Isothermal section at 1673 K of the Mo–Ru–Si

Intermetallics 11 (2003) 1223–1228
www.elsevier.com/locate/intermet
Isothermal section at 1673 K of the Mo–Ru–Si diagram and
crystallographic structures of ternary phases
A. Littnera, M. Francoisa, B. Malamana, J. Steinmetza, M. Vilasia,*, E. Elkaimb
a
Laboratoire de Chimie du Solide Mine´ral, UMR CNRS 7555, Universite´ Henri Poincare´, Boulevard des Aiguillettes BP 239,
F-54506 Vandoeuvre-le`s-Nancy Cedex, France
b
Laboratoire LURE, Bat 209D, Centre Universitaire Paris Sud, BP 34, 91898 Orsay Cedex, France
Abstract
Eﬀorts to improve the high temperature behavior of MoSi2 in oxidizing environments led to the investigation of the Mo–Ru–Si
phase diagram. The isothermal section at 1673 K was determined by X-ray diﬀraction, optical and scanning electron microscopies
and EPMA. Five new silicides were identiﬁed and their crystallographic structure was characterized using conventional and synchrotron X-ray as well as neutron powder diﬀraction. Mo15Ru35Si50, denoted a-phase, is of FeSi-type structure, space group P213,
a=4.7535 (5) A˚, Dx=7.90 g. cm 3, Bragg R=7.13. Mo60Ru30Si10 is the ordered extension of the Mo70Ru30 s-phase with space
group P42/mnm, a=9.45940(8) A˚, c=4.94273(5) A˚, Dx=6.14 g. cm 3, Bragg R=5.75.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: A. Ternary alloy systems; A. Silicides, various; B. Crystallography; B. Thermodynamic and thermochemical properties
1. Introduction
2. Experimental and results
Molybdenum disilicide (MoSi2) is widely used as a
protective coating on heating elements for air furnaces
although it undergoes irreversible damage in an oxidizing atmosphere at temperatures below 600 C. This
deleterious eﬀect, well known as ‘‘the pest phenomenon’’, was discovered by Fitzer in 1955 [1].
In the course of our studies related to the corrosion of
metallic and intermetallic structural materials, we have
focused our work on the improvement of the oxidation
resistance of MoSi2. Therefore, we chose to combine the
disilicide with ruthenium and thus, in order to optimise
the alloy composition, we studied the corresponding
phase diagrams.
In this paper, the results related to the study of the
isothermal section of Mo-Ru-Si system at 1673 K are
reported. Five new ternary intermetallic compounds
were clearly identiﬁed and their crystallographic structure was determined by powder diﬀraction.
2.1. Mo–Ru–Si diagram: determination of the
isothermal section at 1673 K
* Corresponding author. Tel.: +33-3-8368-4652; fax: +33-3-83684611.
E-mail address: [email protected] (M. Vilasi).
0966-9795/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0966-9795(03)00162-6
2.1.1. Syntheses and control of powder materials
Starting materials were the elemental components in
the form of powders with nominal purities > 99.8%.
The molydenum and silicon powders were 325 mesh and
delivered by the CERAC company. Ruthenium was
spongy material displayed by the Johnson Mattey company. Several compositions were prepared by mixing the
pure elements in an agate mortar. The as-obtained mixtures were introduced in silica tubes which were evacuated, partially ﬁlled with Ar (2104 Pa), then sealed.
After annealing for a maximum of 2 days at the adequate temperature, powders were quenched in water,
then rapidly controlled by XRD using an Enraf-Nonius
Guinier camera or a Philips X’Pert Pro diﬀractometer
(Bragg–Brentano –2 geometry; l=Cu Ka1). A part of
each powder was embedded in an epoxy resin, polished
and microanalysed by electron probe (SX 50 Cameca),
using the PAP correction program [2].The results related to single phase are reported on the schematic diagram represented in Fig. 1. It is evidence of the
extension of the stability domains.
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A. Littner et al. / Intermetallics 11 (2003) 1223–1228
Fig. 1. Report of single phase compositions related to Mo–Ru–Si alloys synthesized from powders annealed at 1673 K.
2.1.2. Syntheses of massive materials
Massive materials were necessary in order to
elaborate:
the end-members alloys of the diﬀusion couples
studied at 1673 K,
cylindrical coupons used during the oxidation
tests.
