development of oxidation resistant high temperature...
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102 Special Issue | October 2014
BARC NEWSLETTERFounder’s DayDEVELOPMENT OF OXIDATION RESISTANT HIGH
TEMPERATURE MO BASED ALLOY
Bhaskar Paul and R.C. HubliMaterials Processing Disivision
Abstract
Refractory metal intermetallic composites (RMICs) are being pursued as high temperature material beyond nickel
based alloys. The results on in-situ synthesis of Mo based RMIC and its characterization have been presented in the
present article. Silicothermic co-reduction and reaction hot pressing techniques were explored to prepare Mo-16Cr-
4Si (wt.%) alloy. The multiphase alloy was consisted of Mo3Si and discontinuous (Mo, Cr) (ss) phase with volume
percentage of 28%. The synthesized alloys were characterized with respect to composition, phases, microstructure,
hardness and their oxidation behaviour. The composite shows an excellent balance of low temperature mechanical
properties with promising environmental resistance at temperatures above 1000 oC.
Shri Bhaskar Paul is the recipient of the DAE Young Engineer Award for the year 2012
Introduction
Design of high temperature structural material
having favorable properties such as high temperature
oxidation resistance, strength, creep resistance on the
one hand and room temperature fracture toughness
on the other is a challenging task in materials
science. Maximum operating temperature capability
of superalloys has risen significantly, but eventually
it faces melting point limitation of major alloying
element e.g. Co, Ni. The next choice is the refractory
metals and alloys, because of their high melting
points and high temperature strength. Amongst
the various refractory metal alloys, molybdenum
based alloys are considered as most attractive and
promising due to their superior high temperature
properties such as excellent creep and tensile strength
at elevated temperature, adequate compatibility with
molten metals such as Pb, Pb-Bi eutectic etc. and
exceptionally high melting temperature. The major
barrier to the use of molybdenum based alloys for
high-temperature applications is their catastrophic
behaviour under oxidizing environments. In contrast,
molybdenum silicides have excellent high-temperature
oxidation resistance with high melting point. However
silicides in monolithic form have inadequate damage
tolerance and extremely low fracture toughness and
the suitability for the practical applications of these
materials as structural components, thus are hindered
by these drawbacks. It has been reported in some
literature that the fracture toughness of silicides can
be improved by incorporating a ductile Mo phase
i.e., ductile phase toughening or refractory metal-
intermetallic composites (RMICs). It is known from
the binary phase diagram of Mo-Si system that below
9.0 wt.% of Si, the microstructure consists of Mo and
Mo3Si.The oxidation resistance of Mo3Si has been
found to be enhanced by the addition of Cr due to the
formation of thermally stable impervious oxide layer of
Cr2O3. The multiphase approach has led to the study
of the systems such as Mo-Cr-Si provide a high level of
freedom in selecting compositions of the constituent
phases in order to obtain a more favorable balance
of high temperature strength, creep, good oxidation
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CONTENTS
Special Issue | October 2014 103
BARC NEWSLETTERFounder’s Dayresistance and at the same time damage tolerance
ability particularly at lower temperatures.
The primary objective of this study to investigate
the effectiveness of the Mo-Cr-Si alloy system as an
oxidation resistant alloy.
Preparation of the alloys
The Mo-16Cr-4Si was prepared by two methods:
Hot pressing
Elemental powders of Mo, Cr and Si with a purity
of 99.9% and an average particle size of 5, 8 and 6
microns, respectively, were thoroughly mixed, in the
desired composition ratios of Mo-16Cr-4Si (wt.%)
alloy using a turbo-mixer unit. The mixed powder was
uniaxially pressed under 200MPa into pellets with
dimensions of Φ25mm×20 mm. Green densities of
the samples were measured using the geometrical
dimensions. The compacts were placed into a
graphite die and then hot-pressed under vacuum at
1600oC for 3 to 5 h by applying a pressure of 10MPa.
The hot pressed alloy pellets were grinded from all
sides to remove the graphite (C) layer using standard
metallographic grinding techniques. Sintered
densities were determined through the immersion
method based on the Archimedean principle using
alcohol as the liquid medium. The polished sintered
samples were etched using a mixed solution of 5 ml
HNO3, 10 ml HF, 15 ml H2SO4 and 50 ml H2O, and
the microstructure of the specimens
were observed under SEM. X-ray
diffraction pattern was recorded for
characterizing the phases evolved
after hot pressing.
Co-Reduction method
An alternative self sustaining
synthesis route for synthesis of high
temperature material is “Co-reduction
synthesis route” In this process, metal oxides are co-
reduced simultaneously by a reductant which could
be anyone or a combination of Al, Si, Ca, B, Mg etc.
and the reactions which when triggered goes to
completion because of their own exothermic heat.
This process has many distinct advantages over other
melting processes such as relatively high proportion
of metallic products, low processing cost, fast process
rate, high energy and time efficiency, may or may
not require external heating from high-temperature
furnace, flexibility of batch size etc.
In the present study, attempts have been made to
prepare Mo-16Cr-4Si (wt.%) alloy by co-reduction
smelting technique using Si as reductant.
