Normally it is accepted that the finer grain sizes lead to poor creep properties in a material. Why so?
Greater the grain size causes to generate more cracks on the surface and lowered the creep resistance
Try F. Garofalo, Fundamentals of creep and creep rupture, Macmillan?, circa 1965
1. Diffusion plays important role in creep deformation process. So, to minimise the diffusion grain boundary area needs to be reduced with the aim to improve creep resistance property. Hence, coarse grained material shows better creep resistance properties compared to fine grained material.
2. Grain boundary sliding is one of the dominant mechanisms for creep to occur. So, grain boundary area being more in fine grained material the contribution to creep deformation from grain boundary sliding will be higher in comparison to coarse grained material.
For the above two reasons single crystal is the ultimate choice.
large grain size leads to high creep resistance because it results in reduction of grain boundary and dislocation
The higher (coarser) the grain size, the lower the creep resistance of the material.
Because if creep is due largely to grain boundary sliding then the less gb's there are the less creep there will be. Fewer larger grains in a given volume means that less creep will appear as was intended for singe crystal turbine blades once developed. However, there are other diffusion processes that can dominate at high temperature which explains the rather contradictory responses to your question. Conditions must be considered.
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