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.
Highlights
The material phenomenon of creep, and more precisely, the prediction of long term behaviour based on short term experiments, has become one of the most important challenges to the UK Energy Sector, according to the Materials UK Energy Review 2007 [1]
In order to understand the nature of this change a series of creep tests were undertaken at graded levels of stress and at 700 ◦C, targeted to investigate changing dislocation structures at the onset of yield
The yield point the dislocation density increases to the point where the dislocation spacing is closer than that of the precipitates and the sub-grain structure around the grain boundaries is obliterated by the increased density throughout
Summary
The material phenomenon of creep, and more precisely, the prediction of long term behaviour based on short term experiments, has become one of the most important challenges to the UK Energy Sector, according to the Materials UK Energy Review 2007 [1]. Material creep has been the subject of extensive research for more than half a century, yet no single approach in the field of high temperature creep proposed during this time has been widely accredited with accomplishing the goals established during the Energy Review. More recent methodologies still fail to link the micromechanical behaviour to macroscopic properties and material behaviour
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