2.1.2.1. HF melting of powder mixtures. Ingots of
desired intermetallic compounds were prepared in HF
furnace (HFF) under argon atmosphere. In a ﬁrst step,
a mixture of metallic elements with adequate composition was prepared as described in the above 2.1.1 subsection, and ﬂashed at high temperature (1473 K) for a
few minutes. In a second step, the as-obtained powder
was melted in the induction device. Finally, the ingots of
studied alloys were annealed at 1673 K for 48 h in an
argon atmosphere (2104 Pa).
2.1.2.2. Powder sintering. In the case of molydenum
silicides, melting was not achieved in the HF furnace
and thus, the preparation of bulk material was carried
out via a sintering route. High-pressure sintering was
performed with the use of a high-temperature furnace
(HTFP) equipped with a 10-ton uniaxial press. The
blended metal powders were introduced in a graphite
die, coated with boron nitride (BN) applied by spraying.
BN works as a diﬀusion barrier between graphite and
metallic powders and thus, avoids metallic carbide formation. During the sintering step, the sample was
slowly heated under a dynamic primary vacuum up to
1073 K. Then, argon was introduced in the furnace (105
Pa) and a 15 MPa pressure was applied to the metallic
sample. Then, the temperature was increased to 1673 K
and this temperature was maintained for 4 h, resulting
in the sintering of the sample.
The main information related to bulky alloys is
reported in Table 1. In most cases, the presence of
minor phases is due to losses of matter by volatilization
during the synthesis process.
2.1.3. Elaboration of diﬀusion couples
The end-members of the diﬀusion couples were
polished with a diamond paste to the 1 mm grid and
then pressed together 24 h in the HTFP under 1.5 MPa
at 1673 K. The results of this solid state diﬀusion
experiment are illustrated by Fig. 2(a,b). One can see
that the homogeneity domain of most evidenced intermetallic silicides are characterized by a constant silicon
content.
Moreover, the metallographic characterizations of the
oxidized compounds comﬁrm the previous results.
Indeed, due to the oxidation process, the compound
composition changes in the superﬁcial part of the sample and this leads to the formation of new phases by
solid state diﬀusion. The composition of the latter is
ruled by phase equilibria. As an example, a metallographic section of a particular oxized compound is
shown in Fig. 3.
2.1.4. Results
The outstanding results of these experiments are
summarized in Fig. 4 where the limits of the various
Mo–Ru–Si solid solutions are reported. A series of 18
three-phase domains was clearly identiﬁed. Nevertheless, as indicated by the grey areas on Fig. 4, it
remains composition ranges where equilibria are to be
A. Littner et al. / Intermetallics 11 (2003) 1223–1228
1225
Fig. 2. (a) Partial representation of diﬀusion couple between Ru and MoSi2—annealing time: 24 h; annealing temperature: 1673 K. (b) Diﬀusion
couple between Ru and Mo3Si—annealing time: 24 h; annealing temperature: 1673 K.
conﬁrmed. The uncertainties are due on the one hand,
to the presence of numerous compounds with a narrow
homogeneity domain and on the other hand, to the
ambiguity related to the existence of Ru5Si3. Indeed, in
contradiction to Weitzer et al. [3], Okamoto does not
indicate the presence of the Ru5Si3 compound in his last
assessment of the Ru–Si system [4].
Moreover, this study has also revealed the existence of
new ternary silicides. Among them, ﬁve compounds
were clearly identiﬁed. They are respectively denoted a,
b, g, e and s in Fig. 4, and studied with regard to their
crystallographic structure.
