Weak-beam study of dislocations moving on {100} planes at 800°C in Ni3Al

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Abstract
The deformation substructure in polycrystalline Ni3Al deformed at 800°C has bean characterized. It consists of dislocation loops which are markedly segmented, the portions all being located in {100} planes. Under weak-beam conditions, the dislocation lines adopt different dislocation configurations, according to their character. Provided that the dislocation lines lie within approximately 10° of the exact screw orientation, they are dissociated in the {001} glide plane of the loop. However, they never show any orientational preference, which would have attested to the operation of the cross-slip mechanism (Kear 1966); this is true for a temperature where the strength of Ni3A1 is at a maximum. When they are mixed in character, they are dissociated by a process which involves the motion by climb of the partials. The resulting configuration is sessile; it confirms Flinn's theoretical predictions (1960). The mixed segments often dissociate in {013} planes in such a way that the 1/2⟨110⟩ displacement vector makes an angle of 26.5° with the antiphase boundary normal. The edge segments dissociate by climb and by glide in two consecutive steps. The asymmetrical faulted contrast is interpreted in terms of a significant difference between intrinsic and extrinsic stacking-fault energies. Some of the mechanical properties of Ni3Al are consistent with the occurrence of climb dissociation. However, both the dependence of the critical resolved shear stress upon orientation and the insensitivity of the yield stress to the ageing conditions remain unexplained by diffusive splitting. It is concluded that, at intermediate temperatures, the influence of cross-slip and climb dissociation should be superimposed, and that the lower the temperature, the larger the role played by cross-slip.