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The microstructure of a high entropy alloy with composition of Mo0.5Al1Nb1Ta0.5Ti1Zr1 (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.

The recently developed refractory multi-principle element alloy, AlMo0.5NbTa0.5TiZr, shows an interesting microstructure with cuboidal precipitates of a disordered phase (β, bcc) coherently embedded in an ordered phase (β′, B2) matrix, unlike the conventional Ni-based superalloys where the ordered phase (γ′, L12) is the precipitate phase and the disordered phase (γ, fcc) is the matrix phase. It becomes critical to understand the phase transformation pathway (PTP) leading to this microstructure in order to tailor the microstructure for specific engineering applications. In this study, we first propose a possible PTP leading to the microstructure and employ the phase-field method to simulate microstructural evolution along the PTP. We then explore possible PTPs and materials parameters that lead to an inverted microstructure with the ordered phase being the precipitate phase and the disordered phase being the matrix phase, a microstructure similar to those observed in Ni-based superalloys. We find that in order to maintain the precipitates as highly discrete particles along these PTPs, the volume fraction of the precipitate phase needs to be smaller than that of the matrix phase and the elastic stiffness of the precipitate phase should be higher than that of the matrix phase.

Electron tomography is a three-dimensional technique well suited for characterizing fine-scale microstructural features on the order of 10-100 nm. Typically performed in a transmission electron microscope (TEM), it relies on successive image acquisition at multiple sample tilts. Images may be formed through various techniques including conventional and scanning transmission electron microscopy (CTEM and STEM), energy-filtered TEM (EFTEM), and energy dispersive x-ray spectroscopy (EDS). The variety of available image formation mechanisms has defined electron tomography as a robust fine-scale characterization tool.Conventional TEM tomography has relied on diffraction contrast as the primary imaging mode. However, this contrast may be insufficient for discriminating different phases in a complex microstructure. In these cases, more informative contrast may be obtained through compositional mapping, where characteristic x-ray emission is recorded over an area of interest. FEI's recently developed ChemiSTEM™ technology employs a detection configuration with four silicon drift detectors (SDD), providing a very large collection angle (0.8str), coupled with high incident beam current, resulting in the ability to collect x-ray maps on the order of electron micrographs [1]. Despite such efficiency, few tomographic reconstructions have employed ChemiSTEM™ mapping as the primary imaging mode. This paper will present novel microstructural characterization of the emerging material system of high-entropy alloys (HEAs) using ChemiSTEM™ tomography.HEAs offer an attractive balance of properties including high strength and corrosion resistance [2,3]. These materials often involve microstructures that result from phase separation and also spinodal decomposition, such that the resulting microstructures are three-dimensionally interconnected. It is important that the true distribution of these phases be established so that accurate models of the deformation processes may be developed, and hence it is necessary to invoke direct 3D characterization. The size-scale of the microstructural features (<100 nm) is well suited for electron tomography. Rather than acquire a tilt-series from a thin foil, a needle-shaped specimen was excised using a DualBeam™FIB/SEM, which offered several advantages. Firstly, when tilted about its longitudinal axis, the needle's symmetry avoided the projected thickness variations that occur when tilting thin foils. Secondly, the gradation of the needle's thickness allowed the authors to explore the limits of various reconstruction algorithms and evaluate their efficacy as a function of sample thickness.For this work, a FEI Titan 60-300 ChemiSTEM™ equipped with a quad-detector SDD was used to collect a tomographic tilt series of EDS spectral images from the aforementioned needle of HEA microstructure. Incident beam dwell times were 20 μs/px, with a live time of 600 s. The tomographic tilt series was collected about an axis parallel to the needle from -62° to +62° in 1° increments. In order to collect adequate sign...