2.2. Crystallographic characterization
The crystallographic study was performed using standard and synchrotron X-ray as well as neutron powder
diﬀractions. Concerning powder neutron diﬀraction, the
measurements were carried out at 320 K on the diffractometer D1B (l=2.525 A˚) at ILL Grenoble [5]. For
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A. Littner et al. / Intermetallics 11 (2003) 1223–1228
Table 1
Elaboration of massive intermetallic alloys: studied composition, synthesis technique and ﬁnal products
Prepared composition
Elaboration technique
Obtained alloy and evidenced equilibria
Mo33.3Si-66.7
Mo62.5Si37.5
Mo75Si25
Mo33Si-66Ru1
Mo15Ru35Si50
Mo26Ru47Si27
s (Mo60Ru30Si10)
d (Mo41Ru41Si18)
e (Mo15Ru50Si35)
Mo32.5Ru30Si37..5
HTFP
HTFP
HTFP
HTFP
HFF
HFF
HFF
HFF
HFF
HFF
Mo40Ru32Si28
HFF
Porous ingot of MoSi2 and presence of Mo5Si3 as minor phase
Porous ingot of Mo5Si3 and presence of Mo3Si as minor phase
Porous ingot of Mo3Si and presence of Mo as minor phase
Two-phase alloy: pure MoSi2 with (Mo,Ru)5Si3 type-phase where Ru at.%=7
Main compound: a( Mo15Ru35Si50); minor compound: RuSi at grain boundaries
Main compound: b(Mo26Ru47Si27); minor compound: d(Mo41Ru41Si18) at grain boundaries
Monophased alloy
Monophased alloy
Monophased alloy
Main compound: (Mo,Ru)5Si3 type-phase where Ru at.%=30; minor compound:
b (Mo26Ru47Si27) at grain boundaries
Three phase alloy: s(Mo55Ru28Si17)–b(Mo26Ru47Si27)–Mo37Ru27Si36
2.2.1. Results of the structural study of and phases
2.2.1.1. Characterization of a(Mo15Ru35Si50)-phase.
The indexation of X-ray pattern showed that the a-phase
is an isotype of the cubic FeSi compound. The coordinates of the metallic and semi-metallic sites were reﬁned
thanks to Rietveld analyses which use FULLPROF
program [9]. The results of this accurate evaluation
(Bragg R factor=7.13) as well as the interatomic distances are reported in Tables 2–4, respectively.
Fig. 3. Metallographic section of the (Mo,Ru)5Si3 and b-Mo26
Ru47Si27 two-phase alloy after oxidation test performed at 1623 K—
magniﬁcation of the superﬁcial diﬀusion layer ( 2143).
experiments involving the synchrotron radiation, the
WDIF4C powder diﬀractometer implanted on the
DW22 beamline of LURE was used [6].
The main features related to this preliminary structural study of a, b, g, e and s intermetallic silicides are
reported in Table 2. The results are evidence that b, g, e
are probably new compounds, their complete structural
study being in progress, whereas a and s are clearly
indentiﬁed as isotypes of FeSi [7] and FeCr s-phase [8],
respectively.
2.2.1.2. Characterization of s(Mo60Ru30Si10)-phase.
The observed synchrotron diagram was completely
indexed in the tetragonal cell with the lattice parameters
and space group given in Table 2. Thus, the compound
appeared to be an isotype of the FeCr s-phase [8].
Then, the crystallographic parameters of the latter typestructure were taken into account as starting model for
Rietveld analyses. Finally, the site occupancies were
reﬁned owing to neutron diﬀraction data for which
scattering length of Ru and Mo (bRu=0.73410 12 cm,
Table 3
Atomic coordinates and site occupancy in the a(Mo15Ru35Si50) phase
(P 213)
Site
X
Occupancy
M1
4a
0.131(1)
Ru: 70%
Mo: 30%
M2
4a
0.837(2)
Si: 100%
Table 2
Experimental and results of the structural study of the intermetallic compounds identiﬁed in the Mo–Ru–Si system at 1673 Ka
Phases
Ex. conditions
a(Mo15Ru35Si50)
Standard X-Ray,
D.S. mode. (l=0.71073)
System or space group P213
Lattice param (A˚).
4.7535 (5)
Isotype
FeSi
a
b(Mo26Ru47Si27)
s(Mo60Ru30Si10)
d(Mo41Ru41Si18)
Synchr. (l=0.6923 A˚)+
Neutron (l=2.525 A˚)
P4/mmm
9.2164(1), 2.8872(1)
New structure
Synchr.(l=0.9991 A˚)+
Neutron (l=2.525 A˚)
P42/mnm
9.45940(8), 4.94273(5)
FeCr
Standard X-ray,
Standard X-ray,
D.S. mode. (l=0.71073 A˚) D.S. mode. (l=0.71073 A˚)
Cubic
Ortho
6.647(1)
9.760(2), 6.303(2), 3.174(2)
New structure
New structure
D.S.: Debye–Scherrer mode; Synchr.: synchrotron X-ray diﬀraction.
e(Mo15Ru50Si35)
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A. Littner et al. / Intermetallics 11 (2003) 1223–1228
Fig. 4. Experimental determination of the Isothermal section at 1673 K of the Mo–Ru–Si phase diagram—binary sections are issued from Ref. [10].
