Robert E.A. Williams

Nature of the interfaces between the constituent phases in the high entropy alloy CoCrCuFeNiAl

The interfaces between the phase separated regions in the dendritic grains of laser-deposited samples of the high entropy alloy CoCrCuFeNiAl have been studied using aberration-corrected analytical (scanning) transmission electron microscopy ((S)TEM). The compositional variations have been determined using energy dispersive x-ray spectroscopy (EDS) in (S)TEM. It was found that between B2, consisting mainly of Al, Ni, Co, and Fe, and disordered bcc phase, consisting mainly of Cr and Fe, there is a transition region, approximately 1.5 nm in width, over which the chemical composition changes from the B2 to that of the bcc phase. The crystal structure of this interfacial region is also B2, but with very different sublattice occupancy than that of the adjacent B2 compound. The structural aspects of the interface between the ordered B2 phase and the disordered bcc phase have been characterized using high angle annular dark-field (HAADF) imaging in STEM. It has been determined that the interfaces are essentially coherent, with the lattice parameters of the two B2 regions and the disordered bcc phase being more or less the same, the uncertainty arising from possible relaxations from the proximity of the surfaces of the thin foils used in imaging of the microstructures. Direct observations show that there is a planar continuity between all three constituent phases.

Transmission Electron Microscopy for Physical Metallurgists

Abstract The aim of this chapter is to provide the reader with an outline of the methods most commonly used to characterize samples, taken from studies involving physical metallurgy, using techniques allied with transmission electron microscopy. In the main, rather than provide full and comprehensive details on all methods including various imaging techniques, diffraction methods, and analytical spectroscopies, it is the attempt of the coauthors to outline techniques, and then provide ways in which difficulties in interpretation may be avoided.

Characterizing Sub-lattice Occupancies in B2 Phases in High Entropy Metallic Alloys using Atomic Resolution STEM-XEDS Mapping

In the recent past, there has been considerable emphasis placed on the exploration of high entropy alloys (HEA). These alloys have been defined as ones with five or more, essentially equal atomic concentrations [1, 2]. CoCrCuFeNiAl is an example of an HEA alloy which mainly consists of two phases, namely ordered B2 and disordered bcc. Although this alloy has been the subject of much study and its microstructures characterized using a number of techniques, only recently has aberration-corrected (S)TEM coupled with x-ray energy dispersive spectroscopy (XEDS), involving large collection angles (ChemiSTEMTM), been applied [3]. In this latter study, it was found that, between the B2, consisting mainly of Al, and Ni, Co, and Fe, and disordered bcc phases, consisting mainly of Cr and Fe, there is a transition region, approximately 1.5nm in width, over which the chemical composition changes from the B2 to that of the bcc phase. The crystal structure of this interfacial region is also B2, but with a significantly different sub-lattice occupancy than that of the adjacent B2 compound. In these B2 phases with very differing compositions, and hence sub-lattice compositions, the intensities of both atomic columns in HAADF images and superlattice reflections in diffraction patterns may vary considerably [3]. The origin of these intensity differences is of interest. For example, when the difference in the intensities of atomic columns in each of the sub-lattices is small, this may be interpreted as either the average compositions of the sub-lattices being similar, and/or being a reduced degree of order of the B2 compound. It is obvious that in order to be able to understand the behavior of these alloys, it is necessary that the degree of order be known. The first of these possibilities may be assessed by making direct measurements of the sub-lattice composition, while the second possibility, degree of ordering, may be assessed by plotting these compositions on an ordering tie-line diagram [4]. The current study involves the direct measurement of sub-lattice compositions.

