M.M. LeBlanc

Direct measurements of temperature-dependent laser absorptivity of metal powders

A compact system is developed to measure laser absorptivity for a variety of powder materials (metals, ceramics, etc.) with different powder size distributions and thicknesses. The measured results for several metal powders are presented. The results are consistent with those from ray tracing calculations.

Mapping mesoscale heterogeneity in the plastic deformation of a copper single crystal

Part of a ‘multiscale characterization’ study of heterogeneous deformation patterns in metals is reported. A copper single crystal was oriented for single slip in the (111)[ ] slip system and tested to ∼10% strain in roughly uniaxial compression. The macroscopic strain field was monitored during the test by optical ‘image correlation’. The strain field was measured on orthogonal surfaces, one of which (the x-face) was oriented perpendicular to [ ] and contained the [ ] direction of the preferred slip system. The macroscopic strain developed in an inhomogeneous pattern of broad, crossed shear bands in the x-face. One, the primary band, lay parallel to (111). The second, the ‘conjugate’ band, was oriented perpendicular to (111) with an overall ( ) habit that contains no common slip plane of the fcc crystal. The mesoscopic deformation pattern was explored with selected area diffraction, using a focused synchrotron radiation polychromatic beam with a resolution of 1–3 µm. Areas within the primary, conjugate and mixed (primary + conjugate) strain regions of the x-face were identified and mapped for their orientation, excess defect density and shear stress. The mesoscopic defect structure was concentrated in broad, somewhat irregular primary bands that lay nominally parallel to (111) in an almost periodic distribution with a period of about 30 µm. These primary bands were dominant even in the region of conjugate strain. There were also broad conjugate defect bands, almost precisely perpendicular to the primary bands, which tended to bridge primary bands and terminate at them. The residual shear stresses were large (ranging to well above 500 MPa) and strongly correlated with the primary shear bands; interband stresses were small. The maximum resolved shear stresses within the primary bands were oriented out of the plane of the bands, and, hence, could not recover the dislocation structure in the bands. The maximum resolved shear stresses in the interband regions lay predominantly in {111} planes. The results are compared to the mesoscopic defect patterns found in Cu in etch pit studies done some decades ago, which also revealed a mesoscopic dislocation structure made up of orthogonal bands.

Temperature-dependent 780-nm laser absorption by engineering grade aluminum, titanium, and steel alloy surfaces

Abstract. The modeling of laser interaction with metals for various applications requires a knowledge of absorption coefficients for real, commercially available materials with engineering grade (unpolished, oxidized) surfaces. However, most currently available absorptivity data pertain to pure metals with polished surfaces or vacuum-deposited thin films in controlled atmospheres. A simple laboratory setup is developed for the direct calorimetric absorptivity measurements using a diode-array laser emitting at 780 nm. A scheme eliminating the effect of convective and radiative losses is implemented. The obtained absorptivity results differ considerably from existing data for polished pure metals and are essential for the development of predictive laser-material interaction models.

The effects of tungsten addition on the microtexture and mechanical behavior of tantalum plate

Abstract Microtexture variations in annealed tantalum and tantalum–tungsten alloy plate materials have been studied by orientation imaging microscopy (OIM) and correlated with the mechanical behavior over a strain rate range of 10 −3 s −1 to 3000 s −1 . Plates of nominally pure Ta and Ta–W alloys (2.5, 5 and 10 wt% W), ≈6 mm thick were mechanically tested quasistatically in compression and tension and dynamically via a split Hopkinson pressure bar and punch through shear tests. The results indicate that the unalloyed Ta exhibits anomalous mechanical responses such as inverse barreling or hourglassing in compression and multiple necks in tension, whereas the tungsten containing alloys deform homogeneously. OIM indicates severe texture banding in the pure Ta such that the fraction of grains with near 〈111〉 normals is very high near the centerline of the plate and decreases toward the surfaces. Large deviations from this generic description occur from specimen to specimen. Microtexture analyses of the Ta–W alloys reveals no significant texture gradients but rather a change in overall texture from a 〈111〉 fiber to a 〈100〉 cube texture with the addition of 2.5 wt% tungsten. The evidence suggests that the addition of tungsten results in a more uniform texture and thus homogeneous mechanical response.

