Quantitative investigation of the role of internal stresses and of dislocation pile-ups in tension and fatigue deformation and fracture using a unidirectionally solidified composite

Abstract

Owing to its simplicity, a unidirectionally solidified composite, which combines the presence of strong non-shearable obstacles (the fibres) with the possibility of deriving from observations the same quantitative information as for a single crystal, has been used as a prototype material to study fundamental deformation and fracture mechanisms. This has made possible a quantitative explanation of both the existence of strong unrelaxed internal stresses in a ductile material, the matrix, and the hardening effect (Mott and Nabarro 1940) of the resulting pseudoperiodical shear stress field. With regard to the role of dislocation pile-ups in deformation and fracture, performing either longitudinal or transverse tensile tests has permitted us to obtain, in the presence of an obstacle, different shear-stress normal-stress combinations which cannot be obtained in a single crystal, but may be found in a given grain of a polycrystal where a crack may thus nucleate. In the case of a high shear stress, dislocation pile-ups containing more than 100 dislocations may lead to the fracture of the obstacle; moreover, these observations have shown that the role of dislocation pile-ups is not restricted to the nucleation of the crack itself, but may be extended to the nature of its propagation in the obstacle, thus explaining the paradoxical propagation in a plane containing the direction of the applied stress. In a situation combining a high shear stress with a high normal stress, the presence of the pile-up may lead to the nucleation of a crack in the glide plane itself, in a material as ductile as the nickel-base matrix. In fatigue, the observation of fibre failure at a much lower stress level than in monotonic tensile tests has suggested a deformation model typical of fatigue and involving two double dislocation pile-ups of opposite sign, located in parallel slip planes, in which case the back-stresses exerted on the sources by the emitted dislocations are opposite in sign and thus mutually annihilate, thereby permitting the sources to emit a sufficiently large number of dislocations to break the obstacle.