Kinetics of the sigma-to-alpha phase transformation caused by ball milling in near equiatomic
Fe-Cr alloys
J. Cieślak,
1
B. F. O. Costa,
2
S. M. Dubiel,
1,
* and G. Le Caër
3
1
Faculty of Physics and Nuclear Techniques, AGH University of Science and Technology, PL-30-059 Kraków, Poland
2
Department of Physics, University of Coimbra, P-3004-516 Coimbra, Portugal
3
GMCM UMR CNRS 6626, University of Rennes 1, F-35042 Rennes Cedex, France
Received 9 February 2006; published 23 May 2006
Kinetics of the sigma-phase decomposition in three series of -FeCr intermetallic compounds caused by ball
milling in a vibratory mill was followed by Mössbauer spectroscopy. It can be described by an exponential
decrease with milling time. The decomposition leads to the formation of the -phase as well as that of an
amorphous phase. The former is expected from the phase diagram, but the latter is not. The amount of the
-phase increases with milling time, reaches a maximum characteristic of the milling conditions, and, finally,
after a slow decrease, remains constant. The decrease can be explained in terms of the -phase partial amor-
phization. The amount of the amorphous phase stays rather constant with a small tendency of increasing for
long-term milling times. Its structure seems to evolve with milling time.
DOI: 10.1103/PhysRevB.73.184123 PACS numbers: 61.18.Fs, 61.43.Dq, 64.70.Kb, 81.07.Bc
I. INTRODUCTION
One peculiarity of the Fe-Cr system is the possibility of
the formation of a -phase. Its crystallographic structure was
definitely identified as a close-packed tetragonal—space
group D
4h
14
- P4
2
/ mnm—with 30 atoms in the unit cell,
1
which
are distributed over five crystallographically nonequivalent
lattice sites: I, II, III, IV, and V with occupation numbers 2,
4, 8, 8, and 8, respectively. The sites have rather high coor-
dination numbers—12 for sites I and IV, 14 for sites III and
V, and 15 for site II—and quite different local symmetries.
The phase can be promoted by an isothermal annealing of
near equiatomic bulk alloys in the temperature range
870 K– 1100 K Ref. 2 and has been both of scientific
and technological interest. Its scientific interest follows from
properties very different from those of the -phase from
which it precipitates, and, in particular, weak magnetism.
3
Its
technological interest stems from its high hardness and
brittleness which causes deterioration of useful mechanical
properties in materials of practical importance. Conse-
quently, a major goal is how to avoid, or, at least, to retard its
formation. Whatever is the reason for the interest in the
-FeCr, the kinetics of its formation and/or decay seems to
be of importance both for scientists and engineers. Curiously,
in the available literature there is a pretty good deal of papers
devoted to the kinetics of the -phase formation both in bulk
alloys
4–11
as well as in nanocrystalline ones.
12
However, to
our knowledge, there is none devoted to the reversed
process, i.e., the kinetics of the decay of the -phase in the
Fe-Cr system, despite past studies on the influence of ball
milling on the -FeCr behavior.
13–17
The aim of this paper is thus to study the kinetics of the
sigma-phase decomposition by ball milling in three series of
-FeCr intermetallic compounds, prepared and/or ball-milled
in different conditions, using the Mössbauer spectroscopy as
the main characterization tool.
II. EXPERIMENT
Three series S
cg1
, S
cg2
, and S
nc
of -FeCr intermetallic
compounds were ball-milled in a Fritsch P0 vibratory mill
which is a low-energy type of mill. The first two series cor-
respond to the milling of a coarse-grained Fe
54
Cr
46
alloy
prepared by melting in argon atmosphere appropriate
amounts of iron 99.9+% purity and chromium 99.95%
purity in an induction furnace. The ingots were homog-
enized by remelting them three times. The as-cast alloy had a
bcc structure, i.e., was in the -phase. It was transformed
into the tetragonal -phase by isothermal annealing at 973 K
in vacuum for 100 h. The obtained piece of the -phase was
next powdered with a pestle in an agate mortar into particles
of 90 m average diameter. A mass of 5 g of such powder
was then ball-milled in argon in a Fritsch P0 mill with a
hardened steel vial and a hardened steel ball having a diam-
eter of 5 cm and a mass of 500 g. The vial’s cover was
homemade to permit an easy filling of the vial with argon.
The argon was introduced into the vial every 100 min of
milling. The mill was working with a vibration amplitude of
AM= 2.5 for the first series of samples, and in more energetic
conditions with the maximum amplitude, AM= 3, for the
second series. The third series consists of nanocrystalline
samples of the -Fe
515
Cr
485
alloy and it was ball-milled
with the maximum amplitude AM=3. The nanocrystalline
-FeCr alloy needed to obtain the starting nanocrystalline
-FeCr phase was prepared in a Fritsch P6 planetary mill at
a disk rotation of 520 rpm from elemental powders of iron
99.0+ % purity, grain size 60 m and chromium 99.0+%
purity, grain size 45 m. The starting mass of each powder
was 25 g and the powder-to-ball weight was 1:20. The pow-
der was sealed in the vial under argon atmosphere. The total
milling time of 16 h was interrupted every 30 min for
10 min. The content of oxygen for this sample was
1.0 at. %. The nanocrystalline bcc alloy was next annealed
in vacuum at 973 K for 6 h to transform it into the -phase.
For all three series, the transformation of the -into the
-phase was verified at room temperature RT by both x-ray
diffraction XRD as well as by Mössbauer spectroscopy.
The nanocrystalline -FeCr sample was then subjected to the
same milling procedure as the previous two samples.
A microprobe analysis was used to determine the alloy
compositions. The oxygen contents after milling and in the
PHYSICAL REVIEW B 73, 184123 2006
1098-0121/2006/7318/1841237 ©2006 The American Physical Society 184123-1