  The microstructure of a second-generation nickel base superalloy is studied
using X-ray computed tomography (XCT) and scanning electron microscopy (SEM).
The as-cast material contains 0.15 (+-0.001) vol% voids and these are
distributed in the inter-dendritic region. The volume fraction of the voids
increases to 0.21 (+-0.001) vol% after tensile deformation. Surface
observations show evidence of dislocation emissions from the void surface, a
mechanism possibly facilitates the expansion of the voids and contributes to
the increased void volume fraction. Phenomenological parameters such as stress
triaxiality, often believed to control void growth, are investigated through
crystal plasticity simulation and compared with literature reported data. The
results indicate weak correlation between stress triaxiality and void growth,
but this may be possibly due to the lack of data at higher level of plastic
deformation, which is limited by the ductility of the material. The
distribution of the stress triaxiality field within the sample is heterogeneous
and the peak of the triaxiality field is a function of the ratio between notch
diameter and sample width. A smaller notch diameter to sample width ratio tend
to distribute the triaxiality peaks towards the centre of the sample but also
lead to higher strain localisation, an effect that results in early sample
failure.
