SUPERPLASTICITY AND MICROSTRUCTURE EVOLUTION IN NANONICKEL
A. P. Zhilyaev
Institute for Physics of Advanced Materials
Ufa State Aviation Technical University,
12 K. Marx St., 450000 Ufa, RUSSIA
FAX: 7 (3472) 233-422, Tel.: 7 (3472) 235-244,
Athough superplasticity has been studied for decades, few observations of
this behavior have been reported for pure metals. A problem generally
attributed to the difficulty of maintaining a grain size small enough
for superplastic flow at the temperatures required for superplasticity.
With the produce of nanocrystalline (NC) materials it has been hoped that
superplasticity could be obtained in a number of pure metals due to the
large volume fraction of grain boundaries in nanocrystalline material
should enhance the tendency for superplastic behavior. Two methods of
nanocrystalline materials producing (severe plastic deformation and
elctrodeposition) allow making fully dense samples available for testing.
In this paper, superplasticity was obtained from electrodeposited nickel
with initially nanocrystalline structure at the lowest normalized
superplastic temperature reported for any crystalline material.
A maximum elongation of 895% was obtained at 0.4 TM. Although a mean grain
size of ultrafine-grained (UFG) microstructure obtained by severe plastic
deformation is comparable with grain size of NC nickel at the beginning of
tensile test (about 0.5 mkm), no superplastic behavior of UFG samples was
detected. This controversy is discussed from viewpoint of non-homogeneity
of microstructure in UFG pure nickel.
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