Thomas A. Mason

The influence of oxygen content on the α to ω phase transformation and shock hardening of titanium

In this study the influence of alloy chemistry on the propensity of omega phase formation in two titanium alloys during shock loading is examined. The effect of peak shock stress on the phase stability and substructural evolution of high purity and A-70 (3700ppm oxygen) titanium was probed utilizing velocity interferometer system for any reflector and post shock substructural analysis. While in the high purity titanium the alpha to omega phase transformation was found to occur at 10.4GPa, no transformation was observed in the A-70 material for stresses up to 35GPa. Transmission electron microscopy analysis and neutron diffraction of shock-recovered samples confirmed these results and probed the details of twin and dislocation structures. Debye temperature data are also presented and the Debye-Waller temperatures for the alpha and omega phases in the high purity titanium are calculated. Finally, the compressive, quasistatic reload behaviors of both high purity and A-70 titanium are compared to the as-annea...

Advances in deformation twin characterization using electron backscattered diffraction data

This article reports on recent progress in the effort to develop an automated, crystallographically based twin identification and quantification routine using large sets of spatially correlated electron backscattered diffraction (EBSD) data. The proposed analysis scheme uses information about the most probably occurring twin types and the macroscopic stress state, taken together with the crystallographic theory of deformation twinning, to identify and classify twinned areas in a scanned cross section of a material. The key features of the analysis are identification of potential twin boundaries by their misorientation character, validation of these boundaries through comparison with the actual boundary position and twin-plane matching across the boundary, and calculation of the Schmid factors for the orientations on either side of the boundary. This scheme will allow researchers to quantify twin area fractions from statistically significant regions and, in turn, estimate twinned volume fractions with reasonable reliability.

Deformation twinning in polycrystalline Zr: Insights from electron backscattered diffraction characterization

The response of polycrystalline α-zirconium to various deformation conditions was investigated through electron backscattered diffraction (EBSD) characterization. The range of deformation conditions included quasi-static compression and tension at room and cryogenic temperatures, along with a Taylor cylinder impact experiment. The resultant data provided spatial resolution of individual with system activity as a function of the progression of deformation. Over 300 deformation twins were analyzed to identify the type of twin system and active variant, along with the Schmid factor in the parent orientation. These data supplied information on the distribution of Schmid factor and variant rank as a function of twin system and deformation condition. Results showed significant deviation from a maximum Schmid factor activation criterion and suggest deformation twinning is greatly affected by local internal stress heterogeneities and the sense of the applied stress.

The application of orientation imaging microscopy

From the necessity to obtain large data sets on the statistics of placement of lattice orientation (and phase) in polycrystals, there has emerged a powerful new form of microscopy. Orientation imaging microscopy consolidates ordinary views of the morphological aspects of the microstructure with knowledge of the local lattice orientation. Such detailed information enables the investigator to ask new questions about the microstructures of metal and ceramic alloys. The answers can be challenging.

Investigating Incipiently Spalled Tantalum through Multiple Section Planes and Serial Sectioning

Shock wave interactions within a material create a three‐dimensional damage field of voids and/or strain localizations. Using a direct high explosive experiment, seven identical shots were performed to test the reproducibility of the damage fields between replicate samples. These samples, along with a sample from a plate impact experiment, were metallographically characterized to quantify the spallation void statistics. Results demonstrate that multiple section planes are required to accurately quantify the damage within a specimen. To further understand the three‐dimensional nature of incipient spallation, serial sectioning was performed on a flyer plate experimental sample. Serial sectioning is yielding void and strain localization interactions not available from two‐dimensional slices.

