Jerky Motion of Dislocations in Ni 3 Al and Ni 3 Ga. Experimental Observations and Modelling
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JERKY MOTION OF DISLOCATIONS IN Ni 3AI AND Ni 3 Ga. EXPERIMENTAL OBSERVATIONS AND MODELLING.
GUY MOLENAT*, DANIEL CAILLARD*, ALAIN COURET* AND VACLAV PAIDAR* *CEMES-LOE/CNRS, B.P. 4347, 31055 Toulouse Cedex, France, "**Institute of Physics, Na Slovance 2, 180 40 Praha 8, Czechoslovakia.
ABSTRACT Recent in situ and post mortem observations in Ni3 A1 and Ni 3Ga alloys have yielded consistent results concerning the structure and the movement of screw superdislocations in ( 111) planes. The different stresses involved in the observed cross-slip processes have been computed, and the outline of a new model of stress anomaly has been formulated on the basis of these results. INTRODUCTION Octahedral glide in Ni 3A1 has been studied extensively over the last ten years, because it is a model material for studying the dislocation behaviour and the mechanical properties of many intermetallic alloys and advanced alloys with high technological potentials. In particular, it exhibits an anomalous increase of the yield stress with temperature, the origin of which is still being debated. Several models which have been summarized in comprehensive review articles (see for instance 1)give possible explanations for the yield stress anomaly. These models are consistent with either some aspects of the mechanical properties or microscopic observations. It was however shown recently 2 that no model can account for all the macroscopic and microscopic characteristics observed. It is thus the aim of this paper to discuss recent microscopic TEM observations and new observations, with an emphasis on the consistent results obtained using both post mortem observations and in situ deformations. A mechanism is proposed for the glide of screw dislocations, and the stress necessary to activate this mechanism has been calculated. The outline of a new model for the yield stress anomaly is described. EXPERIMENTAL OBSERVATIONS AND INTERPRETATIONS Experimental observations have been made in Ni 3AI and Ni 3Ga single crystals. The results obtained can be directly compared because these two materials have very similar mechanical properties and dislocation substructures. Post mortem observations have been made in Ni 3Ga crystals in collaboration with Oxford University, and in situ straining experiments were conducted in Ni 3AI crystal supplied by ONERA. The results obtained can be divided into two temperature ranges corresponding to the lower and higher parts of the yield stress anomaly domain. In the low temperature range (300 K-473 K), in situ observations have revealed a high density of rectilinear screw superdislocations dissociated into two superpartials separated by an APB ribbon which lies very often in a plane close to the primary octahedral glide plane. These dislocations are seen to move jerkily over distances which are often equal to the width of the APB ribbon, in such a way that the trailing superpartials take the exact place of the leading superpartials 3, as illustrated in fig. 1.
Mat. Res. Soc. Symp. Proc. Vol. 288. ©1993 Materials Research
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