Abstract :
Nanomaterials can easily be prepared as
thin films and powders, but are much harder to prepare
in bulk form. Nanostructured materials are prepared
mainly by consolidation, electrodeposition, and deformation.
These processing techniques have problems
such as porosity, contamination, high cost, and limitations
in refining the grain size. Since most bulk
engineering metals are initially prepared by casting,
we developed a casting technique, flux-melting and
melt-solidification, to prepare bulk nanostructured
alloys. The casting technique has such advantages as
simplicity, low cost, and full density. In our method,
Ag–Cu alloys were melted in B2O3 flux, which
removed most of the impurities, mainly oxides, in the
melts. Upon solidifying the melt at a relatively slow
cooling rate on the order of 101–102 K/s a large
undercooling of ~0.25 Tm (where Tm is the melting
temperature) was achieved. This large undercooling
leads to the formation of bulk nanostructured Ag–Cu
alloys composed of alternative Ag/Cu lamella and
nanocrystals, both ~50 nm in dimension. Our liquidprocessed
alloys are fully dense and relatively free
from contamination. The nanostructured Ag–Cu alloys
have similar yield strength in tension and in compression.
The as-quenched alloys have yield strength of
400 MPa, ultimate tensile strength (UTS) of 550 MPa,
and plastic elongation of ~8%. The UTS was further
increased to ~830 MPa after the as-quenched alloy rod
was cold drawn to a strain of ~2. The nanostructured
Ag–Cu alloys show a high electrical conductivity
(~80% that of International Annealed Copper Standard),
a slight strain hardening (strain-hardening coefficient
of 0.10), and a high thermal stability up to a
reduced temperature of 2/3 Tm. Some of these behaviors
are different than those found in previous bulk
nanostructured materials synthesized by solid state
methods, and are explained based on the unique
nanostructures achieved by our flux-melting and
melt-solidification technique