Automated Twin Identification Technique for Use with Electron Backscatter Diffraction

Historically, twinning classification has been obtained by optical microscopy, bulk x-ray and neutron diffraction, and transmission electron microscopy (TEM). Recent research has shown that automated electron backscatter diffraction (EBSD) can be used to quantify twin content and thereby greatly improve the reliability of twinning statistics. An automated twin identification technique for use with EBSD has facilitated a greater understanding of deformation twinning in materials. The key features of this automated framework are the use of the crystallographic definition of twin relationships, and the correct identification of the parent orientation in a parent/twin pair. The complex nature of the parent/twin interactions required the use of a voting scheme to correctly identify parent orientations. In those few cases where the voting scheme was unable to determine parent orientation (< 2%) the algorithm allows for manual selection. Twin area fractions are categorized by operative twin systems along with secondary and tertiary twinning. These statistics are reported for deformation and annealing twin populations in deformed a-zirconium and asannealed 316L stainless steel, respectively. These improved twin statistics can help provide insight into the effect of deformation processes on microstructural evolution, as well as provide validation of plasticity models for materials that exhibit deformation twinning.

Anisotropic Plasticity Modeling Incorporating EBSD Characterization of Tantalum and Zirconium

The application of automated EBSD techniques in the context of an overall predictive materials modeling effort incorporating anisotropic properties for tantalum and zirconium is covered in this chapter. The focus will be on the role of microtextural investigations as an integral tool supporting the development and validation of material models that incorporate anisotropic constitutive behavior. Continuum mechanics codes require accurate descriptions of materials behavior to adequately predict large-strain deformation response. The corresponding requirement of characterizing micro structure s after significant deformation places severe requirements on the EBSD system. In this work, a Philips XL30 SEM employing a warm Schottky FEG was used for all data collection; the combination of high resolution with adequate beam current was a necessity for analyzing fine detail amid heavily worked structures. The ability to spatially resolve orientation differences on the order of 100 nm is achievable. All EBSD data collection and analysis was performed with TSL’s OIM™ software, while the popLA code (Kallend et al., 1991) was used for x-ray texture analysis.

USING SCHLIEREN VISUALIZATION TO TRACK DETONATOR PERFORMANCE

Several experiments will be presented that are part of a phased plan to understand the evolution of detonation in a detonator from initiation shock through run to detonation, to full detonation, to transition, to booster and booster detonation. High‐speed multiframe schlieren imagery has been used to study several explosive initiation events, such as exploding bridgewires (EBWs), exploding foil initiators (EFIs or “slappers”), direct optical initiation (DOI), and electrostatic discharge. Additionally, a series of tests has been performed on “cut‐back” detonators with varying initial pressing heights. We have also used this diagnostic to visualize a range of EBW, EFI, and DOI full‐up detonators. Future applications to other explosive events, such as boosters and insensitive high explosives booster evaluation, will be discussed. The EPIC finite element code has been used to analyze the shock fronts from the schlieren images to solve iteratively for consistent boundary or initial conditions to determine the ...

Simulation of the variation of material tensor properties of polycrystals achieved through modification of the crystallographic texture

The material properties of all crystalline metals are intimately related to the underlying structure of the crystal lattice. According to Neumann`s principle, a material`s property must possess the same or higher-symmetry operators as the material`s crystal lattice. This restriction requires, for example, that all properties of a cubic material have at least cubic symmetry with respect to the crystal lattice but also allows the isotropic symmetry. Anisotropic properties of single crystals are clearly functions of the crystallographic lattice orientation. In theory, once the single crystal physical properties have been determined for a material, the bulk response of a polycrystalline sample of the material should be obtainable. Due to the lack of information regarding interior details of microstructures under service conditions, progress is made in theory development by employing a number of assumptions. These include overall isotropy, equiaxed grains, lack of correlation of lattice orientation between neighboring grains, no correlations between grain size, shape or lattice orientation, stress free grain interiors, etc.

Damage Progression in Explosively Loaded Polycrystals

One of the current challenges facing researchers in the field of dynamic properties of materials is the need for a predictive modeling capability for damage and fragmentation. A series of small‐scale, explosively‐driven experiments were designed and executed in order to gain a better understanding of the nucleation and growth of damage under explosive loading. The interaction of varying obliquity detonation waves with the test articles was of particular interest. The material in these tests experiences a combination of hydrostatic and deviatoric stresses that is spatially and temporally varying. Variations in shot geometries and explosive load causes direct variations in the nature of the resulting damage fields in the recovered samples. The characterization of a number of samples from a test series of tantalum discs will be presented and compared to numerical analyses of the experiments. Insights gained from the post‐mortem examination of the discs and the accompanying simulations will be presented.