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Submitted on 1 Jan 1974
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DISCUSSION
J. Friedel
To cite this version:
J. Friedel. DISCUSSION. Journal de Physique Colloques, 1974, 35 (C7), pp.C7-120-C7-120.
�10.1051/jphyscol:1974712�. �jpa-00215868�
C7-120 J. A. VENABLES, C. A. ENGLISH, K. F. NIEBEL AND G. J. TATLOCK
DISCUSSION Chairman : J. FRIEDEL V. VITEK.
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Computer simulation of dislocationbehaviour under uniform stress.
L. LECJEK.
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Dissociation of dislocations in b. c. C.and the Peierls-Nabarro stress.
F. GRANZER.
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Atomistic and elastic calculations in crystals with rocksalt structure.W. BOLLMANN. - A brief report on recent calcula- tions by DEVLIN of stacking fault energy in some F. C. C. noble metals.
A. SLEESWYK. - Atomistic computation of the Carbon diffusion path to dislocations in Nickel, as compared to the Cottrell and Bilby model.
D. BACON. - Extended nodes in anisotropic cubic metals.
P. SWANN. - Summary of stacking fault energy data of primary solid solutions.
M. LORETTO. - Stacking fault energy in some rare earth alloys.
A. PINEAU. - Stacking fault energy as a function of temperature.
DISCUSSION ABSTRACT : J. FRIEDEL.
1. Atomistic computations.
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The computer simula- tion studies by Vitek, Lejcek and Granzer clearly showed that, in metals and ionic solids with simple structures, the core of dislocations is usually spread out along one or a few fairly well defined atomic planes, so that it is useful to use the Peierls-Nabarro description of a continuous planar distribution of infinitesimal dislocations. The hope was expressed by Friedel that the crude but simple concepts about well defined partial dislocations and stacking faults would be kept in mind when studying or using these more refined models. The simple concepts however loose some of their usefulness for splittings of the order of core widths, thus stacking fault energies above typically 50 ergs/cm2, and also for unstable stacking faults such as along (112) in BCC metals.Computations at finite temperatures seem difficult to realize, because of large activation energies, except probably under stresses not much below the Peierls frictional stress. Upper limits for activation barriers could however be computed from the energy of reaso- nable excited configurations.
The values reported by Bollmann of Devlin's computed stacking fault energies in FCC noble metals seem to be in surprisingly good agreement with experi- ments, seeing they were based on a pseudopotential model, and these usually predict that the HCP phase is more stable than the FCC.
In Granzer's computations on ionic solids, the way anharmonic terms were taken into the boundary conditions between the core and the outside region
were not clear to several participants. The long range contraction perpendicular to the stacking fault seemed necessary to balance the effect of the short range expansion, for a ribbon of split dislocation of finite width ; it did not seem therefore to contradict necessa- rily Fontaine's result of a simple short range expansion for an infinite stacking fault.
Finally Sleeswyk's result (in an atomistic computa- tion, Cis not attracted to the core of a dislocation in Ni in the way predicted by Cottrell and Bilby's simple term due to an expansion effect, in a continuous model) seemed to Friedel to be a simple consequence of the fact that Sleeswyk's model includes the analog of an elastic modulus term, i. e. C acts as a harder (and larger) region ; even in a continuous model, one then expects lines of forces quite different from Cottrell and Billy in the short range region.
2. Stacking faults energies.
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The general feeling from the talks of Bacon, Swann and Pineau was that, for simple and pure enough metals, a given experi- mental method gave results of improving accuracy, but that various methods still gave somewhat different answers, especially in the large y range, probably in part because of the loss of accuracy of the model refer- red to above, for this case.In alloys, two further complications arose, as pointed out by Poirier, Coulomb and Friedel : at lower temperatures, pinning by alloying elements ; at higher tempxatures, Suzuki effect could develop whether temperature hysteresis effects are or not observed ; and the only case where one was sure of avoiding both pinning and Suzuki effect was if dislocations freshly developed at low temperatures showed no pinning and no temperature hysteresis, in a temperature range where diffusion was ineffective. These remarks bore especially on measurements of dy/dT reported by Pineau where, in alloys, it is not clear what is due to a vibrational entropy effect and what to a Suzuki effect.
Finally the contrast observed by Loretto in deformed PdCe alloys lead to the feeling that in alloys of two components of different sizes, the misfit produced by beaking the short range order might be partly released by a small average shift across the slip plane which could have a tangential as well as a normal component.
Similar observations seem to have been made in some long range order alloys. Another explanation proposed was that long wave length concentration fluctuations could induce, by slip, a distribution of infinitesimal dislocations along the slip plane to accommodate the change in lattice parameter along the plane ; it is however not clear how the corresponding stresses would interfer with the preexisting stresses due t o the assumed fluctuations.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974712
