R.E.A. Williams

Rapid characterization of titanium microstructural features for specific modelling of mechanical properties

Mechanical properties of α/β Ti alloys are closely related to their microstructure. The complexity of the microstructural features involved makes it rather difficult to develop models for predicting properties of these alloys. Advances in stereology and microscopy permit rapid characterization of various features in Ti alloys including Widmanstatten α-laths, grain sizes, grain shapes, colony structures and volume fractions of different phases. This research documents the stereology procedures for characterizing microstructural features in Ti alloys, including the use of three-dimensional serial sectioning and reconstruction procedures for developing through material measurements. The resulting data indicate the powerful characterization processes now available, and the ability to rapidly assess microstructural features in Ti alloys. The processes were tested using Ti-62222 by serial sectioning the sample and conducting automated stereology protocols to determine features. In addition, three-dimensional reconstruction was completed on a Ti-6242 sample to evaluate lath interactions within the alloy. Results indicate the tremendous potential for characterizing microstructures using advanced techniques.

Epitaxial growth of iridate pyrochlore Nd2Ir2O7 films.

Epitaxial films of the pyrochlore Nd2Ir2O7 have been grown on (111)-oriented yttria-stabilized zirconia (YSZ) substrates by off-axis sputtering followed by post-growth annealing. X-ray diffraction (XRD) results demonstrate phase-pure epitaxial growth of the pyrochlore films on YSZ. Scanning transmission electron microscopy (STEM) investigation of an Nd2Ir2O7 film with a short post-annealing provides insight into the mechanism for crystallization of Nd2Ir2O7 during the post-annealing process. STEM images reveal clear pyrochlore ordering of Nd and Ir in the films. The epitaxial relationship between the YSZ and Nd2Ir2O7 is observed clearly while some interfacial regions show a thin region with polycrystalline Ir nanocrystals.

Investigations of Omega Precipitation in Titanium Molybdenum Alloys by Coupling 3D Atom Probe Tomography and High Resolution (S)TEM

Titanium-base alloys are used in a number of critical components in aerospace and defense, biomedical, automotive, and a range of other industries. These alloys typically exhibit complex multi-phase microstructures spanning across a range of length scales and also involving a large number of alloying additions. The ω phase is commonly observed in many commercial β or nearβ titanium alloys on quenching from the solution treatment temperature in the single beta phase field [1]. These ω precipitates typically have an embrittling effect on the alloy and are therefore considered detrimental for its mechanical properties [2]. However, since ω precipitates are highly refined (nanometer scale) and homogeneously distributed, and due to the fact that they reject β-stabilizing elements, it is possible that they can act as heterogeneous nucleation sites for the precipitation of the equilibrium α phase. This leads to a homogeneous distribution of refined α precipitates that can substantially strengthen the alloy [1,3]. Therefore, the detailed investigation of ω precipitation in the beta matrix of titanium alloys is rather important.

Cation non-stoichiometry in pulsed laser deposited Sr2+yFe1+xMo1-xO6 epitaxial films

Sr2FeMoO6 (SFMO) films were grown on SrTiO3 (100)- and (111)-oriented substrates via pulsed laser deposition (PLD). In order to study the fundamental characteristics of deposition, films were grown in two different PLD chambers. In chamber I, the best films were grown with a relatively long substrate-to-target distance (89 mm), high substrate temperature (850 °C), and low pressure (50 mTorr) in a 95% Ar/5% H2 atmosphere. Although X-ray diffraction (XRD) measurements indicate these films are single phase, Rutherford Backscattering (RBS) measurements reveal considerable non-stoichiometry, corresponding to a Sr2Fe1−xMo1+xO6 composition with x ≅ 0.2–0.3. This level of non-stoichiometry results in inferior magnetic properties. In chamber II, the best films were grown with a much shorter substrate-to-target distance (38 mm), lower temperature (680 °C), and higher pressure (225 mTorr). XRD measurements show that the films are single phase, and RBS measurements indicate that they are nearly stoichiometric. The de...

Investigation of Possible Nucleation Mechanisms for Producing an Ultra-Refined Alpha Phase Microstructure in Beta Titanium Alloys Using High-Resolution Electron Microscopy and 3D Atom Probe Tomography

Titanium alloys contain complex, interdependent microstructural features that can range from nm to mm in length. In order to fully probe the salient microstructural features across this variation in length scale, multiple characterization techniques must be implemented and correlated. For the work to be presented, conventional transmission electron microscopy (CTEM), selected area diffraction (SAD), scanning transmission electron microscopy (STEM) utilizing Z-contrast imaging and apparent atomic resolution EDS mapping as well as 3D atom probe tomography (3DAP) were all correlated to fully characterize the microand nanometer structural features, as well as compositional variations, that might possibly influence alpha phase nucleation. Moreover, the efforts of this research lead to the discovery of a previously unknown phase that is believed to play an impactful role in alpha phase nucleation.

Characterizing Epitaxial Growth of Nd 2 Ir 2 O 7 Pyrochlore Thin Films via HAADF-STEM Imaging and EDX

Much interest has been given to the 5d transition metal oxides because of their strong spin-orbit coupling, leading to the prediction of new exotic phases of matter, including the Weyl semimetal, topological Mott insulator, and spin liquid [1–4]. The pyrochlore iridates with the general formula of A2Ir2O7 have been predicted to show the Weyl semimetal and topological insulator phases under epitaxial strain [5,6]. These theoretical predictions have motivated the study and growth of epitaxial thin films of such pyrochlore iridates. In this work, we report the synthesis of Nd2Ir2O7 and growth of epitaxial thin films using off-axis magnetron sputtering and ex-situ post-growth annealing. Using aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) and energy dispersive X-ray spectroscopy (EDX), the growth mechanisms of Nd2Ir2O7 are characterized. A thermodynamic explanation of the observed mechanisms is presented.

