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Gray, George Thompson III

Publications and source records attributed to Gray, George Thompson III.

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Shock-reshock of zirconium to explore the effect of microstructure on the alpha-omega phase transformation

Phase transformations play an important role in the mechanical behavior of materials subjected to extreme loading conditions. A series of shock-reshock experiments were fielded to determine whether the phase transitions in materials are significantly enhanced or inhibited by preexisting microstructural features. Polycrystalline zirconium samples were shock loaded using gas-gun plate impact and soft recovered to examine microstructure using electron backscatter diffraction (EBSD). Drive conditions were varied to study the (hcp) alpha to (hexagonal) omega solidsolid phase transformation. Recovered samples were then subjected to a second shock loading event to determine the change in material behavior as a function of pre-shock microstructure. Crystallography of phase fragments in the final microstructure showed that prior twinning (formed during the shock to a peak stress below the transition threshold) appeared to suppress omega formation/retention after reshock. Conversely, when a material was initially shocked into the omega phase field, retained-omega was not found to have a large impact on subsequent omega formation during reshock. This suggests that nucleation and growth of omega phase are important processes, and the relative activity of nucleation vs. growth processes is modified by a pre-existing substructure. Additionally, orientation relationships reveal a reverse transformation pathway (omega to alpha) dominates the final microstructure, suggesting significant grain growth in the omega phase field is possible even for dynamic timescales.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effects of heat treatment and build orientation on the evolution of $ϵ$ and $α'$ martensite and strength during compressive loading of additively manufactured 304L stainless steel

In this paper, the effect of heat-treatment and build orientation on martensitic phase transformation in additively manufactured (AM) 304L stainless steel is studied and compared with conventionally produced wrought material. The relationships between observed martensitic transformations and material microstructures and their effects on mechanical strength are established through experimental observations. In situ high-energy X-ray powder diffraction measurements were performed to monitor the evolution of $ϵ$ and $α'$ martensite during compressive loading of stainless steel. Electron backscatter diffraction (EBSD) was used to provide insight on initial grain morphology, crystallographic misorientation within grains, and crystallographic texture. Heat treatment alters the microstructure of AM samples creating different initial conditions. This difference in starting microstructure resulted in variability in martensitic transformation during compressive deformation. The rate of martensitic transformation decreased for AM samples treated with temperatures up to 1100°C, after which the AM microstructures recrystallized, resulting in increased rate of martensitic transformation for those samples treated at higher temperatures. It was also observed that aligning the axis of compression with the AM build direction resulted in a lower rate of strain-induced martensite formation as opposed to aligning the compression axis perpendicular to it. More favorable distribution of crystal orientations in the latter loading orientation promoted martensitic transformation. These and additional experimental observations from EBSD in terms of kernel average misorientation, mean grain orientation spread, and mean crystallite size reveal strong microstructural effects on strength of additively manufactured metallic materials.

304L stainless steel↗
DOE OSTI2018

Orientation dependent spall strength of tantalum single crystals

We report that it is generally recognized that single crystals exhibit orientation-dependent elastic and plastic responses. However, it is less known how the dynamic tensile, or spall, strength depends on crystalline orientation, especially for BCC materials. It has been previously shown that the dynamic tensile strength of FCC materials is highly dependent on their plastic response under compression, with a direct correlation between plastic deformation and spall strength. In BCC materials like Ta, where the primary deformation mechanism at high strain rates is a combination of slip and twinning, the quantitative dependence of spall strength on these deformation mechanisms is less understood. To fill this gap in our knowledge, a series of non-equilibrium molecular dynamics simulations are completed for six tantalum single crystal orientations: <001>, <011>, <111>, <013>, <112>, <149>, to explore the role of directional anisotropy on ductile spallation. Our results show that due to the role of non-Schmid and release effects in BCC Ta, the evolution of plasticity follows a complex trajectory through shock compression, release, and tension. Orientations that contain residual twinning deformation have a reduced spall strength proportional to the amount of twinning present. Lastly, twins and their intersections function as regions of increased stress localization within the system due to compatibility requirements and their interactions with dislocations.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On the dynamic tensile strength of an FCC metal

The tensile response of ductile, polycrystalline metals is often accompanied by the formation of pores within the material, which coalesce and fail a plane within a metal. This large deformation process is broadly progressive, with a physical path consisting of nucleation, growth, coalescence, and failure, which occur individually over short periods of time. Thus distinct micro-mechanisms operate, each influenced by microstructure, loading path, and loading profile, which remains a significant challenge to represent and predict numerically at the macroscale. In a previous study, the influence of loading path on damage evolution in high-purity tantalum has been presented; in this paper, complimentary measurements are made on a pure, FCC copper. Samples were shock loaded to three different peak stresses using both symmetric impact, and two different composite flyer plate configurations, such that upon unloading, the three samples were subject to nearly identical tensile loading. The damage evolution in the soft-recovered copper samples was then quantified using optical metallography and electron-back-scatter diffraction. We shall compare metallurgical observations, velocimetry histories and simulations to discuss the dynamic failure mechanics observed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Signatures of the unique microstructure of additively manufactured steel observed via diffraction

A series of measurements were designed to gain confidence in the interpretation of the peak breadth in diffraction patterns collected from additively manufactured material, which has a novel microstructure in comparison to the well understood microstructure of wrought materials. Stainless steels made with two additive manufacturing techniques were compared to wrought material. Similar patterns observed in the scattering vector dependence for additively manufactured and deformed wrought materials suggested that the broadening in both materials was related to dislocations. Finally, this was confirmed by heat-treatment, during which both materials exhibited recovery due to the annealing of dislocations at the same temperature.

304L stainless-steel↗
DOE OSTI2015

Influence of sweeping detonation-wave loading on damage evolution during spallation loading of tantalum in both a planar and curved geometry

Widespread research over the past five decades has provided a wealth of experimental data and insight concerning the shock hardening, damage evolution, and the spallation response of materials subjected to square-topped shock-wave loading profiles. However, fewer quantitative studies have been conducted on the effect of direct, in-contact, high explosive (HE)-driven Taylor wave (unsupported shocks) loading on the shock hardening, damage evolution, or spallation response of materials. Systematic studies quantifying the effect of sweeping-detonation wave loading are yet sparser. In this study, the damage evolution and spallation response of Ta is shown to be critically dependent on the peak shock stress, the geometry of the sample (flat or curved plate geometry), and the shock obliquity during sweeping-detonation-wave shock loading. Sweepingwave loading in the flat-plate geometry is observed to: a) yield a lower spall strength than previously documented for 1-D supported-shock-wave loading, b) exhibit increased shock hardening as a function of increasing obliquity, and c) lead to an increased incidence of deformation twin formation with increasing shock obliquity. Sweeping-wave loading of a 10 cm radius curved Ta plate is observed to: a) lead to an increase in the shear stress as a function of increasing obliquity, b) display a more developed level of damage evolution, extensive voids and coalescence, and lower spall strength with obliquity in the curved plate than seen in the flat-plate sweeping-detonation wave loading for an equivalent HE loading, and c) no increased propensity for deformation twin formation with increasing obliquity as seen in the flat-plate geometry. The overall observations comparing and contrasting the flat versus curved sweeping-wave spall experiments with 1D loaded spallation behavior suggests a coupled influence of obliquity and geometry on dynamic shock-induced damage evolution and spall strength. Coupled experimental and modeling research to quantify the combined effects of sweeping-wave loading with increasingly complex sample geometries on the shockwave response of materials is clearly crucial to providing the basis for developing and thereafter validation of predictive modeling capability.

36 MATERIALS SCIENCE↗