The values for E 0 in Eq. (11.4) are close to the bulk values found in coarse-grained
materials. The yield stress is influenced by the porosity in a similar way; Figure 11.5
shows the yield stress of nanocrystalline copper and palladium again as function of
porosity.
Owing to the unavoidable grain growth during sintering of the specimens, which
were prepared by powder metallurgical means, in most cases, a varying density is
connected to a varying grain size. As the yield strength is also grain size-dependent,
correlations between porosity and yield stress are usually quite poor. For the
samples used for Figure 11.5, Figure 11.6 provides a correlation between grain
size and porosity. The remarkable increase in grain size with decreasing porosity
Figure 11.4 Young’s modulus of palladium
and copper according to Sanders et al. [4]. The
wide scattering of experimental values
highlights the problem in preparing a
consolidated nanocrystalline specimen and
measuring the porosity. The severe scattering of
the experimental values is probably caused by
different grain sizes.
Figure 11.5 Yield stress of nanocrystalline copper and palladium as a function of the porosity [5].
11.2 Bulk Metallic and Ceramic Materials j303