Sponsored by: SMD Refractory Metals and Materials Committee and Jt. MDMD/EPD Synthesis, Control and Analysis in Materials Processing
Program Organizers: Andrew Crowson, U.S. Army Research Office, Research Triangle Park, NC; Edward S. Chen, U.S. Army Research Office, Research Triangle Park, NC; Prabhat Kumar, Cabot Corp, Boyertown, PA; Willam Ebihara, Picatinny Arsenal, Picatinny, NJ; Enrique J. Lavernia, UC Irvine, Irvine, CA
Tuesday, PM Room: A4-5
February 6, 1996 Location: Anaheim Convention Center
Session Chairpersons: William Ebihara, Picatinny Arsenal, Picatinny, NJ. David H. Lassila, Lawrence Livermore National Laboratory, Livermore, CA
1:30 pm Invited
FACTORS EFFECTING THE MECHANICAL PROPERTIES AND TEXTURE OF TANTALUM: Christoper A. Michaluk, Project Leader, Cabot Performance Materials, County Line Road, Boyertown, PA, 19512
A comprehensive review of factors which influence the microstructure, strength, and texture of tantalum is presented. Data demonstrating the roles of melting procedure (Electron Beam versus Vacuum Arc Remelting), alloy additions, and annealing temperature on mechanical properties, metallurgical structure, and texture is provided. Results reveal that material chemistry has an overbearing effect on the behavior of the final product.
MULTIPLE MECHANISMS THE THERMALLY ACTIVATED FLOW OF TANTALUM: William H. Gourdin, David H. Lassila, University of California, Lawrence Livermore National Laboratory, PO Box 808, L-355, Livermore, CA 94550
We propose a unified, physically based, approach to modeling of BCC and FCC materials which incorporates a two-component Peierls-type mechanism and an FCC obstacle mechanism in series. We have applied our model to unalloyed tantalum with reasonably good success over a wide range of strain rates and a modest range of temperatures. We find that the proposed two component Peierls term is necessary to properly reproduce the initial flow stress over the entire range of strain-rates and temperatures studied. We will discuss the relevance of the proposed model to modeling of shock-prestrained tantalum.
WORK-HARDENING AND DISLOCATION BEHAVIOR OF TANTALUM AND TANTALUM ALLOYS: W. Wasserbach, Max-Planck-Institut fur Metallforschung, Institut fur Physik, HeisenberstraMe l, D-70569 Stuttgart, Germany
High-purity tantalum and tantalum alloy single crystals have been deformed in the temperature region between 77K and 470K. The deformation behavior has been studied by slip-line morphology, Laue X-ray backreflexion, X-ray topography, transmission electron microscopy and measurements of the flow stress. At low deformation temperatures the work-hardening curves are parabolic and the dislocation arrangement consists of primary and secondary screw dislocations. At intermediate temperatures the work-hardening curves exhibit two regimes which are analogous to stage I and stage III of the work-hardening curves of face-centered cubic metals. In stage I bundles of edge dipoles of primary dislocations are observed. In stage III the dislocations are arranged in symmetric pairs of dislocation sheets consisting of crossed grids of primary and secondary dislocations parallel to the primary slip plane. In spite of the different dislocation arrangements, in both stages the macroscopic slip proceeds by glide over large slip distances.
POLYCRYSTAL ELASTO-PLASTICITY: APPLICATION TO PREDICTION OF EARING DURING CUP DRAWING OF TANTALUM SHEET: Dr. Lallit Anand, Manish Kothari, 1-310, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02l39
Constitutive equations for polycrystal elasto-plasticity, which are capable of modeling the initial and evolving anisotropy in ductile materials due to the evolution of crystallographic texture are reviewed, and the implementation of these equations in a finite element program is described. The constitutive equations and computational procedures are used to simulate the formation of earing defects during cup-drawing of body-centered-cubic tantalum sheet. Comparisons of the results of the numerical simulations against existing experimental results on earing shows that the predictions of the number of ears and their positions are in excellent agreement with the experiments. These results should have excellent bearing on the formation of fins and flares in explosively formed penetrators.
