ATOM PROBE ANALYSIS OF β PRECIPITATION IN A MODEL IRON-BASED Fe-Ni-Al-Mo SUPERALLOY

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1 ATOM PROBE ANALYSIS OF β PRECIPITATION IN A MODEL IRON-BASED Fe-Ni-Al-Mo SUPERALLOY M. Miller, M. Hetherington To cite this version: M. Miller, M. Hetherington. ATOM PROBE ANALYSIS OF β PRECIPITATION IN A MODEL IRON-BASED Fe-Ni-Al-Mo SUPERALLOY. Journal de Physique Colloques, 1989, 50 (C8), pp.c8-425-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1989 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 COLLOQUE DE PHYSIQUE Colloque C8, Suppl6ment au noll, Tome 50, novgmbre 1989 ATOM PROBE ANALYSIS OF SUPERALLOY PRECIPITATION IN A MODEL IRON-BASED Fe-Ni-Al-Mo M.K. MILLER and M.G. HETHERINGTON* Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, FN , U.S.A. Department of Metallurgy and Science of Materials, University of Oxford, GB-Oxford OX1 3PH, Great-Britain Abstract - An atom probe field-ion microscopy study has been performed on model Fe-Ni-Al-Mo superalloys. The molybdenum and iron were found to partition preferentially to the a matrix and nickel and aluminum partition to the p' precipitates. The precipitates contained approximately 11% Fe. No evidence was found for molybdenum segregation to the precipitate-matrix interface. INTRODUCTION The good mechanical properties of superalloys at elevated temperatures are due to the presence of small ordered precipitates that are coherent with the matrix. Most commercial superalloys are nickel-based and obtain their properties from L1,-ordered 7' or DO,-ordered 7' precipitates in a face centered cubic matrix. Iron-based superalloys are not as common, although some commercial alloys such as 17-7 PH and Nitralloy N steel have been developed. These Fe-Ni-Al alloys are based on B2-ordered NiAl p' precipitates in a body centered cubic ferrite matrix. However, the elevated temperature range of these alloys is restricted because the ferrite can transform to austenite. The ferrite phase field is extended to higher temperatures in alloys with high aluminum contents. In order to preserve their mechanical properties during service at elevated temperature, the precipitates must resist coarsening. The coarsening rate of these precipitates is dependent on the lattice parameter mismatch of the precipitates and the matrix. Calderon et al. have demonstrated that molybdenum additions to these materials alter the lattice misfit between the p' precipitates and the ferrite matrix!1s21 Also transmission electron microscopy (TEM) observations indicated that molybdenum additions to these alloys stabilized the spherical morphology of the p' precipitates during prolonged aging.['." Calderon et al. suggested that segregation of molybdenum to the precipitate-matrix interface was responsible for this stabilization through changes in the interfacial energy and surface stress. In this preliminary atom probe field-ion microscopy investigation, the chemistry of the precipitates and the matrix and partitioning behavior of molybdenum have been investigated. EXPERIMENTAL The Fe-Ni-Al-Mo materials studied in this atom probe investigation are listed in Table 1. These alloys formed part of the systematic study of Calderon et al!1.21 The atom probe analyses were performed in the Oak Ridge National Laboratory (ORNL) energycompensated atom The specimen temperature during atom probe analysis was 50 to 60K and pulse fractions of 20% were used. Field-ion micrographs were recorded at 50 to 70K with neon as the image gas. Table 1. Composition and aging treatments of the alloys. Composition (at. %) Aging Aging Mo Ni Al Fe Time Temperature Balance 100 h 775 C Balance 50 h 750 C RESULTS AND DISCUSSION A pair of field-ion micrographs of the 1% Mo alloy is shown in Fig. 1. This material exhibited high quality micrographs in which many crystallographic poles were evident, but no precipitates were discerniible in the image despite the previously reported volume fraction of 36%J21 However, in a field-ion micrograph of the 4% Mo alloy, shown in Fig. 2(a), precipitates were evident. The precise shape of the precipitates was not distinct in some portions of the micrograph. A clearer indication of the extent of the precipitates was Article published online by EDP Sciences and available at

