Effect of stress path on the miniaturization size effect for nickel polycrystals

The mechanical behavior of metallic materials deeply depends on the size of samples. For
specimen dimensions decreasing from a few millimeters to a few micrometers, the general
observed trend is a softening of the mechanical behavior in tension which affects the stress
level and the strain hardening. The objective of this work is to provide new experimental
results in order to analyze the miniaturization size effects for various stress paths without
strain gradients across the thickness of the samples. To this aim, experimental tensile tests,
large tensile tests and shear tests have been performed on Ni sheets with various grain
sizes. Results show that the miniaturization softening is affected by triaxiality, the larger
is this parameter, the lower is the mechanical softening. These features seem to be linked
to surface effects which are larger for low triaxiality stress paths. From an industrial point
of view, it is hence possible to improve the forming of microparts using suitable stress
paths.