Uniaxial Stress Deformation Experiment for Validation of 3-D Dislocation Dynamics Simulations

An apparatus has been developed for performing compression deformation experiments on oriented metallic single crystals to provide data for validation of 3-D dislocation dynamics simulations. The experiment is performed under conditions that allow unconstrained motion of the upper and lower compression platen, and thus a relatively uniform state of axial stress is maintained during the deformation. Experiments have been performed on high-purity Mo single crystal and polycrystalline Cu. Various aspects of the experimental procedures and results are presented. Possible usages of the experimental data for the validation of 3-D dislocation dynamics simulations are discussed.

Calculation of the Slip System Activity in Deformed Zinc Single Crystals Using Digital 3-D Image Correlation Data

A 3-D image correlation system, which measures the full-field displacements in three dimensions, has been used to experimentally determine the full deformation gradient matrix for two zinc single crystals. Based on the image correlation data, slip system activity for the two crystals has been calculated. The results of the calculation show that, for one crystal, only the primary slip system is active, which is consistent with traditional theory. The other crystal, however, shows appreciable deformation on slip systems other than the primary. An analysis was conducted verifying the experimental observation that the net result from slip on the secondary slip systems is approximately one third the magnitude and directly orthogonal to the primary system.

The trianvil test apparatus: Measurement of shear strength under pressure

An experimental apparatus has been developed for performing shear tests on specimens held under moderately high hydrostatic pressures (up to the order of 10 GPa). This testing procedure experimentally determines the pressure dependent shear strength of thin foil specimens. This information is necessary for models of materials subjected to extreme pressures and can assist in model validation for models such as discrete dislocation dynamics simulations, among others. This paper reports the development of the experimental procedures and the results of initial experiments on thin foils of polycrystalline Ta performed under hydrostatic pressures ranging from 2 to 4 GPa. Subsequent characterization of the samples held under pressure established that the procedure described herein represents a reliable method to impose nearly uniform hydrostatic pressure on thin foil specimens. Both yielding and hardening behavior of Ta are observed to be sensitive to the imposed pressure.

Thermal and Structural Issues of Target Injection into a Laser-Driven Inertial Fusion Energy Chamber

Abstract Inertial fusion energy (IFE) targets injected into fusion chambers must withstand the demanding acceleration forces and the intense thermal environment of the fusion chamber. For indirect targets, the ultrathin capsule support membrane is the target component that is most sensitive to acceleration forces. Maintaining the deuterium-tritium (DT) temperature, to prevent a significant increase in DT vapor pressure, is the most critical thermal requirement. Secondarily, material selection of the high-temperature laser entrance hole window is required. This paper briefly describes how these requirements are satisfied for a laser-driven IFE plant design.

A Novel Mechanical Method to Measure Shear Strength in Specimens Under Pressure

A new experimental apparatus has been developed for performing shear tests on specimens held under moderately high hydrostatic pressures (on the order of 4 GPa). This testing procedure experimentally determines the pressure-dependent shear strength of thin foil specimens. The experiments provide calibration data for models of materials subjected to extreme pressures such as the Steinberg-Guinan hardening model and can assist in model validation for discrete dislocation dynamics simulations, among others. This paper reports the development of the experimental procedures and the results of initial experiments on thin foils of polycrystalline Ta performed under hydrostatic pressures ranging from 1 to 4 GPa. Both yielding and hardening behavior of Ta are observed to be sensitive to the imposed pressure.

X-ray Microdiffraction Characterization of Deformation Heterogeneities in BCC Crystals

The deformation behavior of BCC metals is being investigated by x-ray microdiffraction measurements (mu XRD) for the purpose of characterizing the dislocation structure that results from uniaxial compression experiments. The high brilliance synchrotron source at the Advanced Light Source (Lawrence Berkeley National Lab) and the micron resolution of the focusing optics allow for the mapping of Laue diffraction patterns across a sample. These measurements are then analyzed in order to map the distribution of residual stresses in the crystal. An important findingis the observation of Laue spot streaking, which indicates localized rotations in the lattice.These may represent an accumulation of same-sign dislocations. Theoretical modeling of the diffraction response for various slip systems is presented, and compared to experimental data. Preliminary results include orientation maps from a highly strained Ta bicrystal and a less highly strained Mo single crystal. The orientation maps of the bicrystal indicate a cell-like structure of dense dislocation walls. This deformation structure is consistent with previous OIM studies of the same crystal. The results suggest that mu XRD may be a particularly useful tool for microscale studies of deformation patterns in a multi-scale investigation of the mechanisms of deformation that ranges from macroscopic deformation tests to high resolution TEM studies of dislocation structures.