Quantifying Ordering Phenomena Through High-Resolution Electron Microscopy, Spectroscopy, and Simulation

Advances in aberration corrected scanning transmission electron microscopy (STEM) have allowed researchers to investigate structure-property relationships at the atomic scale [1–4]. In many systems, ordering phenomena at the atomic level can have a dramatic impact on the structural, electronic, and magnetic properties of the material. By combining experiment, simulation, and data processing, these ordering phenomena can be studied in a quantitative way, opening the door for new insights into the structure-property relationships of a wide variety of material systems. Experimentally, high angle annular dark field (HAADF) STEM and energy dispersive X-ray spectroscopy (EDX) are two very powerful techniques for probing compositional variations at the Ångstrom scale. In order to fully understand and quantify these techniques, image simulation and ionization calculations using the quantum excitation of phonons model can be used [5,6]. For the first time, using a double aberration corrected FEI ThemisTM with a Super-XTM XEDS detector compositional mapping of a Ni-based superalloy (commercially available HL-11) was collected at atomic resolution across a stacking fault, as seen in Figure . Individual spectra in the atomic resolution XEDS maps exhibit low signal-to-noise, usually having very few counts per channel. Based on previous characterization of the fault structure determining structural periodicity[4], the data was summed over a repeating unit cell of the fault structure along the [110] projection, resulting in almost an order of magnitude increase in the peak maxima of the XEDS spectra and even larger increase in integrated peak counts. These modified 3D data cubes were then fed into the Bruker Esprit software package and quantified using experimentally determined Cliff-Lorimer k-factors from a solutionized sample of the same material. By preprocessing these data before quantification, the error in the quantification was significantly reduced, allowing for site-specific determination of solute segregation in and around the fault structure in this Ni-based superalloy, leading to the determination of a novel high temperature strengthening mechanism.

15 Years of Characterizing Titanium Alloys' Microstructure by DBFIB

Over the past 15 years, transformative advances have been realized in the field of three-dimensional microscopy for microand nanometer scale microstructural morphology and phase characterization. This is in no small part due to the development and maturation of the small, dual-beam focused ion beam (DBFIB). OSU accepted delivery of FEI company’s first commercially available small DB-FIB platform in 1999 and immediately began TEM sample preparation by ex-situ lift-out. Shortly after delivery of the instrument, the technique of serial sectioning characterization was attempted and some of the initial serial sectioning data sets were collected during 2000 and 2001 by researchers at The Ohio State University (OSU) and WrightPatterson Air Force Base with FEI company as shown in Figure 1[1].

Viability of HAADF-STEM Imaging Contrast and Simulations as a Measure of B-site Ordering for Double Perovskites

Spintronics has emerged as a promising technology that exploits both the intrinsic spin of the electron and it associated magnetic moment in a solid-state device. Utilization of the spin degree of freedom in metals and semiconductors has potential to create significant technological advances over current, charge-based technologies[1]. Therefore, accurate, atomic level characterization of half-metallic spin injectors and magnetic semiconductors will be essential to realizing these next generation technologies.

HR-STEM Imaging and EELS Characterizing of Nano-Scale Defects in Sputter Deposited Thin Films of Double-Perovskite Sr 2 FeMoO 6 (SFMO) and Sr 2 CrReO 6 (SCRO)

Oxides are promising candidates in the emerging field of spintronics. Some oxide systems of interest include double perovskites like Sr2FeMoO6 (SFMO) and Sr2CrReO6 (SCRO). Sr2FeMoO6 is the most studied half-metallic double perovskite with the potential for room-temperature magnetoelectronic applications due to its Curie temperature being above 400 K[1]. However, Sr2FeMoO6 has not been incorporated into devices that reflect its high spin polarization largely due to lack of understanding of some fundamental questions about the fabrication of Sr2FeMoO6 films. Due to the complexity in the A2BB’O6 double perovskites and their stringent requirements for growth conditions, it is rather challenging to obtain stoichiometric, phase pure, and fully epitaxial Sr2FeMoO6 films with high double perovskite ordering between B and B’ sites; all of these properties are essential to achieve half-metallicity[2-6]. The magnetic properties of these systems have also been found to depend on the amount of disorder present as well as other defects. The major challenge during layer deposition is obtaining the necessary long-range ordering while preventing defect formation due to processing parameters. This requires structural and chemical characterization ranging from identification of defects and morphology as well as correlating microstructure-property relationships. For this work thin films of SFMO and SCRO have been sputter deposited on (100) and (111) STO with varying deposition and sputtering parameters in an attempt to produce optimal electronic properties within the oxide layer. HAADF-STEM imaging and EELS analyses were conducted using an aberrationcorrected FEI TitanTM80-300 STEM to provide microstructural and chemical information. The first HAADF-STEM images of SFMO and SCRO double perovskite ordering are presented as well as characterization of interfacial defects (FIG. 1-3) [7]. Initial EDX results were also collected using a FEI Osiris TEM to map the chemical nature of the defects present. Defect images are also compared with STEM multislice simulations for validation Special care was taken in preparing (100) and (110) cross sectional TEM samples for HR-STEM using a Helios Nanolab for bulk trenching and thinning. The foil was extracted using an Omniprobe 200 micromanipulator and mounted on a Cu grid in a flag orientation for subsequent low kV milling at 500 eV to remove amorphous ion damage and provide optimal surface quality for high resolution analysis.