Search

Abstract: This paper assesses the potential of multivariate statistical analysis (MSA) applied to electron backscattered diffraction (EBSD) data. Instead of directly indexing EBSD patterns on an individual basis, this multivariate approach reduces a large (thousands) set of individual EBSD patterns into a core set of statistically derived component EBSD patterns which can be subsequently indexed. The following hypotheses are considered in this paper: (1) experimental EBSD patterns from a microstructure can be analytically treated as linear combinations of spatially simple components, (2) MSA has an angular resolution on par with standard EBSD, (3) MSA can discriminate between similar and dissimilar phases, and (4) the MSA approach can improve the effective spatial resolution of automated EBSD. [Copyright &y& Elsevier]

Wrought and additively manufactured (AM) 316 L stainless steel samples were exposed to a simulated Brayton cycle supercritical CO 2 (sCO 2) environment, at temperature of 450 °C and pressure of 76 bar in 99.999% CO 2 for 2 weeks. The ground surfaces of wrought and AM 316 L specimens had average oxide thickness of 95 and 51 µm, respectively, composed primarily of Fe and Cr oxides. An as-printed AM 316 L sample was also exposed to the sCO 2 environment and exhibited negligible oxidation during the 2-week exposure because of a thick Cr rich Mn silicate native oxide covering most of surface. • As-printed surface of AM316L material is covered in a 20–1000 nm thick Si/Mn/Cr oxide. • Polished surfaces of AM oxidized at half the rate of a wrought 316 L counterpart in sCO 2. • The as-printed surface experienced minimal oxidation in sCO 2. [ABSTRACT FROM AUTHOR]

Density-functional theory (DFT) is used to identify phase-equilibria in multi-principal-element and high-entropy alloys (MPEAs/HEAs), including duplex-phase and eutectic microstructures. A combination of composition-dependent formation energy and electronic-structure-based ordering parameters were used to identify a transition from FCC to BCC favoring mixtures, and these predictions experimentally validated in the Al-Co-Cr-Cu-Fe-Ni system. A sharp crossover in lattice structure and dual-phase stability as a function of composition were predicted via DFT and validated experimentally. The impact of solidification kinetics and thermodynamic stability was explored experimentally using a range of techniques, from slow (castings) to rapid (laser remelting), which showed a decoupling of phase fraction from thermal history, i.e., phase fraction was found to be solidification rate-independent, enabling tuning of a multi-modal cell and grain size ranging from nanoscale through macroscale. Strength and ductility tradeoffs for select processing parameters were investigated via uniaxial tension and small-punch testing on specimens manufactured via powder-based additive manufacturing (directed-energy deposition). This work establishes a pathway for design and optimization of next-generation multiphase superalloys via tailoring of structural and chemical ordering in concentrated solid solutions. [Display omitted] [ABSTRACT FROM AUTHOR]

Novel materials based on the aluminum oxyhydroxide boehmite phase were prepared using a glycothermal reaction in 1,4-butanediol. Under the synthesis conditions, the atomic structure of the boehmite phase is altered by the glycol solvent in place of the interlayer hydroxyl groups, creating glycoboehmite. The structure of glycoboehmite was examined in detail to determine that glycol molecules are intercalated in a bilayer structure, which would suggest that there is twice the expansion identified previously in the literature. This precursor phase enables synthesis of two new phases that incorporate either polyvinylpyrrolidone or hydroxylpropyl cellulose nonionic polymers. These new materials exhibit changes in morphology, thermal properties, and surface chemistry. All the intercalated phases were investigated using PXRD, HRSTEM, SEM, FT-IR, TGA/DSC, zeta potential titrations, and specific surface area measurement. These intercalation polymers are non-ionic and interact through wetting interactions and hydrogen bonding, rather than by chemisorption or chelation with the aluminum ions in the structure. [Display omitted] • Glycothermal process intercalates bilayer glycol structure in boehmite. • New approach to intercalate nonionic polymers in layered boehmite sheets. • Surface charging properties are made more acidic by glycol bonding. • Nonionic polymers increase surface area during calcination processes. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we demonstrate the fabrication of electrostatically loaded, free-standing Al–0.5wt.%Cu thin-film samples, realizing a near-zero compliance support post. We measure Young’s modulus E =74GPa using cantilevers, in good agreement with grain texture measurements. We measure residual stress σ R ranging from 30 to 60MPa using fixed–fixed beams and find that processing induces significant plastic straining, which leads to residual stress values significantly less than the as-deposited value. Strength of this alloy is at least 172MPa if the film is not severely strained, and the material exhibits no room-temperature fatigue up to 1 billion cycles at this stress level. Notched devices that have been subjected to process-induced plastic straining of ∼4% are weaker and fatigue logarithmically with the number of cycles. We compare deformation processes on the samples using ex situ TEM. The mechanism for the high strength value is attributed to the grain size and the thin surface oxide which constrain dislocation glide, while fatigue of the highly strained material is associated with the appearance of persistent slip bands. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract The thickening behavior of aluminum scandium nitride (Al 0.88 Sc 0.12 N) films grown on Si(111) substrates has been investigated experimentally using X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy, and residual stress measurement. Al 0.88 Sc 0.12 N films were grown with thicknesses spanning 14 nm to 1.1 μm. TEM analysis shows that the argon sputter etch used to remove the native oxide prior to deposition produced an amorphous, oxygen-rich surface, preventing epitaxial growth. XRD analysis of the films show that the AlScN(002) orientation improves as the films thicken and the XRD AlScN(002) rocking curve full width half maximum decreases to 1.34° for the 1.1 μm thick film. XRD analysis shows that the unit cell is expanded in both the a- and c-axes by Sc doping; the a-axis lattice parameter was measured to be 3.172 ± 0.007 Å and the c-axis lattice parameter was measured to be 5.000 ± 0.001 Å, representing 1.96% and 0.44% expansions over aluminum nitride lattice parameters, respectively. The grain size and roughness increase as the film thickness increases. A stress gradient forms through the film; the residual stress grows more tensile as the film thickens, from −1.24 GPa to +8.5 MPa. Highlights • Al 0.88 Sc 0.12 N films of varied thicknesses (14 nm – 1.1 μm) were deposited on silicon. • The argon sputter etch amorphized the substrate surface, preventing epitaxial growth. • As the films thicken the film texture improves and the films coarsen. • The unit cell is expanded due to scandium substitution within the host lattice. [ABSTRACT FROM AUTHOR]

