This dissertation reports a study on grain boundary dependent creep deformation behaviour of Inconei 718 in the region of power-law dislocation creep. It is proposed that grain boundaries may influence the creep deformation not only exclusively by grain boundary sliding, but also by an alternative mechanism yet to be determined. Existence of this new mechanism is justified by the feet that in the past many results have been explained unsatisfactorily on the basis of models of grain boundary sliding, and also that a few available results, although they are incomplete, are supportive of this concept.

The material used in this study was a wrought commercial precipitation strengthened nickel base alloy Inconei 718. This material was heat treated to produce two types of materials which were different only in the microstructures of their grain boundaries. The grain boundaries of one type were free of precipitates (material A) and those of the second type were decorated with 5-phase particles (material B). Creep tests on specimens with these two microstructures were comparatively conducted at a temperature in the range of 600°C to 650°C and at constant applied stress in the range of 745 MPa and 860 MPa. These creep tests provided a strong evidence of grain boundary dependent creep bahaviour which is distinguished the grain-material dependent creep behaviour in the following three major aspects:​ (1) It has a secondary creep rate that is independent of the strength of grain material, (2) It shows much stronger dependence of secondary creep rate on applied stress and effective stress as well, (3) It causes the back stress to be dependent on the applied stress and grain boundary microstmcture (which includes the grain size and the amount of coverage of precipitates on grain boundaries).

The observed results have been also analyzed and it is proposed that creep deformation of this material involves a grain boundary dependent deformation mechanism that is different from the mechanism of grain boundary sliding. It is suggested that the elastic and plastic incompatibility around grab boundaries caused heterogeneity in the distribution of applied stress. This incompatibility is described by a variable R, which considers the strengthening state of the grain interior and of the grain boundaries. Partitioning of the applied stress in both regions is, then, quantitatively given by a factor which includes the effect of R and grain size. Based on these concepts, it is proposed that (1) the incompatibility in material with clean grain boundaries can be eliminated very soon after the stress is applied, and therefore, a uniform distribution of applied stress can be achieved. This may result in a creep behaviour which is dependent on the microstructures of grain interior, (2) In material B, however, the incompatibility can be intensified due tc the high stress concentration around densely spaced particles on grain boundaries. ' produces a heterogeneity in partitioning of the applied stress to the grain interior and the regions of grain boundaries. As a result, the creep deformation is observed to depend on grain boundary microstructure as well. Results reported in the literature can also be satisfactorily explained on the basis of the proposed mechanism.