A hierarchical microstructure due to chemical ordering in the bcc lattice: Early stages of formation in a ferritic Fe-Al-Cr-Ni-Ti alloy

A hierarchical microstructure is obtained in an alloy with composition Fe-8.1Al-12.2Cr-1.9Mo-18.2Ni-2.0Ti (wt.%) processed by melt-spinning. The evolution of the precipitation pathways is investigated using transmission electron microscopy (TEM) techniques, atom probe tomography (APT) and first-principles thermodynamic calculations. As-solidified ribbons exhibit a random dispersion of B2-ordered precipitates (NiAl-type) in an Fe-based matrix. Subsequent aging at 700 degrees C yields nucleation and growth of the L2(1)-phase (Ni2TiAl-type) within the primary B2-precipitates, leading to a microstructure exhibiting three types of hierarchy: (i) a structural hierarchy due to chemical ordering, with a chemically disordered matrix of bcc-Fe (A2), the nearest-neighbor (NN) ordered B2-precipitates (NiAl-type) and the next nearest-neighbor (NNN) ordered L2(1)-precipitates (Ni2TiAl-type) within B2, (ii) a dimensional hierarchy with a continuous bcc-Fe matrix, coherently embedded B2-precipitates, with a size range of 60200 nm and the coherent precipitate substructure, with L2(1)-phase and dimensions of 1520 nm. (iii) A spatial hierarchy where B2-precipitates are embedded in the bcc-Fe matrix and L2(1)-precipitates nucleate and grow only within B2-precipitates. In addition, it is verified that the interface between B2 and L2(1) is coherent and adopts a diffuse structural profile. Monte-Carlo simulations reproduce these observations and it is found that interface energies of B2 and L2(1) reduce from 50 mJ/m(2) at 0 K to 11 mJ/m(2) at 973 K. Kinetic-Monte-Carlo simulations support the interpretation of the experimental results that the L2(1) nucleates within the B2 phase. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.