Phase separation in immiscible silver–copper alloy thin films
Soumya Nag Æ Kristopher C. Mahdak Æ Arun Devaraj Æ
Smita Gohil Æ Pushan Ayyub Æ Rajarshi Banerjee
Received: 14 January 2009 / Accepted: 27 March 2009 / Published online: 15 April 2009
Ó Springer Science+Business Media, LLC 2009
Abstract Far from equilibrium, immiscible nanocrystal-
line Ag–Cu alloy thin films of nominal composition Ag–
40 at.% Cu have been deposited by co-sputter deposition.
Both X-ray and electron diffraction studies indicate that the
as-deposited films largely consist of nanocrystalline grains
of a single alloyed face-centered cubic (fcc) phase. How-
ever, detailed three-dimensional atom probe tomography
studies on the same films give direct evidence of a nano-
scale phase separation within the columnar grains of the as-
deposited Ag–Cu films. Subsequent annealing of these
films at 200 °C leads to two effects; a more pronounced
nanoscale separation of the Ag and Cu phases, as well as
the early stages of recrystallization leading to the break-
down of the columnar grain morphology. Finally, anneal-
ing at a higher temperature of 390 °C for a long period of
time leads to complete recrystallization, grain coarsening,
and a complete phase separation into fcc Cu and fcc Ag
phases.
Introduction
In recent years, there have been a number of studies on far-
from equilibrium alloys consisting of thermodynamically
immiscible alloying elements. These studies have been
well summarized in a recent review [1]. The silver–copper
binary system falls in this category with copper and silver
being mutually immiscible in the solid state, and exhibits
positive enthalpy of mixing in both solid and liquid states
[2]. Therefore, based on thermodynamic considerations,
solid-state alloying is not permitted at equilibrium, and the
mutual solid solubilities of Cu and Ag are typically \0.1%
at 300 °C[3]. The large positive enthalpy of mixing for the
liquid phase in such systems makes it rather difficult to mix
them even in the molten state. Therefore, in many cases,
far-from-equilibrium processing techniques, such as vapor
deposition and mechanical alloying, have been employed
to extend the solid solubility in such systems [4–8]. Since
alloy formation is thermodynamically unfavorable in such
systems, they exist only in a metastable form, limited by
kinetic constraints from phase separating into the equilib-
rium elemental constituents. Phase separation has also been
explored in a number of recent studies reported in the lit-
erature, such as in case of Au–Ag core–shell composite
films [9], in nanostructured stainless steels [10], as well as
by 3D atom probe in bulk nanocrystalline steels [11] and
oxide dispersion-strengthened (ODS) alloys [12].
Previous studies on sputter-deposited Ag–Cu alloy films
report that depending on the processing conditions, these
films either form a supersaturated solid solution [4–6, 13]
or a phase-separated mixture of fcc Cu-rich and fcc Ag-rich
solid solutions [13–16]. Deposition of these films on sub-
strates at or above room temperature [13–15], or on neg-
atively biased substrates [16], results in a higher mobility
of the vapor atoms on the surface of the growing film
leading to phase separation and consequent lowering of the
Gibbs free energy. In contrast, if the Ag–Cu films are
deposited on liquid nitrogen-cooled substrates, the vapor
atoms get rapidly quenched as soon as they arrive at the
film surface, and form supersaturated, metastable solid
S. Nag Á K. C. Mahdak Á A. Devaraj Á R. Banerjee (&)
Center for Advanced Research and Technology and Department
of Materials Science and Engineering, University of North
Texas, Denton, TX, USA
e-mail: rajarshi.banerjee@unt.edu
S. Gohil Á P. Ayyub
Department of Condensed Matter Physics and Materials Science,
Tata Institute of Fundamental Research, Mumbai 400005, India
123
J Mater Sci (2009) 44:3393–3401
DOI 10.1007/s10853-009-3449-0