Dynamic observations in the HVEM of the influence of crystal orientation on the deformation and fracture behaviour of molybdenum single crystals

Abstract

In-situ tensile experiments have been carried out on single crystals of molybdenum with four different orientations for the tensile axis [hkl] and foil plane (h′k′l′), namely [011](011), [100](011), [011](100), and [100](001). In all specimens the primary source of dislocation generation was observed to occur at particles, the specimen orientation influencing only the mode of dislocation propagation from the particles. The operative slip systems are found to be those having both a large Schmid factor and a small angle α between the operative Burgers vector b and the projection of b in the slip plane on to the foil surface, i.e. those for which screw dislocations can escape more easily at the foil surface. Quantitative application of a combined Schmid and surface-orientation factor has been examined in the present work and is discussed. The detailed dislocation interactions observed during straining are very dependent on specimen geometry. In the [011](011) specimen dislocation interaction is limited because only short lengths of edge dislocations propagate significant distances. For other orientations dislocation multiplication occurs by the interaction of mobile screw dislocations with either grown-in dislocations or other gliding dislocations. At higher strains, prolific dislocation activity is associated with the propagation of cracks. The movement of dislocations which have a Burgers vector inclined to the foil surface results in an effective thinning of the foil perpendicular to the foil surface and eventual failure is observed to occur by crack propagation along these thinned regions.