The formation of Al 3 Sc, in 100 nm Al 0.8 Sc 0.2 films, is found to be driven by exposure to high temperature through higher deposition temperature or annealing. High film resistivity was observed in films with lower deposition temperature that exhibited a lack of crystallinity, which is anticipated to cause more electron scattering. An increase in deposition temperature allows for the nucleation and growth of crystalline Al 3 Sc regions that were verified by electron diffraction. The increase in crystallinity reduces electron scattering, which results in lower film resistivity. Annealing Al 0.8 Sc 0.2 films at 600 °C in an Ar vacuum environment also allows for the formation and recrystallization of Al 3 Sc and Al and yields saturated resistivity values between 9.58 and 10.5 μΩ-cm regardless of sputter conditions. Al 3 Sc was found to nucleate and grow in a random orientation when deposited on SiO 2 , and highly {111} textured when deposited on 100 nm Ti and AlN films that were used as template layers. The rocking curve of the Al 3 Sc 111 reflection for the as-deposited films on Ti and AlN at 450 °C was 1.79° and 1.68°, respectively. Annealing the film deposited on the AlN template reduced the rocking curve substantially to 1.01° due to recrystallization of Al 3 Sc and Al within the film. [Display omitted] • The formation of Al 3 Sc is found to be driven by exposure to high temperatures. • Low deposition temperatures lead to low crystallinity films with high resistivity. • {111} textured Al 3 Sc achieved by leveraging temperature and a template layer. • Annealing Al 0.8 Sc 0.2 films reduces resistivity regardless of sputter conditions. • Strong recrystallization effect seen in annealed films with {111} textured Al 3 Sc. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate an ex situ method for analyzing the chemistry of battery electrode particles after electrochemical cycling using the transmission electron microscope (TEM). The arrangement of particles during our analysis is the same as when the particles are being cycled. We start by sectioning LiFePO4 battery electrodes using an ultramicrotome. We then show that mapping of the Fe2+ and Fe3+ oxidation state using energy-filtered TEM (EFTEM) and multivariate statistical analysis techniques can be used to determine the spatial distribution of Li in the particles. This approach is validated by comparison with scanning transmission X-ray microscopy (STXM) analysis of the same samples [Chueh et al. Nanoletters, 13 (3) (2013) 866–72]. EFTEM uses a parallel electron beam and reduces the electron-beam dose (and potential beam-induced damage) to the sample when compared to alternate techniques that use a focused probe (e.g. STEM–EELS). Our analysis confirms that under the charging conditions of the analyzed battery, mixed phase particles are rare and thus Li intercalation is limited by the nucleation of new phases. [Copyright &y& Elsevier]