Grey areas remain uncertain: further investigations are needed.
bMo=0.66110 12 cm) oﬀer a better contrast than the
X-ray scattering factors and allow the diﬀerenciation of
the two elements (Bragg R factor=5.75). The ﬁnal
results of atomic position, metallic atom distribution
and interatomic distances are detailed in Tables 5 and 6.
They reveal that:
Table 4
Selected interatomic distances (A˚) in a (Mo15Ru35Si50) phase (P213)
M1
M2
3M2
3M2
6M1
2.419(10)
2.439(10)
2.713(10)
2.912(7)
M2
M1
3M1
3M1
6M2
2.419(10)
2.439(10)
2.713(10)
2.954(12)
Table 5
Atomic coordinates and site occupancy in the s(Mo60Ru30Si10) phase
(P42/mnm)
Site
x
y
z
Occupancy
M1
2a
0.0
0.0
0.0
Ru: 70%
Si: 30%
M2
4f
0.39970(11)
0.39970(11)
0.0
Mo: 100%
M3
8i
0.46490(11)
0.13081(11)
0.0
Ru: 46%
Mo: 54%
M4
8i
0.74210(13)
0.06441(15)
0.0
Ru: 68%
Si: 32%
M5
8i
0.18368(7)
0.18368(7)
0.25085(34)
Mo: 100%
the compositions deduced from X-Ray
(Mo65.1Ru24.3Si10.6) and neutron (Mo54.3Ru25.6Si20.1) diﬀraction studies are in the accuracy
of the measurements and in agreement with that
determined by EPMA analyses (Mo56Ru27Si17),
the interatomic distances are compatible with
those usually encountred in this kind of interTable 6
Selected interatomic distances (A˚) in s(Mo60Ru30Si10) phase (P42/
mnm)
M1
4 M4
4 M5
4 M2
2.515(1)
2.752(1)
2.812(1)
M2
2 M3
M2
2 M1
4 M4
4 M5
2 M5
2.617(1)
2.684(1)
2.812(1)
2.906(1)
3.059(1)
3.145(1)
M3
M2
2 M4
2 M4
2 M5
2 M5
4 M3
2.562(1)
2.617(1)
2.696(2)
2.701(1)
2.977(1)
2.980(1)
3.064(1)
M3
M4
M1
M4
2 M3
2 M3
2 M5
2 M5
2 M2
2.515(1)
2.588(2)
2.696(2)
2.701(1)
2.739(2)
2.745(2)
2.906(2)
M5
M5
M5
2 M4
2 M4
M1
2 M3
2 M3
2 M2
M2
2.463(2)
2.480(2)
2.739(1)
2.745(1)
2.752(2)
2.977(1)
2.980(1)
3.059(1)
3.145(1)
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A. Littner et al. / Intermetallics 11 (2003) 1223–1228
metallic compounds (as it is also the case in the
-phase) [10].
The characteristic structure of the s-phase can be
described as corner shearing hexagonal antriprims
centred by Mo atoms which form inﬁnite columns.
alous diﬀraction measurements at the absorption edge
of Mo. The expected results should lead to a better
description of atomic distribution on crystallographic
sites and thus, better information for choosing a sublattice model in a procedure of phase diagram calculation.
Finally, the crystallographic study of the other intermetallic silicides will be completed.
3. Concluding remarks
Isothermal section at 1673 K of the Mo-Ru-Si system
was established. Nevertheless, further investigations are
needed to get accurate informations in two particular
composition ranges centred on Mo10Ru60Si30 and
Mo50Ru20Si30, respectively.
The structural study reveals that the a-phase is
an isotype of FeSi with P213 space group and thus,
does not extend the RuSi solid solution with CsCl
type-structure. Energy calculation using the CASTEP
program will be carried out in order to ﬁnd some
justiﬁcations.
Improvement of the structural characterization of the
whole s-solid solution should be achieved by anom-
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[4] Okamoto H. J Phase Equilib 2002;23(4):388.
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[6] LURE W22 beamline. available from: http://www.lure.u-psud.fr/
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