Novel Investigative Preparation of Human Hair

Preparing hair samples for electron microscopy has been problematic for various reasons. Keratinized hair is densely packed and inherently dry with the proteins heavily cross-linked[1]. This eliminates the need for primary and secondary fixation. However, the inability to uniformly stain through the cuticle layers and throughout the central cortex has shown varying results[1]. The hair samples in this investigation were treated with an oxidative permanent colorant and washed in tap water containing low levels of copper (Cu). The amount of Cu in the hair was confirmed using inductively coupled plasma optical emission spectroscopy (ICP-OES). In order to optimize and better understand the effect of sample preparation on maintaining the native hair structure as well as internal chemical composition, analytical electron microscopy (AEM) characterization was performed [2,3]. In particular, techniques such as S/TEM-HAADF imaging and Super-X XEDS were used to investigate the ultrastructure and chemistry of the cross sectional surface of several hair fibers.

Characterizing Atomic Ordering in Intermetallic Compounds Using X-ray Energy Dispersive Spectroscopy in an Aberration-Corrected (S)TEM

Intermetallic compounds have been the subject of considerable interest as structural and functional materials in a wide range of applications. They are used as engineered materials themselves or as second phase components in high performance applications. It is important that these materials be fully characterized to permit effective alloy development to meet the requisite balance of properties for a given application. One important parameter is the degree to which these compounds are ordered. This is particularly important in a new series of materials known as high entropy alloys (HEA), or compositionally complex alloys (CCA), which contain typically four to six alloying elements, each at or near to equi-atomic concentrations. Many of these CCAs contain an intimate mixture of a disordered bcc phase and an ordered phase with the B2 crystal structure. These ordered phases contain reasonable concentrations of many of the alloying elements in the alloy, and so it is of interest to know how the elements are partitioned to the two sublattices in the ordered structure and to what degree anti-site defects are tolerated, i.e., what is the degree of order in these compounds. An approach to the determination of this latter parameter has been afforded by the combination of aberration-corrected S(TEM) instruments, where electron probe sizes less than interatomic spacings may be achieved coupled with x-ray energy dispersive spectrometers (XEDS) making use of silicon drift detectors(SDD) and large collection angles. This approach has been adopted in the present research, making use of an FEI ThemisTM instrument equipped with Super-XTM XEDS. Experimentally, the compositions of the individual sublattices are determined from spatially-resolved XEDS measurements and subsequent data analysis, and these compositions are plotted onto an Ordering Tie-Line diagram[1], from which the degree of order is deduced.

Characterization of Nano-scale Instabilities in Titanium Alloys Using Aberration-Corrected Scanning Transmission Electron Microscope

Due to the refined nature of microstructures that can be effectively manipulated by the application of various thermal/mechanical processes, metastable beta titanium alloys have attracted considerable attention in recent days. Usually, such refinement involves the precipitation of the intragranular hcp structure alpha phase. In authors recent studies, it has been shown that the size, morphology and number density of these alpha precipitates in Ti-5Al-5Mo-5V-3Cr (Ti-5553, wt.%) can be significantly influenced by the nano-scale structural and compositional instability present in parent bcc structure beta phase, more specifically in this alloy, the metastable hexagonal structure isothermal omega phase [1-3]. In these latter studies, it was found that either the compositional and/or stress field associated with the isothermal omega phase may contribute to an increased driving force for alpha nucleation [3]. Recent technological improvement in TEM’s, spectroscopy detectors and cameras, specifically probe aberration corrected STEM instruments, have enabled atomic resolution z-contrast high angle annular dark fieldscanning transmission electron microscopy (HAADF-STEM) imaging capable of characterizing atomic column configurations with a sub-angstrom probe [4] and provides novel insights of new nano-scale instabilities in titanium alloys.

Characterization of Stannous Fluoride Uptake in Human Dentine by Super-X XEDS and Dual-EELS analysis

Stannous fluoride (SnF2) is a common additive to dental products and has been shown to reduce the dental hyper-sensitivity in patients. In order to elucidate and better understand the permeability and mass transport mechanisms, analytical electron microscopy (AEM) characterization was performed on human dentin exposed to SnF2 [1]. In particular, techniques such as S/TEM-HAADF imaging, Super-X XEDS and dual-EELS have been used to investigate the ultrastructure and chemistry of the inner dentine tubule surface. Results on the characterization of the Sn-reacted product on the inner surface of dentine microtubules, as well as the role of dentine “nano-tubules” that branch off from the primary microtubule will be discussed.

