|
Researchers from the Chemical Metallurgy
Research Center of the CNRS (CECM, Vitry-sur-Seine, France) and the Laboratoire
d’Ingénierie des Matériaux et des Hautes Pressions
of the CNRS (LIMHP, Villetaneuse, France) have just demonstrated the exceptional
mechanical properties of nanocrystalline copper. This material displays
a near-perfect elastoplastic behavior never before observed.
This research appears in the April 11, 2003, issue of the journal, Science.
The CECM and LIMHP teams carried out tensile tests on fully dense large-scale
bulk nanocrystalline samples, a material that has a grain size smaller
than the characteristic length scales normally encountered: about one
hundred nanometers as opposed to several microns1 . These tests consist
of following the evolution of the deformation of the material depending
on the applied stress. These are the first tensile tests on a material
with such fine grains (an average of 80 nanometers) made on representative
tensile testing samples (35 mm long with a diameter of 3.5 mm). Two special
techniques were used to obtain this type of material: the massive production
of ultrafine metal powder using a cryogenic evaporation-condensation technique
(developed at the CECM) and differential cold extrusion (developed at
the LIMHP).
The mechanical properties obtained are exceptional: nanocrystalline copper
is three times more resistant than normal copper and deforms homogeneously
with no apparent necking by a steady deformation flow until sample failure
occurs. Researchers were thus able to observe a near-perfect electroplasticity
for the first time. Deformation normally takes place heterogeneously in
traditional materials, increasing with applied stress and leading to premature
crack propagation and, ultimately, failure.
This behavior could be explained by high atomic diffusion and the small
size of the grains which would make it possible to activate superplastic-type
mechanisms at room temperature: the grains would slide around in a fluid
flow. Superplasticity occurs at higher temperatures for traditional materials.
The behavior observed opens some very interesting prospects for plastic
formation of materials at room temperature.
1 - A nanometer is equivalent to one thousandth of a micron.
Researchers contact:
Chemical Sciences Department
(CECM)
Yannick Champion
Tel: +33 1 56 70 30 30
e-mail: yannick.champion@glvt-cnrs.fr
Engineering Sciences Department
(LIMHP)
Patrick Langlois
Tel: +33 1 49 40 34 17/34 27
E-mail: langlois@limhp.univ-paris13.fr
Press contact :
Muriel Ilous
Tel : +33 1 44 96 43 09
e-mail : muriel.ilous@cnrs-dir.fr
|