Sven C. Vogel

Study of slip mechanisms in a magnesium alloy by neutron diffraction and modeling

Abstract Internal strains within a polycrystalline magnesium alloy plate have been measured during tensile and compression testing in situ by neutron diffraction. Using an elasto-plastic self-consistent simulation code, information about the operation of slip and mechanical twinning modes as a function of strain has been obtained.

Rietveld texture analysis from TOF neutron diffraction data

One of the advantages of a multidetector neutron time-of-flight diffractometer such as the high pressure preferred orientation diffractometer (HIPPO) at the Los Alamos Neutron Science Center is the capability to measure efficiently preferred orientation of bulk materials. A routine experimental method for measurements, both at ambient conditions, as well as high or low temperatures, has been established. However, only recently has the complex data analysis been streamlined to make it straightforward for a noninitiated user. Here, we describe the Rietveld texture analysis of HIPPO data with the computer code Materials Analysis Using Diffraction ( MAUD ) as a step-by-step procedure and illustrate it with a metamorphic quartz rock. Postprocessing of the results is described and neutron diffraction results are compared with electron backscatter diffraction measurements on the same sample.

Texture measurements using the new neutron diffractometer HIPPO and their analysis using the Rietveld method.

In this paper we describe the capabilities for texture measurements of the new neutron time-offlight diffractometer HIPPO at the Los Alamos Neutron Science Center (LANSCE). The orientation distribution function (ODF) is extracted from multiple neutron time-of-flight histograms using the full-pattern analysis first described by Rietveld. Both, the well-established description of the ODF using spherical harmonics functions and the WIMV method, more recently introduced for the analysis of time-of-flight data, are available to routinely derive the ODF from HIPPO data. At ambient conditions, total count time of less than one hour is ample to collect sufficient data for texture analysis in most cases. The large sample throughput for texture measurements at ambient conditions possible with HIPPO requires a robust and reliable, semiautomated data analysis. HIPPO’s unique capabilities to measure large quantities of ambient condition samples and to measure texture at temperature and uni-axial stress are described. Examples for all types of texture measurements are given.

Quantitative texture analysis with the HIPPO neutron TOF diffractometer

One of the design goals of the neutron time-of-flight (TOF) diffractometer HIPPO (High Pressure–Preferred Orientation) at LANSCE (Los Alamos Neutron Science Center) was efficient quantitative texture analysis. In this paper, the effects of the HIPPO detector geometry and layout on texture analysis, particularly the shape and dimensions of the detector panels, are investigated in detail. An equal-channel angular-pressed (ECAP) aluminium sample with a strong texture was used to determine the methodological limitations of various methods of quantitative texture analysis. Several algorithms for extracting the orientation distribution function (ODF) from the TOF spectra are compared: discrete orientations at arbitrary positions, harmonic methods in Rietveld codes (MAUD and GSAS) and discrete methods in MAUD. Because of the detector geometry, the sharpest texture peaks that can be represented are 12–15° in width, resulting in an optimal texture resolution of 25–30°. Due to the limited resolution and incomplete pole-figure coverage, harmonic expansions beyond L = 12 (where L is the maximum degree of the harmonic expansion) introduce subsidiary oscillations, which are consistently identified as artifacts. Only discrete methods provide a quantitative representation of the texture. Harmonic methods are adequate for a qualitative description of the main texture component. The results of the analysis establish HIPPO as an efficient instrument to determine preferred orientations in relatively short measuring times.

Thermal Stability of Cu-Nb Nanolamellar Composites Fabricated via Accumulative Roll Bonding

In situ annealing within a neutron beam line and ex situ annealing followed by transmission electron microscopy were used to study the thermal stability of the texture, microstructure, and bi-metal interface in bulk nanolamellar Cu/Nb composites (h = 18 nm individual layer thickness) fabricated via accumulative roll bonding, a severe plastic deformation technique. Compared to the bulk single-phase constituent materials, the nanocomposite is two orders of magnitude higher in hardness and significantly more thermally stable, e.g., no observed recrystallization in Cu at temperatures as high as 85% of the melting temperature. The nanoscale h = 18 nm individual layer thickness is maintained up to 500°C, the lamellar structure thickens but is maintained up to 700°C, and recrystallization is suppressed even up to 900°C. With increasing temperature, the texture sharpens, and among the interfaces found in the starting material, the {112}Cu || {112}Nb interface with a Kurdjumov-Sachs orientation relationship shows the greatest thermal stability. Our results suggest that thickening of the individual layers under heat treatment coincides with thermally driven removal of energetically unfavorable bi-metal interfaces. Thus, we uncover a temperature regime that maintains the lamellar structure but alters the interface distribution such that a single, low energy, thermally stable interface prevails.

