DISCUSSION

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Submitted on 1 Jan 1974

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DISCUSSION

S. Amelinckx

To cite this version:

S. Amelinckx. DISCUSSION. Journal de Physique Colloques, 1974, 35 (C7), pp.C7-77-C7-77.

�10.1051/jphyscol:1974706�. �jpa-00215862�

JOURNAL DE PHYSIQUE

Colloque C7, suppltment au no 12, Tome 35, Dtcembre 1974, page C7-77

DISCUSSION

Chairman

S. AMELINCKX

A. SLEESWYK.

Creation of intrinsic-extrinsic stacking fault pairs.

D. ROWCLIFFE.

Deformation Mechanisms in transition metal carbides.

J. L. MARTIN.

Glide mechanisms in TaC.

G .

TENDELOO.

Dissociated antiphase boun- daries in ordered alloys.

J. L. STRUDEL.

Shearing mechanisms in a two- phase alloy with coherent ordered precipitates

partial dislocation arrays and microtwins.

DISCUSSION

S. AMELINCKX.

During the discussion session five papers were pre- sented in relation with the theme of the review papers of the morning session.

Sleeswyk pointed out that the mechanics for the creation of intrinsic-extrinsic fault pairs is still not fully understood. In particular he drew the attention to the fact that the formation of an extrinsic fault in the FCC structure requires that the two limiting Shockley par- tials should be in the reverse succession from that for an intrinsic fault. He presented a model for the intrinsic- extrinsic fault pairs, in which one of the outer partials of the triple ribbon is in fact the superposition of two partials separated by one or a small number of planes.

The resultant Burgers vector is then the same as that of the two other partials. A similar model has been pro- posed by S. kmelinckx (to be published in Treatise on Dislocatio~zs, by F . R. N. Nabarro).

Deformation mechanisms in Transition Metal Car- bides were discussed by D. J. Rowcliffe. At room tem- perature, the slip plane seems to depend on the compo- sition

in stoichiometric group V-carbides the primary slip plane is { 11 1 ) whilst it is { 110 ) for the non- stoichiometric carbides. It is also { 110 ) in the group IV-carbides. At high temperature this composition dependence disappears and the primary slip system is always { 11 1 ) < 110 >, above the brittle to ductile transition. These phenomena can be explained in terms of a model in which dissociation of dislocations normal to the { 11 1 ) plane is assumed to take place. Another

transition metal carbide was treated by J. L. Martin who discussed Glide mechanisms in TaC. From the experimental relationship between strain-rate, stress and temperature in a wide temperature range it is deduced that an athermal component contributes to the stress. The activation volume of the deformation pro- cess is found to be close to 100 b3 and to be relatively independent of stress. The activation energy is found to be close to that for carbon diffusion in the carbide (i. e. 6 eV/atom). The deformation process is then described in terms of the viscous glide of dissociated dislocations, the velocity of which is limited by the diffusion rate of carbon vacancies out of the dislocation core. Weak-beam images reveal dissociation of the dislocations as required by the model.

Van Tendeloo then elaborated on a relatively new concept in dissociation phenomena

Dissociated anti- phase boundaries in ordered alloys. He showed that in the X-phase (Fe,,Cr,,Ti,) and in ordered Ni3Mo groups of APB's are attached to dislocations. The case of Ni3Mo was studied in detail and it was shown that groups of one, two or three APB's are attached to a single dislocation, depending on the Burgers vector of the dislocation. In the absence of dislocations the APB's disappear always in groups of four. It was shown by means of EDAX analysis that compositional changes are associated with the APB's in particular an excess of nickel was found.

Lattice relaxation was found to take place as well along thermal as along deformation produced APB's.

The final paper of this session was given by Strudel on Shearing mechanisms in a two-phase alloy with cohe- rent ordered precipitates

partial dislocation arrays and, microtwins. In nickel base superalloys, submitted to deformation in the temperature range 600°-900 0C various stacking fault and partial dislocation struc- tures were observed and analysed. In sufficiently large deformation rates it is found that twinning occurs in the alloy

microtwins nucleate either in the matrix or in the precipitates, depending on temperature and composition. The pole mechanism proposed by Sleeswyk can account for the observed microtwins.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974706