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.