Neutron-diffraction study and modeling of the lattice parameters of a NiAl-precipitate-strengthened Fe-based alloy

The lattice misfit between the body-centered cubic α-Fe matrix and the B2-ordered NiAl-type β′ precipitates is a parameter of significant importance in controlling the creep resistance of precipitate-strengthened ferritic steels. However, the measurement of the lattice misfit is complicated due to the fact that the fundamental reflections of α and β′ phases almost completely overlap. In this study, neutron diffraction is used to determine the lattice parameters of these two phases in a Fe–18.9 Al–9.8 Cr–13 Ni–1.8 Mo (atomic percent, at.%) alloy as a function of temperature. The accuracy of the measurement at room temperature is verified by high-energy synchrotron X-ray diffraction. The comparison between these two techniques is discussed in terms of the difference in superlattice intensity. Furthermore, using the phase compositions determined by atom probe tomography, models are proposed to predict the lattice parameters of both phases at room temperature as a function of their compositions. The results are in very good agreement with those obtained experimentally.