February 6, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: TBA
AN OVERVIEW OF DISLOCATION MOBILITY IN CRYSTALS-FOUR DECADES IN RETROSPECT: V. R. Parameswaran, Institute for Aerospace Research National Research Council Canada, Ottawa, Ontario K1A OR6; R. J. Arsenault, Metallurgical Materials Laboratory, Dept. of Materials Nuclear Engineering, University of Maryland, College Park, MD 20742-2115
Studies carried out in the past four decades on dislocation motion in crystals under different velocity regimes are briefly reviewed. The evolution of the techniques starting from slow deformation under quasi-static loading leading to high-strain-rate techniques, such as torsion and impact stress pulses and ultrasonic methods, are recapitulated. High speed dislocations subjected to lattice drag effects and their anomalous behaviour at relativistic velocities predicted by Weertman, are also briefly reviewed, with their implications in high-strain-rate deformation of materials.
DETERMINATION OF THE MAGNITUDES OF PEIERLS STRESS FOR DIFFERENT CRYSTALS: Jian N. Wang, Chemistry & Materials Sciences, L-370, Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551-9900
The classical derivation of Peierls stress is critically examined. It is found that in Peierls- Nabarro treatments there may exist an error in representing the atom positions across the slip plane. With the correction of this error, it is shown that 1) the misfit energy of a dislocation and the lattice friction to dislocation motion have an expected periodicity of the Burgers vector b rather than the unexpected periodicity of b/2 predicted by the original model, and 2) the Peierls stress formula for a dislocation in a lattice in which atoms just above and below the slip plane face each other may not be different from that for a dislocation in a lattice in which atoms alternate across the slip plane, as suggested before. Comparison with experimental data and atomistic calculations suggests that the delived new equation can be used to predict the magnitude of the Peierls stress for predominant slip systems in different crystals, and the accuracy of the prediction may degenerate greatly in the cases that the spreading of the dislocation core is non- planar and the application of an external stress changes the core structure significantly.
THE BEHAVIOR OF THE BORDONI PEAK IN ULTRA HIGH PURITY CU AND ITS IMPLICATION FOR DISLOCATION MECHANISM: S. Okuds Faculty of Engineering, Ibaraki University, Hitachi, Ibaraki 316, Japan
The size of the Bordoni peak is usually believed to decrease after high temperature annealing above recrystallization, because of a decrease in dislocation density. Recently, however, in ultra high purity Cu ( higher than nominal 7- nine ) the Bordoni peak was found to grow even after annealing up to 800deg. C. This was true for specimens heavily deformed by drawing and also for specimens deformed by fatigue at low amplitude. The size of the peak can well be explained as caused by kink pair formation on dislocations which form networks after high temperature annealing. The decay of the peak in ordinary pure Cu after annealing should be attributed to impurity pinning. Implication of the results for dislocation mechanism and arrangement will be discussed in detail.
ON THE DEFORMATION MECHANISM IN HARPER- DORN CREEP: Jian N. Wang, Chemistry & Materials Sciences, L- 370, Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551- 9900
Harper- Dorn creep, a Newtonian dislocation creep, has been observed in many metallic, ceramic and silicate materials. Based on an early Weertman's model for power law creep, a micro physical model for Harper- Dorn creep is developed. Two assumptions are made. One is that a steady dislocation density is established not only by the applied stress but by the Peierls stress as well. The other is that the flow process is dislocation glide plus climb with the climb being rate- controlling. It is shown that the predicted dependencies of dislocation density and strain rate on the Peierls stress are in very good agreements with experimental data on a wide range of materials.
3:20 pm BREAK
DYNAMIC DEFORMATION RESEARCH AND BIOSPHERIC PROTECTION: THE INTERMESHING OF MATERIALS SCIENCE AND ENVIRONMENTAL MANAGEMENT: Arthur H. Purcell, Preventive Environmental Management, 1745 Selby Ave., Los Angeles, CA 90024
Materials research and environmental protection are inextricably linked. Materials research leads to advances in material performance; and the better a material performs, the greater the opportunity for applying it to advanced resource conservation and environmental protection systems. Similarly, superior materials performance means reduced long- term resource and environmental impacts of material creation, use, and ultimate discard. Dynamic deformation research has particularly relevant implications for environmental management, since it directly supports the critical pollution prevention strategy of source reduction. Increasing durability, strength, and mechanical diversity (including recyclability) of materials leads directly to reduced energy, materials, and environmental intensiveness, and also facilitates utilization of alternative materials that can entail both enhanced environment/materials compatibility and reduced resource exploitation stress on the biosphere.
KINETICS OF RECRYSTALLIZATION IN ADIABATIC SHEAR BANDS, Joy A. Hines, Kenneth S. Vecchio, University of CA-San Diego, Materials Science Group, Dept. of AMES, MC-0411, LaJolla, CA 92093
Small crystallized grains have been observed to form within the adiabatic
shear bands of a variety of metals and alloys. Adiabatic shear bands form
under conditions of high strain rate and high shear strain; however,
recrystallization behavior under these circumstances is somewhat unclear. In
this study, the kinetics of two existing models for recrystallization,
strain-induced boundary migration and subgrain coalescence, are compared with
the shear band deformation and cooling times and the resulting recrystallized
grin sizes in adiabatic shear bands of several materials. This comparison shows
that the resulting recrystallized microstructure could not have occurred
statically during cooling down of the shear band and must have occurred during
deformation. The existing diffusion-dominated models for recrystallization
will be shown to be inadequate to describe dynamic recrystallization under the
present conditions of high strain, high strain rate adiabatic shear banding. It
is hypothesized that an additional mechanical effect associated with subgrain
rotations contributes to dynamic recrystallization under these conditions.
Preliminary calculations of subgrain rotations during deformation will be
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