Phase transformation and morphological features in a cold-worked CrMnFeCoNi high entropy alloy with Al addition

Addition of Al to equiatomic high entropy alloy CrMnFeCoNi alloy with a single phase FCC structure is known to promote formation of a BCC phase. In addition, tetragonal intermetallic phase sigma was also found to form in a (CrMnFeCoNi)92Al8 alloy when subjected to processing by caliber rolling (8 passes) at room temperature followed by annealing at 800 circle C. Microstructure of the as-rolled alloy is complex, containing a high dislocation density and planar faults (including twins) which can be parallel and perpendicular to the rolling direction. Morphological features of the transformations induced by straining by caliber rolling have been determined by transmission electron microscopy (TEM). Two main morphologies have been determined. In one type of morphology, bundles of elongated domains of B2 ordered BCC phase form preferentially on {111} matrix planes, inside which sigma phase particles nucleate on interface with the matrix. In another type of morphology called "aggregate" here, "spines" of elongated grains of sigma phase form, over which domains of B2 phase nucleate driven by change in solute concentration ahead of the interface. The sigma phase forms by nucleation on planar faults in the matrix with its (001) plane, forming the known orientation relationship (OR) {001}sigma 11 {111}FCC and (110)sigma 11 (110)FCC. Since the planar faults in a caliber rolled alloy form parallel as well as perpendicular to the rolling direction, spines of the sigma phase elongated in three mutually perpendicular directions are shown to exist. OR of the B2 phase with the matrix is Kurdjumov-Sachs. The B2 phase matched pairs of close packed planes with sigma phase, but no particular OR was found. Elemental concentration profiles across interfaces suggests that sigma and B2 phases grow into the matrix. All the evidence suggests that the sigma phase nucleates on the planar faults in the matrix, and then B2 phase forms by elemental concentration changes in the matrix caused by growth of the sigma phase, forming the aggregate morphology.