In the current study, dislocation activity and storage during creep deformation in a nickel based superalloy (Waspaloy) was investigated, focussing on the storage of geometrically necessary (GND) and statistically stored (SSD) dislocations. Two methods of GND density calculations were used, namely; EBSD Hough Transformation and HR-EBSD Cross Correlation based methods. The storage of dislocations, including SSDs, was investigated by means of TEM imaging. Here, the concept of GND accumulation in soft and hard grains and the effect of neighbouring grain orientation on total dislocation density was examined. Furthermore, the influence of applied stress (below and above Waspaloy yield stress) during creep on deformation micro-mechanism and dislocation density was studied. It was demonstrated that soft grains provided pure shear conditions at least on two octahedral (111) slips for easy dislocation movement reaching the grain boundary without significant geometrically necessary accumulation in the centre of the grain. Hence, the majority of the soft grains appeared to have minimum GND density in the centre of the grain with high GND accumulation in the vicinity of the grain boundaries. However, the values and width of accumulated GND depended on the surrounding grain orientations. Furthermore, it was shown that the hard grains were not favourably oriented for octahedral slip system activation leading to a grain rotation in order to activate any of the available slip systems. Eventually, (i) the hard grain resistance to deformation and (ii) neighbouring grain resistance for the hard grain reorientation caused high GND density on a number of octahedral (111) slip systems. The results also showed that during creep below the yield stress of Waspaloy (500 MPa/700C), the GND accumulation was relatively low due to insufficient microscopic stress level. However, the regions near grain boundaries showed high GND density.Whereas, in addition to the movement of pre-existing dislocations (SSD and GND) at higher mobility rate under 800 MPa/700C above yield creep condition, large numbers of dislocations were generated and moved toward the grain boundaries. This resulted in much higher GND density but narrower width of high intensity GND near the grain boundaries. It is concluded that although GND measurement by means of EBSD can provide a great insight of dislocation accumulation and its behaviour, it is critical however to consider SSD type which is also contributes to the strain hardening of the materials.

Current creep models are derived using data from constant stress (or load) creep tests and are capable of accurately predicting creep behaviour when applied conditions are constant or near constant. However, analyses of creep curve shapes for the nickel based superalloy Waspaloy, when applied stress and/or temperature vary greatly during testing, have shown that predictive methods based purely on strain, time or lifefraction are insufficient and cannot predict observed creep rates. This is important when considering stress concentration features where stress relaxation due to creep can significantly alter the distribution of stress and thus affect fatigue life. When both stress and temperature are changed during a creep test, dislocation movement must proceed through a dislocation network formed under different conditions, resulting in greater than expected creep rates. It is proposed that this is due to a reduction in effective internal stress due to changes in dislocation structure.

Abstract. Creep tests of the polycrystalline nickel alloy Waspaloy have been conducted at Swansea University, for varying stress conditions at 700 • C. Investigation through use of Transmission Electron Microscopy at Cambridge University has examined the dislocation networks formed under these conditions, notably those with stresses above and below the yield stress. This paper highlights how the dislocation structures vary throughout creep and proposes a dislocation mechanism theory for creep in Waspaloy. In particular, the roles of recovery, tertiary gamma prime particles and dislocation foresting are examined, and related back to observations from the Wilshire fits. The virgin (untested) material has been forged and heat treated, containing some recrystallised material together with areas of more heavily deformed and recovered material clustered around the grain boundaries. Observations from tests below the 0.2% proof stress show relatively low dislocation densities away from grain boundaries and dislocation movement can be seen to be governed by interactions with the γ precipitates. In contrast, above the 0.2% proof stress, TEM observations show a substantially greater density of dislocations. The increased density provides an increment of strength through forest hardening. At stresses above the original yield point, determined by the precipitates, the creep rate is controlled by inter-action with the dislocation forest and results in an apparent activation energy change. It is proposed that the activation energy change is related to the stress increment provided by work hardening, as can be observed from Ti, Ni and steel results.