Veronica Livescu

Response and representation of ductile damage under varying shock loading conditions in tantalum

The response of polycrystalline metals, which possess adequate mechanisms for plastic deformation under extreme loading conditions, is often accompanied by the formation of pores within the structure of the material. This large deformation process is broadly identified as progressive with nucleation, growth, coalescence, and failure the physical path taken over very short periods of time. These are well known to be complex processes strongly influenced by microstructure, loading path, and the loading profile, which remains a significant challenge to represent and predict numerically. In the current study, the influence of loading path on the damage evolution in high-purity tantalum is presented. Tantalum samples were shock loaded to three different peak shock stresses using both symmetric impact, and two different composite flyer plate configurations such that upon unloading the three samples displayed nearly identical “pull-back” signals as measured via rear-surface velocimetry. While the “pull-back” sig...

The High Strain Rate Deformation Behavior of High Purity Magnesium and AZ31B Magnesium Alloy

The deformation in compression of pure magnesium and AZ31B magnesium alloy, both with a strong basal pole texture, has been investigated as a function of temperature, strain rate, and specimen orientation. The mechanical response of both metals is highly dependent upon the orientation of loading direction with respect to the basal pole. Specimens compressed along the basal pole direction have a high sensitivity to strain rate and temperature and display a concave down work hardening behavior. Specimens loaded perpendicularly to the basal pole have a yield stress that is relatively insensitive to strain rate and temperature and a work hardening behavior that is parabolic and then linearly upwards. Both specimen orientations display a mechanical response that is sensitive to temperature and strain rate. Post mortem characterization of the pure magnesium was conducted on a subset of specimens to determine the microstructural and textural evolution during deformation and these results are correlated with the observed work hardening behavior and strain rate sensitivities were calculated.

Impact of Defects in Powder Feedstock Materials on Microstructure of 304L and 316L Stainless Steel Produced by Additive Manufacturing

Recent work in both 304L and 316L stainless steel produced by additive manufacturing (AM) has shown that in addition to the unique, characteristic microstructures formed during the process, a fine dispersion of sub-micron particles, with a chemistry different from either the powder feedstock or the expected final material, are evident in the final microstructure. Such fine-scale features can only be resolved using transmission electron microscopy (TEM) or similar techniques. The present work uses electron microscopy to study both the initial powder feedstock and microstructures in final AM parts. Special attention is paid to the chemistry and origin of these nanoscale particles in several different metal alloys, and their impact on the final build. Comparisons to traditional, wrought material will be made.

Local Mechanical Property Evolution During High Strain-Rate Deformation of Tantalum

Damage within ductile metals is often linked to local heterogeneities. In ductile metals, damage typically occurs after plastic deformation, which evolves the microstructure and its properties in ways that are not easily measured in situ. This is particularly true for materials subject to dynamic loading. Here, we use a combination of spherical nanoindentation testing and electron microscopy to quantify changes in local dislocation slip resistance as a function of grain orientation in polycrystalline tantalum subjected to high strain-rate deformation. A nanoindentation data analysis technique is used to convert spherical nanoindentation data into stress–strain curves. This technique works with microstructural characterization at the indentation site and involves two steps: (1) determination of the functional dependence of the indentation yield strength (Yind) on the crystal orientation in the undeformed condition, and (2) use of nanoindentation and EBSD measurements on the deformed samples to determine changes in the local slip resistance. In this work, undeformed Ta had indentation yield values that varied by as much as 40% depending on the crystal orientation. The dynamically deformed Ta displayed a large variance in the strain hardening rates as a function of grain orientation. Soft grains (those with low Taylor Factor) were found to harden significantly more as compared to hard grains (those with a high Taylor Factor). These data are discussed in terms of grain interactions where the hard grains impose additional work on neighboring soft grains due to constraint at the boundaries.

Defect and Damage Evolution Quantification in Dynamically-Deformed Metals Using Orientation-Imaging Microscopy

Orientation-imaging microscopy offers unique capabilities to quantify the defects and damage evolution occurring in metals following dynamic and shock loading. Examples of the quantification of the types of deformation twins activated, volume fraction of twinning, and damage evolution as a function of shock loading in Ta are presented. Electron back-scatter diffraction (EBSD) examination of the damage evolution in sweeping-detonation-wave shock loading to study spallation in Cu is also presented.

GEOMETRY OF DAMAGE IN SHOCK LOADED COPPER AND TANTALUM

Cavities of coalesced voids have been found in recovered samples of Tantalum in high‐explosive‐driven experiments. The boundaries of these cavities are imprinted with details of the coalescence and void growth processes. One way of quantifying these details is to measure the roughness of the surfaces. In this work, we calculate the roughness of 2D cross sections of such cavity surfaces from micrographs by analyzing the images with the box counting technique. Spall plane damage driven by flyer plates in Copper samples is also analyzed. The different length scale regimes found will be discussed.

Structure/Property Behavior of Additively Manufactured (AM) Materials: Opportunities and Challenges

The certification and qualification paradigms required for additively manufactured (AM) metals and alloys must evolve given the absence of any broadly accepted “ASTM- or DIN-type” AM certification/qualification processes or fixed AM-material produced specifications. This is in part due to the breath of the evolved microstructures produced across the spectrum of AM manufacturing technologies including powder bed and directed energy systems. Accordingly, design and microstructure optimization, manufacture, and thereafter implementation and insertion of AM-produced materials to meet the wide range of engineering applications requires detailed quantification of the structure/property behavior of AM-materials, across the spectrum of metallic AM methods, in comparison/contrast to conventionally-manufactured metals and alloys. The scope of this talk is a discussion of some present opportunities and challenges to achieving qualification and certification of AM produced metals and alloys for engineering applications.

Capturing the Complexity of Additively Manufactured Microstructures

The underlying mechanisms and kinetics controlling damage nucleation and growth as a function of material microstructure and loading paths are discussed. These experiments indicate that structural features such as grain boundaries, grain size distribution, grain morphology crystallographic texture are all factors that influence mechanical behavior.

Characterization of Carbon Epoxy-Filled Composite

Please find attached a summary of the characterization work performed at Los Alamos between 2014 and 2015 on epoxy-filled carbon composite material.

Microstructural Data for Model Development and Validation

The dynamic deformation of metallic polycrystalline materials leading to ductile damage and failure events involves a complex series of physical processes which are poorly understood. This lack of understanding prevents us from properly formulating and offering the appropriate physically based theories for accurate and robust representation of the ductile damage and failure response of ductile materials. This paper briefly describes and illustrates a coupled experimental and computational methodology to develop greater physical insight linking the structural details of the material to its formation of damage sites. Results from examinations of both tantalum and copper are presented to illustrate the types of methodologies that will be needed in the future to better understand the critical physical processes occurring in polycrystalline metallic materials leading to their catastrophic failure.