Characterizations and Property evaluation:
Microstructural characterization
Fig-1a and Fig-1b show the microstructures of the co-
reduced and hot pressed Mo-16Cr-4Si alloy, showing
two different contrast regions, namely white and black.
The quantitative microanalyses in these regions were
carried out by EDS for evaluating the compositional
variation. From the composition of the two phases, it
is seen that the first phase (phase-A) appearing light
(white) is made up of a solid solution phase, basically
of Mo and Cr, containing small amount of Si. The
second phase (phase-B, appearing dark) is made up
of (Mo, Cr)3Si intermetallic phase, which has been
confirmed by the XRD analysis.
Fig.1: BSE image of Mo-16Cr-4Si prepared by (a) hot pressing (b) silicothermic co-reduction
104 Special Issue | October 2014
BARC NEWSLETTERFounder’s DayOxidation Studies
Specimens were cut from the prepared alloy button
followed by polishing and ultrasonically cleaning in
acetone. For isothermal oxidation studies, polished
sintered samples of approximate size of 10 × 5 × 5 mm
were introduced into the furnace in an alumina crucible
when the furnace temperature reached the set value.
For isothermal experiments, samples were oxidized
for different time intervals up to 50 h. Each sample
was carefully weighed before and after exposure to
determine the weight changes during the oxidation.
Phases present on the surface of the oxidized samples
were characterized by XRD. The morphology and
nature of oxide layer were investigated by observing
the surface in SEM.
Fig. 2a shows the data of weight change per unit area
with time, obtained during isothermal oxidation at
different temperatures. Initially the rate of mass loss
is high, because of the volatilization of MoO3 at all
the isothermal conditions. Later on, the oxidation
rates decrease substantially due to the formation of
the protective mixed SiO2 and Cr2O3 layer over the alloy
surface. Formation of the cristobalite phase of SiO2
(matched with JCPDS 820403) and Cr2O3 (matched
with JCPDS 841616) have been confirmed by XRD
analysis of surface oxide layer of the alloy after 50 h,
shown in the inset of Fig. 2b.
Fig. 2b shows the SEM images of the oxidized surfaces
of the alloy after oxidation at 1000oC for 5 and 50 h.
The morphologies of the oxide scales are particulate
Fig-2 (a) weight change per unit area with time at different temperatures (b) SEM images of the oxidized surfaces of the alloy after oxidation at 1000oC for 5 and 50 h. Inset shows the XRD of the oxidized surfaces of the alloy after oxidation at 1000oC for 50h.
Fig-3: (a) Vickers indentation profiles at 3 N load (b) Vickers indentation profiles at 3 N load at phase A showing pile up (c) the crack arresting capability of phase A
Special Issue | October 2014 105
BARC NEWSLETTERFounder’s Dayin nature. The oxidized surfaces are free of cracks;
however, the porosities are detected until the initial 5h.
With increase in time, the coagulation/agglomeration
of oxide particles tries to close the pores, and makes
the oxide layer more protective.
Mechanical Testing
The room temperature flexure strength and the
fracture toughness of the alloy determined using
standard equations were found to be 615 ± 15 MPa
and 10.7 ± 0.5 MPa.m1/2, respectively. The average
values of hardness are 6800 ± 15 MPa and 11300 ±
10 MPa for phase A and B, respectively. The profiles of
a 3 N Vickers indentation along the corners and the
faces at both phases are shown in Fig-3a. No pile-up is
observed around indentation at phase B while a clear
≥500 nm high pile-up is present along the faces of
indentation at phase A (Fig-3b). The pile up is due to
the plastic flow of the material, showing the evidence
of ductility of phase A. The fracture toughness
increases due to the crack arresting capability of phase
A, which is clearly seen in Fig-3c. Crack intercepts by
the primary Mo phase, hinder the catastrophic fracture
through the formation of unbroken ductile-particle
ligaments in the crack wake. The fracture toughness of
the system could be enhanced by the presence of the
ductile phase; it is either by crack blunting, branching,
deflection or combinations of these. Crack deflection
or branching alters the mode of loading (I to mode II)
so crack propagation is hindered resulting in increase
in fracture toughness. Due to the crack hindrance
(Fig. 3) and plastic deformation (Fig. 3) of the Mo-
phase, together with crack deflection and interfacial
de-bonding the toughness of the alloy is enhanced.
Conclusion
On the basis of the present experimental results the
following conclusions can be drawn:
1. The technical feasibility of preparation of Mo based
oxidation resistant alloy of nominal composition
Mo-16Cr-4Si (wt.%) was demonstrated through
systematic investigation by direct silicothermy and
hot pressing.
2. The alloy is included by a ductile refractory solid
solution phase of Mo and Cr containing 0.9-1.9
wt% Si and an intermetallic matrix of (Mo,Cr)3Si.
3. The multiphase alloy is consisted of Mo3Si and
discontinuous (Mo, Cr) (ss) phase with volume
percentage of 28%.
4. Due to the crack hindrance and plastic deformation
of the Mo-phase, together with crack deflection
and interfacial de-bonding the toughness of the
alloy is enhanced.
5. The alloy exhibited better oxidation resistant as
compared to single phase Mo alloy.