Mechanical tests of nanocrystalline specimens
Faculty of Materials Science and Engineering, Warsaw University of
Nanocrystalline materials must be often examined using
very small specimens, and the reasons why this is so are: many
technological processes permit us to prepare only small specimens.
Often we need to examine small components of electro-mechanical
Problems arising in the strength examinations of
micro-specimens can be classified into 5 groups: cutting the
specimens, mounting the specimens, accuracy in controlling the loading
force and resulting strain, strain measurement, force measurement.
The production of nanomaterials by plastic straining is difficult
because of strain localisation in bands. The present study was
concerned with the effects of various straining parameters on the
microstructure and mechanical properties obtained. The effect of heavy
deformation and associated refinement of the microstructure was
achieved by the three methods: high-pressure torsion, equal channel
extrusion, and multiple rolling.
The materials examined belonged to various classes. It has
been found that in single-phase materials, plastic deformation results
in a nanocrystalline structure being formed. The strongest refinement
of the grains was achieved when using high-pressure torsion. The
average grain sizes ranged here from 20 to 100nm. This method has
however a drawback in that the samples obtained have small volumes.
Equal-channel extrusion gives greater sample volumes but the grain
size usually exceeds 100nm. In the intermetallic matrix materials,
the heavy deformation brought about the partial or total lost of the
long-range order and an increase of the material plasticity. The
ordering was recovered and the plasticity eliminated by subjecting the
material to annealing.