Three-dimensional characterisation of the microstructure of an high entropy alloy using STEM/HAADF tomography

The microstructure of a high entropy alloy with composition of Mo 0.5 Al 1 Nb 1 Ta 0.5 Ti 1 Zr 1 (the digits refer to molar volumes) has been characterised directly in three dimensions using TEM dark field (DF) imaging and by recording tilt pair micrographs using STEM high angle annular DF (HAADF) imaging. The microstructure contains disordered bcc precipitates that appeared as orthogonal stacks of plate-like features. A tapered needle sample was prepared in a focused ion beam/SEM and was used to acquire a 180° tomographic dataset of STEM/HAADF images in 2° increments. The tilt series images were registered, and the algebraic reconstruction technique was used to reconstruct the three-dimensional microstructure. The bcc precipitates were segmented using a combinative approach involving two threshold techniques. The precipitates were then visualised using commercial software, which revealed surprisingly the existence of both cuboidal and plate-like morphologies. Colouring each precipitate according to its morphology (determined using the omega-2 moment invariant) revealed a precipitate arrangement where plate-like features appeared parallel to each cuboid face.