Hierarchical microstructure of ferritic superalloys
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.
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.