Tishomingo is a chemically and structurally unique iron with 32.5wt.% Ni that contains 20% residual taenite and 80% martensite plates, which formed on cooling to between −75 and −200°C, probably the lowest temperature recorded by any meteorite. Our studies using transmission (TEM) and scanning electron microscopy (SEM), X-ray microanalysis (AEM) and electron backscatter diffraction (EBSD) show that martensite plates in Tishomingo formed in a single crystal of taenite and decomposed during reheating forming 10–100nm taenite particles with ∼50wt.% Ni, kamacite with ∼4wt.%Ni, along with martensite or taenite with 32wt.% Ni. EBSD data and experimental constraints show that Tishomingo was reheated to 320–400°C for about a year transforming some martensite to kamacite and to taenite particles and some martensite directly to taenite without composition change. Fizzy-textured intergrowths of troilite, kamacite with 2.7wt.% Ni and 2.6wt.% Co, and taenite with 56wt.% Ni and 0.15wt.% Co formed by localized shock melting. A single impact probably melted the sub-mm sulfides, formed stishovite, and reheated and decomposed the martensite plates. Tishomingo and its near-twin Willow Grove, which has 28wt.% Ni, differ from IAB-related irons like Santa Catharina and San Cristobal that contain 25–36wt.% Ni, as they are highly depleted in moderately volatile siderophiles and enriched in Ir and other refractory elements. Tishomingo and Willow Grove therefore resemble IVB irons but are chemically distinct. The absence of cloudy taenite in these two irons shows that they cooled through 250°C abnormally fast at >0.01°C/yr. Thus this grouplet, like the IVA and IVB irons, suffered an early impact that disrupted their parent body when it was still hot. Our noble gas data show that Tishomingo was excavated from its parent body about 100 to 200Myr ago and exposed to cosmic rays as a meteoroid with a radius of ∼50–85cm. [ABSTRACT FROM AUTHOR]

Abstract: We have determined metallographic cooling rates of 9 IVB irons by measuring Ni gradients 3μm or less in length at kamacite–taenite boundaries with the analytical transmission electron microscope and by comparing these Ni gradients with those derived by modeling kamacite growth. Cooling rates at 600–400°C vary from 475K/Myr at the low-Ni end of group IVB to 5000K/Myr at the high-Ni end. Sizes of high-Ni particles in the cloudy zone microstructure in taenite and the widths of the tetrataenite rims, which both increase with decreasing cooling rate, are inversely correlated with the bulk Ni concentrations of the IVB irons confirming the correlation between cooling rate and bulk Ni. Since samples of a core that cooled inside a thermally insulating silicate mantle should have uniform cooling rates, the IVB core must have cooled through 500°C without a silicate mantle. The correlation between cooling rate and bulk Ni suggests that the core crystallized concentrically outwards. Our thermal and fractional crystallization models suggest that in this case the radius of the core was 65±15km when it cooled without a mantle. The mantle was probably removed when the IVB body was torn apart in a glancing impact with a larger body. Clean separation of the mantle from the solid core during this impact could have been aided by a thin layer of residual metallic melt at the core-mantle boundary. Thus the IVB irons may have crystallized in a well-mantled core that was 70±15km in radius while it was inside a body of radius 140±30km. [Copyright &y& Elsevier]

Abstract: Nanostructured polymer blends prepared via anionic ring opening polymerizations of cyclic monomers in the presence of a pre-made polymer melt exhibit a number of special properties over traditional polymer blends and homopolymers. Here, we report on a simple and versatile method of in situ polymerization of macrocyclic carbonates in the presence of a maleic anhydride polypropylene (mPP) matrix and a surface-active compatibilizer (i.e. PC grafted onto a mPP backbone generated in situ) to yield a micro- and nanostructured polymer blends consisting of a polycarbonate (PC) minor phase, and a polypropylene (PP) major phase. By varying the processing conditions and concentration of the macrocyclic carbonate it was possible to reduce the size of the PC dispersions to an average minor diameter of 150nm. NMR and TEM characterizations indicate that the PC dispersions do not influence crystal content in the PP phase. Overall, the results point to a simple strategy and versatile route to new polymeric materials with enhanced benefits. [Copyright &y& Elsevier]

We have investigated the wetting and reaction behavior of two less common transition element additives to braze alloys to learn more about their reaction and bonding mechanisms. Alloys of Ag with different amounts of Hf or Zr were reacted in a controlled atmosphere furnace with sapphire, alumina of 99.6 and 96% purity, and fused silica. We determined contact angles during heating and examined cross sections after cooling using electron analytical techniques. Different interfacial microstructures were obtained when the active metal was varied in an otherwise constant system. The Hf-containing alloys reacted with polycrystalline Al2O3 and had a dispersion of Hf-containing particles near the interface. If there were a reaction layer with the Al2O3 it was below the resolution of the analysis. By contrast, Zr reaction products were more evident and appeared to have diffused away from the interface into the alloy. The microstructural observations are interpreted using available thermodynamic data and known phase relations. [Copyright &y& Elsevier]