Correlative STEM-Cathodoluminescence and Low-Loss EELS of Semiconducting Oxide Nano-Heterostructures for Resistive Gas-Sensing Applications

Recent advances in resistive-type oxide gas sensors have been made primarily by the combination of multiple materials into nano-heterostructures that often show enhanced or unique properties compared to each pure material constituent [1,2]. It has been shown to be especially useful to use highly crystalline one-dimensional nanorods and nanowires decorated with either discrete oxide particles or a continuous coating generating a core-shell structure [3]. The operating principle of these sensors is simply measuring a change in resistance of a film deposited between two or more electrodes, which varies with the number of charge carriers accepted or donated between the surface and the nearby gas molecules. Two different resistance-dominating mechanisms exist in these sensors when the nanowires are deposited as a random network film [1]. In both mechanisms a depletion layer is formed at the surface and a larger depletion layer creates a higher resistance. The first mechanism considers the potential energy barrier at the interface between two nanowires that an electron must overcome to move through the film. The second mechanism considers electrons moving along the axis of a nanowire and the constriction of the cross-sectional area that does not lie in the depletion region. This can be equated to pushing a current through a smaller diameter wire, which increases the resistance. The second axial mechanism can be engineered through decoration or coating of, for example, p-type Cr2O3 onto an n-type SnO2 nanowire. The p-n junctions created can affect this mechanism greatly, enhancing the sensor response and even making the material more selective toward specific gases and reducing cross-interference.

Characterization of Alpha/Beta Interface Structure in a Titanium Alloy Using Aberration-Corrected Scanning Transmission Electron Microscope

In body centered cubic (bcc) β titanium alloys, the precipitate most commonly found in the β matrix is the hexagonal closed packed (hcp) α phase, with the two often in the Burgers orientation [1]. The anisotropy present in the α/β interfacial structure influences the morphology of the α precipitate, and structural defects, such as dislocations and ledges present in the α/β interface, may also affect precipitate-dislocation interactions, and so change the deformation mechanisms. Recently, three different fine scale alpha microstructures in a metastable beta titanium alloy, Ti-5Al-5Mo-5V-3Cr (Ti5553), have been investigated by Zheng et al, classified as refined, more-refined, and super-refined α microstructures [2-5]. The refined α microstructures (number density ≤10/mm), shown in Fig. 1, were produced by step-quenching from above the β transus to 600°C, and rapidly heating (rate ~100°C/min) the as-quenched material to 600°C [2, 3]. It is observed a) that the nucleation and growth of the α plates is extremely rapid, and b) that they align along three specific crystallographic orientations. In order to understand the mechanism of formation of the refined α microstructure in these alloys, a detailed study of the α/β interface structure in the early stages of α phase precipitation, is required.

Characterizing Atomic Ordering of High Entropy Alloys Using Super-X EDS Characterization

High entropy alloys (HEA), more recently referred to as compositionally complex alloys (CCA), are a new group of alloys receiving a great deal of attention because of the potentially remarkable balance of properties they are expected to exhibit. They offer new pathways to lightweighting in structural applications, new alloys for intermediate and elevated temperature components, and new magnetic materials[1,2]. To realize their potential, however, requires considerable alloy development that will rely on application of integrated computational materials (science and) engineering (ICME), which requires accurate computational models predicting their performance in addition to a detailed knowledge of, for example, their deformation mechanisms. Often, these alloys consist of a mixture of ordered and disordered phases, and because of the compositional complexity, it is necessary to know the nature (i.e., degree of order, site occupancy, and presence of anti-site defects) in the ordered phases if effective models of the deformation behavior are to be developed[3]. These various metrics require accurate compositional measurements at the atomic scale.