Luke N. Brewer

Misorientation Mapping for Visualization of Plastic Deformation via Electron Back-Scattered Diffraction

The ability to map plastic deformation around high strain gradient microstructural features is central in studying phenomena such as fatigue and stress corrosion cracking. A method for the visualization of plastic deformation in electron back-scattered diffraction (EBSD) data has been developed and is described in this article. This technique is based on mapping the intragrain misorientation in polycrystalline metals. The algorithm maps the scalar misorientation between a local minimum misorientation reference pixel and every other pixel within an individual grain. A map around the corner of a Vickers indentation in 304 stainless steel was used as a test case. Several algorithms for EBSD mapping were then applied to the deformation distributions around air fatigue and stress corrosion cracks in 304 stainless steel. Using this technique, clear visualization of a deformation zone around high strain gradient microstructural features (crack tips, indentations, etc.) is possible with standard EBSD data.

A diffusion-multiple approach for mapping phase diagrams, hardness, and elastic modulus

This paper seeks to illustrate the power of a diffusion-multiple approach in efficient mapping of phase diagrams and materials properties for multicomponent alloy systems, showing that many ternary phase diagrams can be mapped from a single diffusion multiple. The diffusion profiles also allow evaluation of diffusion coefficients. Moreover, the composition variations created in the diffusion multiple allow efficient mapping of hardness and elastic modulus as a function of composition and phases. Examples will be given for precious metal systems currently used in high-temperature sensors. For these systems, the diffusion-multiple approach also produces a tremendous cost saving compared to making heats of various alloys, preparing samples, and measuring properties. This approach uses several microanalytical techniques, such as electron probe microanalysis, electron backscatter diffraction, and instrumented nanoindentation.

Interface modification for increased fracture toughness in reaction-formed yttrium aluminum garnet/alumina eutectic composites

The validity of controlling interfacial toughness in reaction-formed composites was explored using solid-state reaction processing and microanalysis techniques. A variety of rare-earth oxides was added to a yttrium aluminum garnet (YAG)/alumina powder mixture and then melted in air. Some melts retained the crystallography and microstructure of the pure, binary YAG–alumina eutectic. Using scanning transmission electron microscopy in conjunction with energy dispersive X-ray spectroscopy, rare-earth ions were observed both to segregate to the YAG/alumina interface and to form a third phase. Some evidence of increased crack deflection at these interfaces was observed via indentation fracture.

Reactive Ni/Ti nanolaminates

Nickel/titanium nanolaminates fabricated by sputter deposition exhibited rapid, high-temperature synthesis. When heated locally, self-sustained reactions were produced in freestanding Ni/Ti multilayer foils characterized by average propagation speeds between ∼0.1 and 1.4 m/s. The speed of a propagating reaction front was affected by total foil thickness and bilayer thickness (layer periodicity). In contrast to previous work with compacted Ni–Ti powders, no preheating of Ni/Ti foils was required to maintain self-propagating reactions. High-temperature synthesis was also stimulated by rapid global heating demonstrating low ignition temperatures (Tig)∼300–400 °C for nanolaminates. Ignition temperature was influenced by bilayer thickness with more coarse laminate designs exhibiting increased Tig. Foils reacted in a vacuum apparatus developed either as single-phase B2 cubic NiTi (austenite) or as a mixed-phase structure that was composed of monoclinic B19′ NiTi (martensite), hexagonal NiTi2, and B2 NiTi. Single-phase, cubic B2 NiTi generally formed when the initial bilayer thickness was made small.Nickel/titanium nanolaminates fabricated by sputter deposition exhibited rapid, high-temperature synthesis. When heated locally, self-sustained reactions were produced in freestanding Ni/Ti multilayer foils characterized by average propagation speeds between ∼0.1 and 1.4 m/s. The speed of a propagating reaction front was affected by total foil thickness and bilayer thickness (layer periodicity). In contrast to previous work with compacted Ni–Ti powders, no preheating of Ni/Ti foils was required to maintain self-propagating reactions. High-temperature synthesis was also stimulated by rapid global heating demonstrating low ignition temperatures (Tig)∼300–400 °C for nanolaminates. Ignition temperature was influenced by bilayer thickness with more coarse laminate designs exhibiting increased Tig. Foils reacted in a vacuum apparatus developed either as single-phase B2 cubic NiTi (austenite) or as a mixed-phase structure that was composed of monoclinic B19′ NiTi (martensite), hexagonal NiTi2, and B2 NiTi. Singl...

Multivariate statistics applications in phase analysis of STEM-EDS spectrum images

Spectrum imaging (SI) methods are displacing traditional spot analyses as the predominant paradigm for spectroscopic analysis with electron beam instrumentation. The multivariate nature of SI provides clear advantages for qualitative analysis of multiphase specimens relative to traditional gray-scale images acquired with non-spectroscopic signals, where different phases with similar average atomic number may exhibit the same intensity. However, with the improvement in qualitative analysis with the SI paradigm has come a decline in the quantitative analysis of the phases thus identified, since the spectra from individual pixels typically have insufficient counting statistics for proper quantification. The present paper outlines a methodology for quantitative analysis within the spectral imaging paradigm, which is illustrated through X-ray energy-dispersive spectroscopy (EDS) of a multiphase (Pb,La)(Zr,Ti)O(3) ceramic in scanning transmission electron microscopy (STEM). Statistical analysis of STEM-EDS SI is shown to identify the number of distinct phases in the analyzed specimen and to provide better segmentation than the STEM high-angle annular dark-field (HAADF) signal. Representative spectra for the identified phases are extracted from the segmented images with and without exclusion of pixels that exhibit spectral contributions from multiple phases, and subsequently quantified using Cliff-Lorimer sensitivity factors. The phase compositions extracted with the method while excluding pixels from multiple phases are found to be in good agreement with those extracted from user-selected regions of interest, while providing improved confidence intervals. Without exclusion of multiphase pixels, the extracted composition is found to be in poor statistical agreement with the other results because of systematic errors arising from the cross-phase spectral contamination. The proposed method allows quantification to be performed in the presence of discontinuous phase distributions and overlapping phases, challenges that are typical of many nanoscale analyses performed by STEM-EDS.

