Sponsored by: Jt. SMD/MSD Nuclear Materials and MSD Flow and Fracture Committees and FEMS (Federation of European Materials Societies)
Program Organizers: R.J. Arsenault, Department of Materials Science and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115; David Cole, CRREL, 72 Lyme Rd., Hanover, NH 03755; Todd Gross, Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824; Gernot Kostorz, Institut für Angewandte Physik, ETH Hönggerberg, CH-8093 Zürich, Switzerland; Peter Liaw, Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200; Sivan Parameswaran, NRC-Institute for Aerospace Research, Ottawa, Canada K1A 0R6; Howard Sizek, Inco Alloys International Inc., Huntington, WV 25705-1771
Tuesday, AM Room: Orange County 3
February 6, 1996 Location: Anaheim Marriott Hotel
Session Chairpersons: S. Parameswaran, NRC-Institute for Aerospace Research, Ottawa, Canada K1A 0R6; H. Ishii, Department of Mechanical Engineering, Shizuoka University, Hamamatsu City, Japan
8:30 am Invited
DIPOLE ORIENTATION JUNCTIONS AND BORDONI RELAXATION: J.J. Gilman, Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA 90024
In 1949, Bordoni reported internal friction peaks in cold- worked Ag, Al, Cu, and Pb at temperatures equal to roughly one- third of their Debye temperatures; and frequencies of 10- 40 kHz. These peaks have since been observed in several other metals, and have been interpreted in terms of the nucleation of dislocation kink- pairs lying in a Peierls- Nabarro crystal potential well. An alternative interpretation is based on the recognition that dislocation dipoles have two orientations relative to a plane that is perpendicular to the glide plane and which contains the axis of the dipole. Furthermore, along a given dipole the orientation can change creating a dipole orientation junction (DOJ) between the two differently oriented pieces of dipole. The DOJs can hop from one atomic plane to another. In the presence of an applied shear stress this causes anelastic relaxation. The activation barrier that resists this is small; hence the low temperature of the relaxation peak. There are two kinds of dipole lines (a vacancy type, and an interstitial type); hence the two peaks. The height of the activation barrier for DOJ motion is determined approximately by the stacking fault energy in the specimen metal. Measured parameters confirm this.
9:00 am Invited
DEFECT GENERATION UNDER SHOCK-WAVE DEFORMATION: M.A. Meyers, Department of AMES, University of California, San Diego, San Diego, CA 92093; G. Ravichandran, California Institute of Technology, Pasadena, CA; V.F. Nesterenko, Lavrentiev Institute of Hydrodynamics, Novosibirsk, Russia
Different models for the generation of point, line, and interfacial defects under shock-wave deformation are reviewed with emphasis on the pioneering work of Weertman on high-velocity dislocations and plastic-wave and shock-wave dislocation-based mechanisms. Recent results of predictions of the decay of the deviatoric component of stress behind the shock front are presented. A constitutive description of the slip-twinning transition is proposed. The possible influence of non-linear, non-equilibrium temperature fluctuations on the process of defect generation is analyzed. Research supported by the U.S. Army Research Office Contract ARO DAAL 03-92-G-0108 and NSF Grant DMR 9396132.
9:30 am Invited
MICROMECHANISMS OF THE DISLOCATION MOTION IN CRYSTALS WITH DEEP PEIERLS RELIEF: V.I. Nikitenko, Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia
Problems of the influence of point defects on dislocation mobility are analyzed. The two- level intermittent loading technique is applied to study kink motion along a dislocation line in Ge, Si, and SiGe crystals that are characterized by different ratio of the influence of point defect barriers and Peierls potential on the dislocation motion. The experimental data are analyzed in the framework of a model, considering the joint interaction of point defects with a dislocation. It is shown that Cottrell atmosphere not only determines the barriers for the kink motion but stimulates the kink pair return to formation centers as well as the specific mode of kink drift along the dislocation line due to the motion in the field of random forces. It is determined by step-like changes of a dislocation energy with attachment of a point defect to the dislocation core or its detachment caused by kink motion and can lead to the nonlinearity of the kink drift or even to localization of kinks.
10:00 am Invited
ON THE ROLE OF DISLOCATIONS IN HEAVILY STRAINED MATERIALS: J.Th.M.De Hosson, Department of Applied Physics, Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, Otmar Kanert, Department of Physics, University of Dortmund, D-44221 Dortmund, Germany
As a tribute to professor Hans Weertman upon the occasion of his 70th birthday this paper deals with the role of dislocations in materials deformed at high strain rates. In contrast to classical crystal deformation experiments which occur typically at strain rates of 10-5s-1 to 10-1s-1, ballistic deformation experiments involve strain rates of 104s-1 to 106s-1. These high strain rates yield completely different deformation mechanisms. This paper describes TEM and HREM study of the dynamic compacted samples of high Tc superconducting ceramic materials such as YBa2Cu3 at various E/M values, i.e. the ratio between the mass density of the explosives and the mass density of the material to be compacted. The E/M values for these samples increases from E/M - 0.7 to E/M=2.1. Higher E/M values correspond to longer pulse duration and slightly higher peak pressures. Apart from the well established <100>{100} glide system, the role of a novel [110](10) and [010](100) glide systems is studied. All glide systems are found to interact with the ferroelastic domains of the material, each in a different way. Possible advantageous effects of the novel dislocations on the superconducting properties are also considered. A comparison is made with in-situ NMR experiments on the deformation-induced generation of point defects in ceramic materials and metals.
