Coarsening and morphology of β′ particles in Fe-Ni-Al-Mo ferritic alloys

Studies have been carried out of the effect of aging at 973 K on the ordered NiAl β′ precipitate particles in a series of Fe-Ni-Al alloys with and without the addition of Mo. The lattice parameter disregistry between matrix and precipitates is affected by the Mo content. The rate of precipitate coarsening during isothermal aging is slowest when the misfit is minimized. The dependence of coarsening upon misfit is more pronounced in alloys with low precipitate volume fractions. Raising the precipitate volume fraction increases β′ coarsening rates more than predicted by theory. The lattice parameters of both the β′ particles and the matrix are functions of aging time. Compositional changes, segregation of Mo to the particle-matrix interface and, at long aging times, loss of coherency are proposed as causes of the observed lattice parameter changes. The addition of Mo to the Fe-Ni-Al alloys causes the β′ particles to remain spherical even after extended periods of aging whereas the particles become somewhat cubic or rectangular parallelepiped in form in the Mo-free alloys. The size distributions of the β′ particles agree in a general way with the predictions of the Brailsford and Wynblatt and Voorhees and Glicksman models, but are somewhat less skewed and do not show the predicted sharp cutoff in particle size on the high radius side.