Hierarchical microstructure of ferritic superalloys
A structural hierarchy due to chemical ordering, dimensionality and spatial arrangement of the constituent phases was obtained in a precipitation strengthened ferritic alloy. Nearest-neighbor ordered B2-NiAl precipitates were coherently embedded in the disordered bcc-Fe matrix. Throughout the solid-state aging heat treatment a coherent substructure of the next-nearest-neighbor ordered L21-Ni2TiAl phase formed only within the primary B2-NiAl precipitates.
A new approach of alloy design was presented by the formation of coherent; two-phase precipitates embedded in a disordered matrix. The L21-ordered Ni2TiAl phase could be stabilized by the addition of Ti to B2-ordered NiAl. Ordered-disorder transitions were only occurring on the Al-sublattice of the NiAl-phase and the system could thus be described as a pseudo-binary system. The lattice mismatch between bcc-Fe, B2-NiAl, and L21-Ni2TiAl was sufficiently small to generate coherent microstructures.
Fig.1a) Energy-filtered TEM image of the hierarchical microstructure obtained by solid-state aging for 10 h at 700 ºC. The elemental map of Fe is shown in blue, Ni in red and Ti in green.
C.H. Liebscher, V.R. Radmilovic, U. Dahmen, N.Q. Vo, D.C. Dunand, M. Asta, G. Ghosh Acta Materialia 92 (2015) 220
Fig.1a) Energy-filtered TEM image of the hierarchical microstructure obtained by solid-state aging for 10 h at 700 ºC. The elemental map of Fe is shown in blue, Ni in red and Ti in green.
C.H. Liebscher, V.R. Radmilovic, U. Dahmen, N.Q. Vo, D.C. Dunand, M. Asta, G. Ghosh Acta Materialia 92 (2015) 220
In the as-quenched state nanometer sized B2-NiAl precipitate formed in the bcc-Fe matrix. In the course of the solid-state aging heat treatment at 700 ºC, the L21-Ni2TiAl phase nucleated only within the primary B2-precipitates. After 10 h of aging the coherent precipitate substructure was fully developed, as illustrated in Fig.1a. The interfaces were highly isotropic and orient towards a cube-on-cube orientation.
Aberration-corrected scanning TEM (STEM) confirmed the coherency between B2 and L21, and that the interface was fluctuating in position with a width of ~4 nm (Fig.1b). This interface broadening was confirmed by cluster-expansion based first-principle thermodynamic calculations. In this context, a decrease of the B2/L21-interface energies from 50 mJ/m2 at 0 K to 11 mJ/m2 at 973 K were determined from Monte-Carlo simulations. Kinetic-Monte-Carlo simulations also supported the observation of L21 nucleating within the B2-precipitates.
Fig. 1b) Aberration-corrected STEM image of a B2/L21-interface. The coloring scheme is based on the nearest neighbor intensity differences of the Al-sublattice to highlight the differences in ordering and the interface fluctuation. An intensity difference of 0 represents perfect B2- (red) and >2.5 L21-order (green).
C.H. Liebscher, V.R. Radmilovic, U. Dahmen, N.Q. Vo, D.C. Dunand, M. Asta, G. Ghosh Acta Materialia 92 (2015) 220
Fig. 1b) Aberration-corrected STEM image of a B2/L21-interface. The coloring scheme is based on the nearest neighbor intensity differences of the Al-sublattice to highlight the differences in ordering and the interface fluctuation. An intensity difference of 0 represents perfect B2- (red) and >2.5 L21-order (green).
C.H. Liebscher, V.R. Radmilovic, U. Dahmen, N.Q. Vo, D.C. Dunand, M. Asta, G. Ghosh Acta Materialia 92 (2015) 220