3 possible by taking a micrograph while the specimen was field evaporating as shown in Fig. 2(b). The precipitates were found to image slightly darker than the a matrix. The results of the atom probe composition determinations of the matrix are summarized in Table 2. These results indicate that the matrix is substantially depleted in nickel and aluminum as would be expected due to the NiAI precipitates. The matrix of the 1% Mo alloy was slightly enriched in molybdenum; whereas, the matrix of the 4% Mo alloy was depleted in molybdenum. Table 2 Composition of the a Matrix Alloy Mo Ni Al Fe 1% Mo 1.2 k i 0.4 Balance 4% Mo 3.7 i i i 1.0 Balance A composition profile through the 1% Mo alloy is shown in Fig. 3. The presence of two NiAl precipitates is clearly evident. The profiles indicate that the moiybdenum partitions preferentially to the a matrix and is absent from the precipitates. The results from a number of selected area analyses of precipitates in the 1% and 4% Mo alloys are summarized in Table 3. The atom probe analyses clearly indicated that molybdenum and iron partitioned to the CY matrix, whereas nickel and aluminum partitioned to the p' precipitates. In addition to the high aluminum and nickel levels, the p' precipitates contained a significant iron content of approximately 11%. These atom probe results contradict the 50% Ni, 30% Al and 20% Mo composition estimated from the structure factor calculations performed in an attempt to explain some features of the TEM contrast.12] Table 3. Average composition of the Precipitates Alloy Mo Ni Al Fe 1% Mo i % Mo 0.08 i i i k 0.2 In addition to the NiAl precipitates, some brightly-imaging FeMo precipitates were also observed in the 4% Mo alloy. The composition of one of these FeMo precipitates analyzed was 48.5% Mo, 49.3% Fe, 1.5% Al and 0.7% Ni. This high molybdenum content indicated that some of the molybdenum was consumed in these precipitates and explains the low molybdenum - level of the matrix in the 4% Mo alloy. This type of precipitate was not observed in the 1% Mo alloy. Possible segregation of molybdenum to the matrix-precipitate interface was investigated from evaporation sequences. Two examples of character plots are shown in Fig. 4. The preferential partitioning of the molybdenum to the iron-rich matrix is apparent, but in neither example is there any evidence of molybdenum enrichment at the precipitate-matrix interface. CONCLUSIONS Atom probe analyses have determined that molybdenum and iron partition preferentially to the a matrix and nickel and aluminum partition to the 8' precipitates in an Fe-Ni-Al-Mo alloy. The precipitates have a iron content of approximately 11%. No evidence was found for molybdenum segregation to the precipitatematrix interface. Acknowledgments The authors would like to thank Prof. J.R. Weertman of Northwestern University for supplying the materials used in this investigation and K.F. Russell for her technical assistance. This research was sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. REFERENCES 1. H.A. Calderon, J.R. Weertman and M.E. Fine, Proc. Td International Conference on Solid-Solid Phase Transformations, Phase Transformation '87, Cambridge, July 1987, ed. G.W. Lorimer, pub. Institute of Metals, England, 1988, pp H.A. Calderon, M.E. Fine and J.R. Weertman, Metall. Trans., (1988) M.K. Miller, J. de Physique, (1986)

4 Fig. 1. Neon field-ion micrographs of Fe- 10% Ni- 15% Al- 1% Mo alloy aged 100 h at 775OC. Fig. 2. Neon field-ion micrographs of Fe- 15% Ni- 20% AI- 4% Mo alloy aged SO h at 700 C. -(a) at best image voltage and (b) during field evaporation.

5 :, I: 'I.: :, I: DISTANCE Fig. 3. Composition profile through Fe- 10% Ni-15% Al-1% Mo alloy showing two NiAl p precipitates. The distance scale is in blocks of 50 ions. Fig. 4. Character plots of the Fe- 10% Ni- 15% Al- 1% Mo alloy. No segregation of molybdenum to the precipitate matrix interface was evident.