About the 1996 TMS Annual Meeting: Tuesday Morning Sessions (February 6)

TANTALUM: High Strain Rate Deformation: Session III

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, AM Room: A4-5

February 6, 1996 Location: Anaheim Convention Center

Session Chairpersons: Rodney Clifton, Brown University, Providence, RI; Mark Meyers, University of California at San Diego, La Jolla, CA

8:30 am Invited

HIGH STRAIN-RATE, HIGH-TEMPERATURE RESPONSE OF TANTALUM: S. Nemat-Nasser, CEAM, Department of AMES, UCSD, La Jolla, CA 92093-0416

Recent advances in high strain-rate, high-temperature experimental techniques have allowed the measurement of the isothermal (quasi-isothermal) flow stress of many ductile materials, over a broad range of strain rates and temperatures. With this technique, we have studied the response of tantalum at strain rates from a few hundred to 40,000/s, and at temperatures of -200 to 750deg.C. The flow stress under adiabatic conditions is compared with the flow stress under isothermal conditions, at prescribed strain rates and initial temperatures. The experiments have revealed a dramatic change in the workhardening of tantalum, when the temperature exceeds a critical value (Tc), estimated to be 150-175deg.C. Above this Tc, the workhardening rate increases dramatically, such that it compensates for the thermal softening accompanying the high strain-rate plastic flow. Hence, the material continues to deform adiabatically at high strain rates, at non-diminishing flow stress. Experiments with the hat-shaped specimens verify this fact. Preliminary TEM observations suggest that a large number of dislocation loops are created when screw dislocations jog. It is speculated that high temperature activates this mechanism, increasing the number of loops which can then serve as barriers to dislocations, leading to greater workhardening.

9:00 am

HIGH-STRAIN, HIGH-STRAIN-RATE BEHAVIOR OF TANTALUM: V.F. Nesterenko, Department of AMES, UCSD, La Jolla, CA, 92093; M.A. Meyers, J.C. LaSalvia, Department of AMES, UCSD, La Jolla, CA, 92093; Institute for Mechanics and Materials. M.P. Bondar, Lavrentyev Institute of Hydrodynamics, Russian Academy of Science, Novosibirsk, 630090 Russia; Y.J. Chen, Department of AMES, UCSD, La olla, CA 92093; Y.L. Lukyanov, Lavrentyev Institute of Hydrodynamics, Russian Academy of Science, Novosibirsk, 630090 Russia

Ta was subjected to large strains ([[gamma]]~0-10) at high strain rates ( 10⁴s^-1) using the radial collapse of a thick- walled cylinder accelerated by a co- axially placed explosive charge. The resulting strains and temperatures achieved produced significant microstructural changes: (i) elongated dislocation cells ([[gamma]]<2, T<600K);(ii) subgrains (2<[[gamma]]<6,600K<T<800K);(iii) dynamically recrystallized micrograins (6<[[gamma]]<8, 800K<T<900K); and (iv) statically recrystallized grains ([[gamma]]>10, T>1000K). Grain- scale localization produced by anisotropic plastic flow and localized recovery and recrystallization was observed at strains [[gamma]]>4. Plasticity analysis predicts significant residual tensile "hoop" stresses near the central hole region upon unloading; profuse ductile fracturing of the Ta was observed within this region. Supported by the U.S. Army Research Office.

9:20 am

TANTALUM MICROSTRUCTURES FOR HIGH STRAIN RATE DEFORMATION: SHOCK LOADING, SHAPED CHARGES AND EXPLOSIVELY FORMED PENETRATORS: L.E. Murr, S. Pappu, C-S. Niou, C. Kennedy, Department of Metallurgical and Materials Engineering, The University of Texas, El Paso, TX 79968; M.A. Meyers, Department of AMES, UC San Diego, LaJolla, CA 92093

In applications of tantalum involving shaped charges and explosively formed penetrators (EFP), a strong shock is first imposed on the liner. In the case of shaped charge slug formation, the residual microstructures as observed in transmission electron microscopy (TEM) are considerably different from those in an EFP. Furthermore, the EFP microstructures are distributed around the EFP slug, while those in the shaped-charge slug are uniform throughout the slug. This uniformity, however, includes dynamic recovery and recrystallization near the slug center with elongated dislocation cells and more regular cells radiating outward from the slug center. These same microstructures are distributed around the EFP slug, with a much lower incidence of dynamic recrystallization. There is no evidence for deformation twins in the shaped charge or EFP microstructures, but very noticeable twinning has been observed in plane-wave shock loaded Ta around 45 GPa peak pressure. Systomatic study of twinning suggests that twins do not form below about 30 GPa peak pressure, and this may account for their absence in the EFP. In the shaped charge slug, the actual post-shock deformation completely obliterates any twins as a consequence of dynamic recovery and recrystallization. Research supported in part by U.S. Army Contract DAAA21-94-C-0059 through Picatinny Arsenal (ARDEC).

9:40 am

HIGH STRAIN RATE DEFORMATION BEHAVIOR OF TANTALUM: R.W. Armstrong, X.J. Zhang, University of Maryland, College Park, MD 20742; C. Feng, U.S. Army Research and Development Center, Dover, NJ 07801; J.D. Williams, University of Illinois, Urbana- Champaign, IL 61801; F.J. Zerilli, Naval Surface Warfare Center Indian Head Division, Silver Spring, MD 20903

Split Hopkinson Pressure bar measurements have been made at ambient temperature on different tantalum materials and are related to predictions made with a dislocation mechanics based constitutive equation analysis (J.D. Williams et al. "Dynamically Deformed Tantalum: Stress-Strain Measurements, Dislocation Microstructures, and Constitutive Equation Consequences",4th Intern. Symp. on Plasticity Preprint, 1993, Balto. MD). Items of interest are evidence of added dislocation generation at SHPB strain rates and, also, the role of interstitial solutes on the strain hardening behavior and ductility (F.J. Zerilli, R.W. Armstrong, "Description of Tantalum Deformation Behavior", J. Appl. Phys. 68,1580,1990).