Forensic analysis of bioagents by X-ray and TOF-SIMS hyperspectral imaging

Hyperspectral imaging combined with multivariate statistics is an approach to microanalysis that makes the maximum use of the large amount of data potentially collected in forensics analysis. This study examines the efficacy of using hyperspectral imaging-enabled microscopies to identify chemical signatures in simulated bioagent materials. This approach allowed for the ready discrimination between all samples in the test. In particular, the hyperspectral imaging approach allowed for the identification of particles with trace elements that would have been missed with a more traditional approach to forensic microanalysis. The importance of combining signals from multiple length scales and analytical sensitivities is discussed.

Risks of "Cleaning" Electron Backscatter Diffraction Data

Collecting good data is an important task, but handling the data correctly is important also. How to handle data largely depends on what the analyst is going to do with it. Electron backscatter diffraction (EBSD) is no exception.

Competitive Abnormal Grain Growth between Allotropic Phases in Nanocrystalline Nickel

The central driving force for the study of nanoscale materials is the expectation that these materials will behave in substantially different ways from their more traditional, microscale counterparts. In particular, there has been significant effort to understand the changes in thermodynamic phase stability as a function of nanoparticle or nanocrystallite size. It has been shown that allotropic stability, melting temperature, and lattice parameters all change for particles below a certain size. For example, metals such as chromium, molybdenum, and tungsten are observed in the face-centered cubic (fcc) structure instead of their normal body-centered cubic (bcc) structure, for particles less than 30 nm in diameter. When processed through traditional means, nickel is almost always observed with the fcc crystal structure. However, there have been reports of nanocrystalline nickel, produced by chemical routes, thermal reduction and pulsed laser deposition (PLD), existing with the hexagonal close-packed (hcp) crystal structure. In all of these examples, the supposition and observation have been that the hcp phase is less stable than the fcc phase and that annealing of the material would cause a direct phase transformation to the more stable form. The work of Vergara and Madurga on nickel thin films created by PLD has shown specifically that films with an initially single-phase hcp crystal structure can be systematically transformed as a function of time and temperature to single-phase fcc structure. Here we show that on annealing at temperatures between 523–623K for times ranging from 10 to 840min, fcc Ni films produced by PLD exhibit abnormal grain growth and that these abnormal grains exhibit either the hcp or the fcc structure. That is, the hcp and fcc abnormal grains coexist and grow competitively with increasing annealing time. In addition, the defect structure developed to accommodate the excess free volume released from the consumed grain boundaries is dependent on the crystal structure. We hypothesize that it is this difference in the defect accommodation mechanisms within the hexagonal and cubic lattices that allow this metastable arrangement to exist.

Analyses of eutectoid phase transformations in Nb-silicide in situ composites.

Nb-silicide in situ composites have great potential for high-temperature turbine applications. Nb-silicide composites consist of a ductile Nb-based solid solution together with high-strength silicides, such as Nb5Si3 and Nb3Si. With the appropriate addition of alloying elements, such as Ti, Hf, Cr, and Al, it is possible to achieve a promising balance of room-temperature fracture toughness, high-temperature creep performance, and oxidation resistance. In Nb-silicide composites generated from metal-rich binary Nb-Si alloys, Nb3Si is unstable and experiences eutectoid decomposition to Nb and Nb5Si3. At high Ti concentrations, Nb3Si is stabilized to room temperature, and the eutectoid decomposition is suppressed. However, the effect of both Ti and Hf additions in quaternary alloys has not been investigated previously. The present article describes the discovery of a low-temperature eutectoid phase transformation during which (Nb)3Si decomposes into (Nb) and (Nb)5Si3, where the (Nb)5Si3 possesses the hP16 crystal structure, as opposed to the tI32 crystal structure observed in binary Nb5Si3. The Ti and Hf concentrations were adjusted over the ranges of 21 to 33 (at.%) and 7.5 to 33 (at.%) to understand the effect of bulk composition on the phases present and the eutectoid phase transformation.

Zinc oxide growth morphology on self-assembled monolayer modified silver surfaces.

Using organic molecules to direct inorganic crystal growth has opened up new avenues for controlled synthesis on surfaces. Combined with soft lithography to form patterned templates, self-assembled monolayers (SAMs) have been shown to be a powerful approach for the assembly of inorganic nanostructures. In this work, we show that the surface free energy of SAM-modified silver, which depends on end groups and deposition method of SAMs, has a dramatic effect on the nucleation and growth of crystalline ZnO, a technologically important material, from supersaturated solutions. For SAMs with inert methyl end groups, ZnO nucleation is inhibited. For SAMs with chemically active (carboxylic or thiol) end groups, the ZnO morphology is found to be three-dimensional nanorods on low-surface-energy surfaces and two-dimensional thin films on high-energy surfaces.