3:00 pm BREAK
3:15 pm Invited
INFLUENCE OF LARGE-STRAIN DEFORMATION ON THE MICROSTRUCTURE, TEXTURE, AND MECHANICAL RESPONSE OF TANTALUM BAR: G.T. (Rustv) Gray III, Stuart I. Wright, Shuh Rong Chen, Sherri Bingert, Paul L. Maudlin, Los Alamos National Laboratory, Los Alamos, NM 87545
Numerous studies to date have established the influence of impurities, crystallographic texture, temperature, and strain rate separately or collectively on the constitutive response of annealed tantalum, in particular plate stock. However, fewer detailed studies have examined the evolution of crystallographic texture and the mechanical response of tantalum bar or rod material following prestrains to strains > 1. In this talk the influence of large plastic prestraining on the microstructure evolution, texture evolution, and mechanical response of high-purity tantalum bar material will be presented. Tantalum cylinders annealed at 1200deg.C were quasi-statically upset forged, with intermediate lubrication, to true strains of 0.4, 1, and 2. Microstructural and texture banding within the starting tantalum bar was characterized in detail and found to evolve during large-strain forging leading to significant differences in mechanical response. Aspects of defect storage, work-hardening response, and texture evolution in tantalum bar as a function of forging strain will be discussed. Work performed under the auspices of the U.S. Department of Energy.
MODELING DYNAMIC BEHAVIOR AND TEXTURE EVOLUTION IN PURE Ta: Scott E. Schoenfeld, U.S. Army Research Laboratory, AMSRL-WT-TD, Aberdeen Proving Ground, MD 21005; Said Ahzi, Clemson University, Department of Mechanical Engineering, Clemson, SC 29634
In order to model high temperature, high strain rate deformation and texture evolution in commercially pure Ta, two distinct descriptions for the thermal elastic visco-plastic behavior of Ta single crystals will be considered along with associated polycrystal averaging schemes. The first description will incorporate a temperature dependent description for pencil glide on the planes of maximum resolved shear stress. Calculated stress-strain data and texture evolution for these two models will be compared with experimental data taken over a range of temperature and stain rates.
GRAIN SUBDIVISION AND THE DEVELOPMENT OF LOCAL ORIENTATIONS IN ROLLED TANTALUM: D.A. Hughes, Center for Materials and Appl. Mechanics, Sandia National Laboratory, Livermore, CA 94550
The formation of dislocation boundaries and their local surroundings determine the properties of deformed tantalum including texture, recrystallization, flow stress and formability. The deformation microstructure formed in tantalum has been observed and quantitatively characterized using transmission electron microscopy following rolling to large reductions. The observed structure includes higher angle lamellar boundaries parallel to the rolling plane surrounding either groups of cells or arrays of loosely knit dislocations and equiaxed subgrains. The distribution of local orientations between individual dislocation boundaries and their angle/axis pairs have been measured using convergent beam Kikuchi analysis and are compared to the rolling texture. The sequence of near neighbor orientations shows that individual grains subdivide into three to four different texture components separated by sharp boundaries. The arrangement of local orientations is much more diverse than suggested by simple models and single crystal studies. This work supported by U.S. DOE under contract No. DE-AC04-94AL8500.
SITE COMPETITION OF IMPURITIES AND GRAIN BOUNDARY STABILITY IN TANTALUM: Genrich L. Krasko, U.S. Army Research Laboratory, Materials Directorate, AMSRL-MA-CC, Aberdeen Proving Ground, MD 21005-5069
Metalloid impurities have a very low solubility in tantalum, and therefore prefer to segregate at the grain boundaries (GBs). In order to analyze the energetics of the impurities on the tantalum GB, the LMTO calculations were performed on a simple 8-atom supercell emulating a typical (capped trigonal prism) GB environment, and the so-called "environment-sensitive embedding energies" were calculated for B, C, N, O, P, and S, as a function of the electron charge density due to the host atoms at the impurity site. The calculations showed that, at the electron density typical of a GB, C has the lowest energy (followed by N and B) and thus would compete with the other impurities for the site on the GB, tending to displace them from the GB. The above energies were then used in a modified Finnis-Sinclair embedded atom approach for calculating the cohesive energies and the equilibrium interplanar distances in the vicinity of a (lll) 3 tilt GB plane, both for the clean GB and that with an impurity. These distances were found to oscillate, returning to the value corresponding to the equilibrium spacing in bulk BCC tantalum by the 10th-12th plane off the GB. The results on both the GB relaxation and GB cohesive energies suggest that C, N and B are intergranular cohesion enhancers, while O, P and S result in decohesion effects.
EFFECT OF DATA SET SIZE ON CONSTITUTIVE MATERIAL MODELING OF TANTALUM: Craig M. Lopatin, Craig L Wittman, Alliant Techsystems Inc, 600 2nd Street N.E., Hopkins, MN 55343
Analytical and numerical techniques were used to derive the constants for the
Johnson-Cook material model from data sets of various sizes for pure tantalum.
The numerical curve fitting routine was based on a commercial graphics package
which adjusts all the constants simultaneously to obtain the best fit.
Numerical solutions are then compared to those given by solving for each term
in the model separately in a standard analytical approach. Recommendations are
given for the minimum number of data points needed to obtain an accurate
|Search||TMS Annual Meetings||TMS Meetings Page||About TMS||TMS OnLine|