Near-neighbor mixing and bond dilation in mechanically alloyed Cu-Fe
V. G. Harris, K. M. Kemner, B. N. Das, N. C. Koon, and A. E. Ehrlich
U.S. Naval Research Laboratory, Washington, D.C. 20375
J. P. Kirkland
SFA Inc., Landover, Maryland 20785
J. C. Woicik
National Institute of Standards and Technology, Gaithersburg, Maryland 20899
P. Crespo and A. Hernando
Instituto de Magnetismo Aplicado, Las Rozas, 28230 Madrid, Spain
A. Garcia Escorial
Centro Nacional de Investigaciones Metalurgicas – Centro Superior de Investigaciones Cientificas, 28040 Madrid, Spain
Received 5 January 1996; revised manuscript received 4 April 1996
Extended x-ray-absorption fine-structure EXAFS measurements were used to obtain element-specific,
structural, and chemical information of the local environments around Cu and Fe atoms in high-energy ball-
milled Cu
x
Fe
1 -x
samples x =0.50 and 0.70. Analysis of the EXAFS data shows both Fe and Cu atoms reside
in face-centered-cubic sites where the first coordination sphere consists of a mixture of Fe and Cu atoms in a
ratio which reflects the as-prepared stoichiometry. The measured bond distances indicate a dilation in the bonds
between unlike neighbors which accounts for the lattice expansion measured by x-ray diffraction. These results
indicate that metastable alloys having a positive heat of mixing can be prepared via the high-energy ball-
milling process. S0163-18299610033-3
I. INTRODUCTION
In recent years there has been renewed interest in extend-
ing the mutual solubility of Fe and Cu. One reason is the
observation of giant magnetoresistance in heterogeneous
samples which have small ferromagnetic particles coherently
suspended within a nonmagnetic noble metal matrix.
1,2
The
easiest way to obtain such a structure in the Cu-Fe system is
the controlled heat treatment of a solid solution. However,
obtaining a solid-solution of Cu-Fe is not a trivial matter.
The Cu-Fe equilibrium phase diagram indicates little or no
miscibility of either constituent at room temperature, and
only 4 at. % Fe dissolves into Cu and 10 at. % Cu into
Fe near their respective liquidus lines.
3
However, extended
regions of metastable solubility can be obtained via vapor-
quenching techniques.
4,5
Recently, an alternative path to
vapor-quenching, high-energy ball-milling HEBM, has
been proposed to form solid solutions of combinations of
elements having a positive heat of mixing.
HEBM is a process which utilizes the energy of ballistic
collisions between particles of a charge material and the sur-
face of hardened steel balls and/or the walls of a shaking
container to mix, fragment, and ultimately amorphize or dis-
solve the charge materials. HEBM has become a popular
technique in recent years for the solid-state amorphization of
binary metal systems,
6,7
metal-metalloid systems,
8–10
and
even single-component systems.
11
Thermodynamic and ki-
netic models which explain these processes have been
proposed.
12–15
It had been widely held that a negative heat of
mixing was required in order to experience single-phase,
atomic mixing i.e., alloying in the HEBM binary metal
systems. However, recently HEBM has been reported to
form metastable alloys, or more correctly supersaturated
solid solutions, of combinations of elements which do not
exhibit appreciable solubility in their equilibrium phase
diagrams.
15–21
The alloying phenomenon in these materials
has been explained by Yavari, Desre, and Benameur
15
to
arise when small fragments, created by codeformation, ob-
tain a critical tip radii, whereupon capillary forces bring
about the dissolution of the tip region. Subsequently, the
solute content changes within the local spinode increasing
the critical radius causing the more rapid dissolution of the
region and the eventual complete mixing of one component
into the other.
The reports concerning the formation and characterization
of HEBM alloys having a positive heat of mixing have relied
largely upon electron microscopy and x-ray-scattering tech-
niques to establish the onset and degree of mixing. However,
when crystallites become increasingly strained and reduced
in size, as they do in HEBM, diffracted intensities experience
a reduction in amplitude from Debye-Scherrer broadening,
leading to the smearing of the diffraction features often be-
yond visual detection. These effects may lead to a misinter-
pretation of these data and consequently a misunderstanding
of the physical state of the sample.
In order to investigate if alloying occurs in HEBM
samples having a positive heat of mixing, we have used
x-ray diffraction in combination with extended x-ray-
absorption fine-structure EXAFS measurements to study
both the long-range and the short-range structure and chem-
istry in HEBM Cu
x
Fe
1 -x
x =0.5 and 0.7 samples. These
compositions are well outside the miscibility regions in the
Cu-Fe equilibrium phase diagram and are not expected to
form solid solutions under steady-state conditions.
PHYSICAL REVIEW B 1 SEPTEMBER 1996-II VOLUME 54, NUMBER 10
54 0163-1829/96/5410/692912/$10.00 6929 © 1996 The American Physical Society