A Correlative Study of an Iron-Base Superalloy Using Transmission Electron Microscopy and Atom Probe Tomography

Abstract

Many heat resistant ferritic steels have superior thermal expansion and conductivity properties compared to Ni-based alloys but are somewhat limited in their applications above ~600 o C by their mechanical properties. Efforts are being made to use precipitation strengthening to improve these properties and thus the efficiency of such alloys [1]. One such effort uses coherent (Ni,Fe)Al precipitates in a ferritic matrix, Fig. 1, and has shown good initial results. Transmission electron microscopy (TEM) has been used to understand globally the precipitate microstructure as it evolves with heat treatment. In order to understand the mechanisms by which such precipitation occurs, it is important to know how the composition variations in the structure on the scale of the precipitates evolve with heat treatment. Atom probe tomography (APT) [2] excels at such atomicscale characterization and is a strong complement to the imaging capabilities of TEM. The local electrode atom probe (LEAP) [3] has been used to characterize the nanoscale microstructure in this alloy in order to determine the chemistry of the third phase (yellow arrow) precipitated inside the (Ni,Fe)Al B2 phase, which is shown as the darkly-imaging phase in Fig. 1b. The Fe-18.1at.% Ni-22.9at.% Al-9.5at.% Cr alloy was produced by vacuum induction melting followed by solution annealing at 1200°C with air cooling. For atom probe analysis, rods of this material were sharpened by electropolishing to have a radius of curvature of less than 100 nm at the apex. Three phases in the microstructure were identified consistent with prior work [1]. A primary Fe-rich phase (A2), a secondary NiAl phase (B2), and a third Fe-rich precipitate. Precipitates of the primary phase within the secondary phase were not seen to form within ~15 nm of large scale primary regions, i.e., there is a denuded zone or precipitate free zone near the primary phase A2 phase. Fig. 2 shows a LEAP analysis with a volume of (80x80x500) nm made up of 110 million atoms. On the right side of Fig. 2, two composition profiles are shown. The top one is across a narrow B2 region between two A2 regions showing the ~50/50 composition of the NiAl phase in this region. The lower profile reveals that the very small (~5nm) precipitates (yellow arrow in Fig. 1) inside the NiAl phase appear to be secondary Fe-rich phase. Through this correlative microscopy effort with TEM and APT, a greater understanding of the precipitation sequence and evolution of the microstructure is obtained.

Characterization of microstructures, mechanical properties, and oxidation behavior of coherent A2 + L21Fe-Al-Ti

“…This observation complies with that made for Fe-Al-Ni-Cr alloys where marked changes in the compositions of the coexisting phases lead to a deterioration of the properties, though in this case a severe embrittlement of the alloys was observed due to the formation of second and third generation precipitates during cooling. 29,30 It has been found in the case of other Fe-Al-X systems that coherent microstructures which include L2 1 are not stable because this phase formed only in a metastable manner, and so it has to be left open whether stable coherent microstructures can be generated in Fe-Al-X alloys by which mechanical properties can be improved.…”

“…[16][17][18][19] Inspection of the Fe-Al-Ti system shows that a twophase equilibrium between disordered aFe,Al (A2) and L2 1 -ordered Fe 2 AlTi exists, 19,20 where the mismatch of lattice constants is sufficiently small to generate coherent A2 + L2 1 microstructures. 21 Aligned/coherent twophase microstructures with A2, B2, D0 3 , or L2 1 have been reported to occur in the Fe-Al-X systems with X = Si, 22 Ti, 21,23,24 V, 25 Co, 26 Ni(+Cr), [27][28][29][30]7,31,32 and Ta. 6 By generating such microstructures, Fe-Albased alloys may be considerably strengthened at high temperatures, but only few relevant investigations have been performed.…”

Fe–Al materials for structural applications at high temperatures: Current research at MPIE

“…Though the formation of the coherent microstructures significantly improves the strength, notably creep strength, at elevated temperatures, marked changes in the solid solubility in A2 and B2 with temperature cause considerable disadvantages. In the Fe -Al -Ni -Cr alloys decreasing solid solubility with decreasing temperature results in the formation of nano-sized second-and third-generation precipitates [122] which cause severe embrittlement at low temperatures [96,97]. Similar changes in solid solubility with temperature lead to disintegration of coherent A2 + B2 microstructures during creep at high temperatures in Fe -Al -Ti alloys [121].…”

Atom-Probe Microscopy LEAPs the Chasm to Mainstream Applications

“…Clearly the precipitates are iron-rich (>50% Fe) and are distributed throughout the NiAl phase. Figure 7 Left -lower magnification DF-TEM [6]. Middle -Higher magnification DF-TEM [6].…”