10:00 am BREAK

10:15 am Invited

VERY HIGH STRAIN RATE BEHAVIOR OF TANTALUM: Kevin Duprey, Rodney Clifton, Division of Engineering, Brown University, Providence, RI 02912

Pressure-shear plate impact experiments are performed to determine the flow stress of tantalum at very high strain rates on the order of 10⁶s^-1. These results are used to determine if there is a change in the rate controlling mechanism of the flow stress of tantalum at high strain rates, as is observed in other metals. The tantalum used in these experiments is examined using transmission electron microscopy to determine if there is any texturing of the material, and also to examine the dislocation structure both before and after the deformation.

10:45 am

MECHANICAL PROPERTIES AND CONSTITUTIVE RELATIONS FOR TANTALUM AND TANTALUM ALLOYS UNDER HIGH-STRAIN-RATE DEFORMATION: Shuh Rong Chen, G.T. Gray III, Mail Stop G755, Los Alamos National Laboratory, Los Alamos, NM 87545

Tantalum and its alloys has received increased interest for ballistic applications. The stress-strain behavior of several tantalums of various compositions and manufacturing sources were investigated as a function of temperature from 77K to l273K, and strain rate from l0^-3s^-1 to 8000s^-1. The yield stress was found to be sensitive to the test temperature, the impurity and solute contents; however, the strain hardening remained very similar for "pure" Ta but increased with alloying. Powder-metallurgy tantalum with various levels of oxygen content produced via different processing paths were also investigated. Similar mechanical properties compared to conventionally processed tantalums were achieved. Constitutive relations based upon the Johnson-Cook, the Zerill-Armstrong and the Mechanical Threshold Stress (MTS) models were evaluated. Parameters for fitting these models to various tantalums and its alloys were derived. The capabilities and limitations of each model for large-strain applications will be examined. Work performed under the auspices of the U.S. Department of Energy.

11:05 am

MECHANICAL BEHAVIOR AND MICRO-TEXTURE ANALYSES OF ANNEALED TANTALUM AND TANTALUM-TUNGSTEN ALLOY PLATE: David H. Lassila, Erik Randish, Gilbert F. Gallegos, University of California, Lawrence Livermore National Laboratory, P.O. Box 808, L- 35, Livermore, CA 94550

At LLNL a wide range of mechanical testing was performed on numerous annealed tantalum and tantalum-tungsten alloy plate materials. These tests include, dynamic tensile testing, uniaxial testing in compression at strain rates of 10^-3s^-1 and 3000s^-1. In general we found the stress-train response of all of the materials to be reasonably well behaved., i.e. test results were reproducible for a given plate and general trends in behavior such as work hardening as a function of loading direction were consistent. However, on a smaller scale the tantalum test materials exhibited inhomogeneous mechanical behavior evidenced by non-uniform plastic strains in the off loading axes. For example, all compression samples were found to exhibit non-uniform radial expansion along the compression samples were found to exhibit non-uniform radial expansion along the compression axis. Metallographic examinations and micro-texture analysis of the microstructure of the test materials indicate that the inhomogeneous mechanical behavior is related to microstructural banding in the plate materials.

11:25 am

THE MICROSTRUCTURE OF Ta and Ta-W ALLOYS DEFORMED AT HIGH STRAIN RATES: C.L. Briant, R.H. Batcheler, Division of Engineering, Brown University, Providence, RI 02912. D.H. Lassila and W. Gourdin, Lawrence Livermore National Labs, Livermore, CA 94551

This paper will report on a study of the microstructure of pure Ta, Ta-2.5W, and Ta-10W alloys that have been subjected to deformation over a range of strain rates between approximately 10^-1 scc.^-1 and 10³ scc.^-1. Prior to testing the samples had either been annealed or subjected to shock loading. The high strain rates were applied by testing the samples in compression in a split-Hopkinson bar. The samples were examined by both optical metallography and transmission electron microscopy; the analysis of the dislocation structure from the latter technique will be the focus of this talk. The results show that in the annealed samples the Ta and Ta-2.5W alloys contain parallel screw dislocations with <111> Burgers vectors. In the Ta-lOW alloy, regions were observed with long, straight parallel dislocations overlapping in specific directions. Deformation greatly increased the density of dislocations and showed the development of a cell structure. The formation and role of twins both during shock loading and deformation will be discussed.

11:45 am

WORK HARDENING AND THERMAL SOFTENING PROPERTIES OF TANTALUM AND TANTALUM ALLOYS: Craig L. Wittman, Jon P. Swensen, Dave A Dehmer, Alliant Techsystems Inc., 600 2nd Street N.E., Hopkins, MN 55343

Tantalum and tantalum Alloys of tantalum-2.5% tungsten and tantalum-10% tungsten have been tested in the annealed and cold worked condition at strain rates ranging from 10³ to 10³s^-1 and temperatures from 78-1000K. The work hardening and thermal softening response as the tungsten content is increased will be shown Additionally the work hardening and thermal softening responses due to various oxygen levels (30-300ppm) in pure tantulum and tantalum-2 5% tungsten will be presented.