Abstract
Miniaturised tensile tests coupled with in-situ scanning electron microscopy are used to deduce the grain boundary properties of a nickel-based superalloy at 750 °C. This allows the damage initiation, evolution and failure processes to be observed directly. The significant variation in ductility – consistent with the limited number of grain boundaries being present – is rationalised using a crystal plasticity approach calibrated by experiments on single crystals loaded along the <001>, <011>, and <111> directions. Quantitative strength and toughness values for the grain boundaries are estimated using a cohesive zone method. The modelling approach is used to determine an approximation of the size of the representative volume element (RVE) needed for volume-averaged behaviour.
Highlights
The grain boundaries of nickel-based superalloys e in common with many polycrystalline alloys e can be their Achilles' heel [1e4]
Compression experiments are used: experimental curves for the single-crystal samples tested at 750C under the loading directions and at a strain rate of 1 Â 10À4, 4 Â 10À5, 1 Â 10À5, and 4 Â 10À6/s are shown as data points in Fig. 3(a) and (b) respectively
The results show that the explicit micro-models are able to rationalise the change in ductility as a function of the microstructure and the grain boundary architecture
Summary
The grain boundaries of nickel-based superalloys e in common with many polycrystalline alloys e can be their Achilles' heel [1e4]. Grain boundaries can impair the macroscopic response, because they can support crack initiation and quasi-brittle behaviour, which is of practical relevance for their application in high temperature systems [3,5e7]. Because the orientation of a grain boundary plane changes along its length, voids and cavities forming around the more stressed parts of a grain boundary do not necessarily interlink [12,13], preventing continuous and rapid intergranular crack growth. Other effects, such as environmentally-assisted grain boundary degradation [2,14], are affected by the local grain boundary architecture. The effect of the atomic scale arrangement at the interfaces has been studied: alloys with low S boundaries show improved ductility and strength [16]
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