Simultaneous in situ-neutron diffraction studies of the anode and cathode in a lithium-ion cell

In situ neutron diffraction analysis was employed to study the behavior of the cathode and anode materials in a commercial Li-ion cell (Saehan Enertech, Inc) using the exact configuration of the commercial product. Accurate lattice parameters were refined for the LiCoO 2 type cathode based on measurements collected as a function of the state of charge. Simultaneous structural characterization was possible on the graphitic anode as well. The simultaneous direct correlation of structural information for both the anode and cathode with the electrochemical data provided a highly detailed picture of the behavior of the active cell materials that ultimately underlie the cell performance.

Dauphiné twinning as evidence for an impact origin of preferred orientation in quartzite: An example from Vredefort, South Africa

Large meteorite impacts cause great changes to geologic features on Earth, ranging from cratering and ejecta blankets to phase transformations in minerals. In this report we describe the effect of a shock wave on the crystallographic orientation of quartz crystals in quartzite from the Vredefort impact site in South Africa. Preferred orientation of bulk quartzite samples was measured by time-of-flight neutron diffraction. With a random distribution of c-axes, a weak but distinct difference between the orientation distribution of positive and negative rhombs was observed, as illustrated with pole figures and inverse pole figures. Results for the natural Vredefort sample are compared with deformation

Neutron diffraction study of the contribution of grain contacts to nonlinear stress‐strain behavior

[1] Repeatable, hysteretic loops in quasi-static loading measurements on rocks are well known; the fundamental processes responsible for them are not. The grain contact region is usually treated as the site of these processes, but there is little supporting experimental evidence. We have performed simultaneous neutron diffraction and quasi-static loadingexperimentsonaselectionofrockstoexperimentally isolate the response of these contact regions. Neutron diffraction measures strain in the lattice planes of the bulk of the grain material, so differences between this strain and the macroscopic response yield information about grain contact behavior. We find the lattice responds linearly to stress in all cases, oblivious to the macroscopic unrecoverable strains, curvature, and hysteresis, localizing these effects to the contacts. Neutron diffraction shows that the more granular rocks appear to distribute stresses so that the same strain appears in all the grains, independent of crystallographic orientation. INDEX TERMS: 3909 Mineral Physics: Elasticity and anelasticity; 3902 Mineral Physics: Creep anddeformation;3954MineralPhysics:Xray,neutron,andelectron spectroscopy and diffraction; 3994 Mineral Physics: Instruments and techniques; 3694 Mineralogy and Petrology: Instruments and techniques.Citation: Darling, T. W., J. A. TenCate, D. W. Brown, B. Clausen, and S. C. Vogel (2004), Neutron diffraction study of the contribution of grain contacts to nonlinear stress-strain behavior, Geophys. Res. Lett., 31, L16604, doi:10.1029/2004GL020463.

In situ analysis of LiFePO4 batteries: Signal extraction by multivariate analysis

Electrochemical reaction behavior of a commercial Li-ion battery (LiFePO4-based cathode, graphite-based anode) has been measured via in-situ neutron diffraction. Multivariate analysis was successfully applied to the neutron diffraction dataset facilitating in the determination of Li bearing phases participating in the electrochemical reaction in both the anode and cathode as a function of state-of-charge (SOC). Analysis resulted in quantified phase fraction values for LiFePO4 and FePO4 cathode compounds as well as identification of staging behavior of Li6 ,L i12, Li24 and graphite phases in the anode. An additional Li-graphite phase has also been tentatively identified during electrochemical cycling as LiC48 at conditions of ~5 to 15% SOC.

Anisotropy in hexagonal close-packed structures: improvements to crystal plasticity approaches applied to magnesium alloy

Abstract Due to its polarity, twinning in strongly textured hexagonal close packed (HCP) structures can be maximized or minimized under particular loading conditions. The resulting anisotropy can be dramatically demonstrated for magnesium with a fibre, for example. The stress–strain behaviour from compression loading parallel to the fibre produces a ‘parabolic’ stress–strain curve, but a ‘sigmoidal’ curve when loaded normal to the fibre. When modelling anisotropy in HCP structures with crystal plasticity, contemporary researchers usually fit hardening parameters to only these two extreme cases, i.e., maximized or minimized twinning activity, presuming that the same parameters would interpolate the correct behaviour under any other transitional stress direction. A comparison with experiments presented in this paper demonstrates that this assumption is not fully accurate, whether using the phenomenological Voce hardening model or the dislocation density based hardening model in the VPSC (visco-plastic self-consistent) framework. This indicates that slip-twin interactions are not properly captured in these models. Through a simple phenomenological implementation, we show that dislocation transmutation by twinning is an important aspect of slip-twin interactions that improve the predictability of the above crystal plasticity models for HCP structures.