Effect of Temperature and Composition on NiAl Precipitation and Morphology in Fe-Ni-Al Alloys
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CURRENT requirements in steam power generation plants require materials that can withstand creep and prolonged high temperature conditions [1033 K (760 C) and 35 MPa].[1–3] A promising option for this application is the use of alloys based on the NiAl intermetallic, since they present good mechanical properties at high temperatures[4] and a comparatively reduced weight due to the high Al content.[5] Unfortunately, the very limited tensile ductility of these materials at room temperature limits their potential application.[6,7] The NiAl intermetallic can partially dissolve Fe in its structure, causing the Fe atoms to take the Al atom sites preferentially.[8] The addition of C. FERREIRA-PALMA, E. CONTRERAS-PIEDRAS, M.L. SAUCEDO-MUN˜OZ, V.M. LOPEZ-HIRATA, J.L. GONZA´LEZVELA´ZQUEZ, and H.J. DORANTES-ROSALES are with the Departamento de Ingenierı´ a en Metalurgia y Materiales, ESIQIE, Instituto Polite´cnico Nacional, UPALM, Av. Instituto Polite´cnico Nacional S/N, C.P. 07738, Mexico, Mexico. Contact e-mail: [email protected] N. CAYETANO-CASTRO is with the Centro de Nanociencias y Micro y Nanotecnologı´ as, Instituto Polite´cnico Nacional, Av. Luis Enrique Erro S/N, C.P. 07738, Mexico, Mexico. Manuscript submitted March 30, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Fe has proved to slightly increase the ductility of NiAl-based alloys at room temperature by promoting the precipitation of the c phase,[9] but not enough to allow their structural use. Other materials like the Ni-based superalloys are also known for their high temperature properties. The excellent creep resistance of these alloys has been attributed to the high coherency that exists between the c matrix and c¢ precipitates.[10] The beneficial effects of a coherent matrix-precipitate interface have already been described by Krug[11] and Grose.[12] Nevertheless, Ni-based superalloys are comparatively costlier. As an alternative, ferritic alloys strengthened with NiAl precipitates are of interest because of their microstructural similarities with the Ni-based superalloys. Specifically, these alloys present coherent precipitates of the ordered b¢ (NiAl) phase with B2 structure, embedded in an a (Fe-rich) matrix.[13,14] Besides the lower cost of having an Fe-rich matrix, these alloys are known for their good thermal conductivity and low thermal expansion.[15] Improved environmental resistance in these alloys can be achieved by the addition of Cr. This also improves the precipitate coarsening resistance in alloys with low volumetric fractions of NiAl precipitates.[16,17]
Regarding b¢ precipitation in the Fe-Ni-Al system, so far only certain temperatures[18] or compositions[19–22] have been assessed, and it is necessary to carry out a broader study to understand the effect of temperature and composition on the precipitation and morphology of this phase. Usually it would be necessary to fabricate several alloys of different compositions. Besides the experimental complexity, this method could be problematic if the phase transformations are particularly se
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