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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...

Over the last decade dual beam FIB has proved increasingly useful in a laboratory setting. Researchers have used dual beam FIB's in a wide variety of experiments, including but in no way limited to fabrication of nano pillars for micromechanical testing, modification of circuits during use, and the creation of tooling for nanoscale machining. The field of high resolution microscopy in particular has seen huge benefits from FIB processing in the form of high quality orientation specific lamella. One drawback of the technique is the formation of damage caused as a result of the impact of fast gallium ions on samples. This damage takes the form of the formation of lattice defects (e.g., dislocation loops) and/or amorphous layers on the surfaces of the FIB'd samples. The latter form of damage can degrade the quality of high resolution images and so the mechanism of its formation has been the subject of considerable study. However, these various studies of amorphization during FIB processing have largely focused on the effect of gallium ions on electronic materials. This concentration of research is likely due to several primary factors, the extensive use of and investment by the semiconductor industry in FIB research and the geometrical factors complicating measurement of amorphous damage. In our research, we have found that there are considerable differences between the amounts of amorphous damage between, on the one hand, semiconductors and, on the other hand, metallic materials. The present paper focuses on attempts to identify the origin for these differences.The main problem in studying the formation of amorphous damage is the consistency in results as a function of the specifics of sample geometry. Most often, thin FIM lamellae are used, and the problem with the use of such samples is the difficulty in distinguishing between "primary" damage and redeposition of material that has been initially milled from the sample surface [1]. In the present study, needle shaped samples have been employed as these allow for consistent measurements of damage formation (as a function of sample thickness, which can be readily determined from the local diameter of the needle), and by positioning of a needle in close proximity to one being milled, the phenomenon of re-deposition can also be examined. Multiple needles may be made using a single FIB trenching operation. In this study one 28 µm * 1.5 µm * 4.5 µm foil was sufficient to create six posts suitable for needle shaping and one slab suitable for creation of a TEM lamella. Redeposition was noted to be a major concern when making closely spaced needles, in Figure 1 . Needles made at 30 kV using NiAl and commercially pure metals such as Ti, Ni, Al, and Fe all contained disordered layers with average thicknesses ranging from a ≈ 2 nm to ≈ 8 nm.

High entropy alloys (HEAs) are a relatively new class of materials garnering a great deal of attention due to their remarkable balance of properties, including high strength, toughness, ductility, and corrosion resistance. In contrast to conventional alloy systems, HEAs are based on 4 or more principal elements with near equimolar concentrations and tend to have simple microstructures due to the preferential formation of solid solution phases attributed to the high configurational entropy of mixing [1]. HEAs appear to offer new pathways to lightweighting in structural applications, new alloys for intermediate and elevated temperature components, and new magnetic materials. However, to realize this potential requires considerable alloy development that will rely on application of integrated computational materials (science and) engineering (ICME). The development of accurate computational models to predict alloy performance requires a detailed knowledge of the morphology and interconnectedness of microstructural features. Due to spinodal decomposition, these alloys often consist of a mixture of ordered and disordered phases on the nano-scale, and because of the compositional and structural complexity, it is desirable to characterize the morphology as well as the chemical nature of the precipitates for inclusion in ICME models.Electron tomography is a powerful characterization technique capable of resolving microstructural features at length scales as small as a few nanometers when coupled with the high spatial resolution available with modern scanning transmission electron microscopes (STEM). For STEM tomography specifically, an electron beam is transmitted through a sample at incremental degrees of rotation and images are recorded at each tilt. The images are then processed to reconstruct a three-dimensional representation of the area of interest. Tomographic reconstructions, in general, may be produced by any set of images collected though an angle of rotation with the quality of the reconstruction depending directly on the angular tilt range collected. A previous tomography study on the AlMo0.5NbTa0.5TiZr HEA was reconstructed from only high angle annular dark-field (HAADF) STEM images and successfully resolved the nano-scale precipitates in the microstructure [2]. While HAADF contrast was sufficient to resolve and segment the nano-scale precipitates in this HEA, the reconstruction provided only morphological information and no compositional data. In order to obtain a novel perspective of the 3D elemental segregation in the HEA, this paper will present the use of Super-X™ energy dispersive xray spectroscopy (XEDS) spectral images (SI) utilized in conjunction with HAADF-STEM images for an improved tomographic reconstruction of the AlMo0.5NbTa0.5TiZr alloy.Rather than using a conventional thin foil STEM sample geometry, a tapered needle-like sample was created using a FEI Helios™ DualBeam FIB. This geometry was selected to eliminate thickness variations that occur with larger degrees of tilting with flat samples...

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