fe-Ni-Al-Mo system is being studied because a) it has potential as the base of an Fe-based superalloy, if stable NiAl precipitates can be formed, and b) it is a useful model system with which to study the effects of interfacial and strain energy on coarsening processes. The modelling of the kinetic processes requires a detailed and accurate characterization of the precipitate morphology and composition. It is rare that a single technique is capable of completely characterizing a microstructure and in this study results from TEM and APFIM are presented. The Fe-Ni-Al-Mo materials reported in this investigation were Fe- 10at.%Ni- 15 %Al- 1 %Mo and Fe- 15 %Ni- 20 %Al- 4 %Mo. These alloys formed part of the systematic study of Calderon et al [1,2] and more details of the atom-probe analysis can be found in Miller and Hetherington [3]. All compositions are in atomic percent. RESULTS Two field-ion microscope (FIM) images of the 1% Mo alloy aged for 100 h at 775°C are shown in Fig. 1. No precipitates were observable in the images from these materials, even though the TEM micrographs shown in Fig. 2suggest that, after aging at 700"C, there should be a high volume fraction of precipitates in the alloys (36% [2]). Atom-probe experiments, however, revealed that many particles were present in the material, as shown in the composition profile in Fig. 3. Since they are invisible in the FIM, this indicates that a) their imaging characteristics are similar to the matrix and b) they are coherent with the matrix. It has been observed that Mo atoms image brightly in steels. Therefore, if Mo segregates to the matrix/precipitate interface, the interface might be expected to be decorated with brightly-imaging atoms. No such behaviour was observed. The 4% Mo alloy aged for 50h at 750°C did not show the same imaging characteristics. and two types of precipitate were observed. Though indistinct, a 03'precipitate is shown in Fig. 4(a). The precise shape of the 3' precipitate can seen more clearly while the specimen was field evaporating, as shown i
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