10:30 am BREAK
10:40 am
ON THE INCREASED STRAIN RATE SENSITIVITY OF THE FLOW STRESS OF COPPER AT HIGH STRAIN RATES: H.D. Chandler, University of The Witwatersrand, Johannesburg, Private Bag 3, WITS 2050, South Africa
Experimental work on the mechanical behavior of fcc metals indicates that when the strain rate increases to about 103sec-1 there is an apparent sharp increase inn the flow stress (measured at constant strain) with strain rate. This has been interpreted in terms of a change in deformation mechanism, e.g. as a change from dislocation glide control to a dislocation drag mechanism. However, much of the recent work on the subject has explained this change in terms of dislocation glide accompanied by an increase in the rate of dislocation production. A similar approach using a glide rate equation together with a structure evolution description is adopted for the present work. However, instead of the dislocation increase being modelled by the evolution equation, it is regarded as initially being forced by the dictates of the glide equation. This leads to a simpler description of the effect requiring fewer empirical values. There is good correlation between modelled behavior and available experimental results on copper.
11:00 am
HIGH STRAIN RATE DEFORMATION MECHANISMS IN ALPHA TITANIUM: D.R. Chichili, K.T. Ramesh, K.J. Hemker, Department of Mechanical Engineering, The John Hopkins University, Baltimore, MD 21218
The present work addresses the deformation mechanisms that control the high strain-rate mechanical behavior of alpha titanium. The mechanical properties have been measured using a combination of quasistatic and dynamic Kolsky bar compression tests, and data for strain-rates ranging from(10-5 to 104s-1) has been acquired. At the macroscopic level, the material shows a strong strain hardening behavior for fixed strain-rates and rate sensitivity of the flow stress for fixed strains. Kolsky bar specimens have been recovered after being subjected to a single known stress pulse, and the deformed specimens have been investigated using both optical and transmission electron microscopy (TEM). This recovery technique allows us to relate the observed microstructural mechanisms with the measured mechanical properties. Macrotwins are evidenced in the optical micrographs. TEM is employed to characterize these deformation twins and to identify the nature and density of dislocations that are present after deformation at different strain-rates. To date, the density of dislocations and the number of active twin planes has been found to increase with the strain-rate. These studies are ongoing, and a more detailed picture of the relationship between the macroscopic behavior and the microstructural mechanisms will also be discussed.
11:20 am
CONCERNING THE EVALUATION OF THE INTERNAL STRESS IN DISLOCATION DYNAMICS: C.V. Iswaran, R.E. Reed-Hill, M.J. Kaufman, Department of Material Sciences and Engineering, University of Florida, Gainesville, FL 32611
The measured flow stress during plastic deformation, in the absence of any strain aging phenomena, consists of two parts: a thermally activated effective stress, [[sigma]]*, and an athermal internal stress, [[sigma]]E. On the one hand, as far as theories of dislocation dynamics are concerned, [[alpha]]* is by far more interesting, and a vast amount of literature exists on the subject of the thermally activated movement of dislocation line segments through barriers posed by obstacles such as point defects to such motion. On the other hand, the experiments measure the total flow stress, [[sigma]]F, which includes [[sigma]]E. In addition, we have also shown, that for a number of polycrystalline materials, data in the literature conform to the power law, providing the effective stress is evaluated by subtracting the correct value of the internal stress from the flow stress.
11:40 am
DISLOCATION MECHANISM OF SHOCK-INDUCED FRACTURE AND MARTENSITE FORMATION IN AUSTENITIC STEEL: E. Zaretsky, Pearlston Center for Aeronautical Engineering Studies, Department of Mechanical Engineering, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva, 84105, Israel
An interferometric (VISAR) technique was used to measure Hugoniot Elastic
Limit, dynamic viscosity and spall strength of austenitic Fe-Cr-Ni steel at
temperatures of 173 - 293 K. The samples were impacted by 3-mm steel
projectiles with velocities of 100-350 m/sec. The recovered samples were
studied metallographically. The material behavior during shock-induced
martensitic transformation at temperatures of 173 -240 K was found to be
similar to the dynamic response of the material near its dynamic fracture
threshold at 293 K. The suggests the same governing mechanism in both
phenomena. A model capable of explaining the phenomena is suggested. It is
based on the motion and multiplication of Shockley-type partial dislocations in
presence of shock-induced shear stress, resulting in the development of a
lens-like colony of stacking faults. At room temperature the perfect
dislocations interact with the faults of the colony, transforming it into the
colony of Frank loops. Under a tensile stress this transformation results in
the nucleation of vacancy discs which coalesce into voids. At lower
temperatures the tensile stress favors the transformation of the stacking fault
lens into a lenticular martensite crystal.
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