[1] 3rd year
B.TECH student of Metallurgical and Materials Engineering Department, NIT
Durgapur,[2]  PhD Research
Scholar at IIT Kharagpur , [3]Professor and HOD of Metallurgical and
Materials Engineering Department ,IIT Kharagpur                                               

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Nb-silicide based
alloys exhibit superior high temperature strength than the commercial nickel
based super alloys.  Attempt has been
made to understand the nature of the eutectic and eutectoid reactions in
Nb-rich portion of the Nb-Si binary system to improve the low-temperature
ductility by microstructural control. The constitutional phases of the
arc-melted Nb-Si-Ti-Mo based alloy are Nbss and Nb5Si3
and the phases are confirmed by scanning electron microscopy (SEM) and X-ray
diffraction (XRD).The Nb5Si3
network of dendrites of hypereutectic composition may act as strong barrier to
oxidation resistance at high temperatures. The top surface has been
characterized by SEM and XRD after oxidation, which was exposed at 1100 °C in
air for 12 hrs. Addition of Mo and Ti enhance oxidation resistance.

XRD, SEM, Oxidation

1. Introduction

jet engine e?ciency strongly depends upon the maximum
temperature in the engines, i.e., the inlet temperature of the high-pressure
turbine [1]. As the hot-end components of gas turbine engines, nickel based
super alloys have been very close to its maximum temperature limit (~11000C)
which has reached or exceeded 85% of its melting point .So the improvement of
the new materials was required urgently for higher temperature structural
constituents of gas turbine engines. It has been shown from recent research that
Nb-Si-based alloys show great potential to overcome the operating temperature
barrier of Ni superalloys and to improve the efficiency of jet engines [2,3].
Many materials researchers have been attracted by Nb silicide alloys due to their
high melting point, comparatively lower density and extremely good high-temperature
strength. Nb-Si system ultra-high temperature intermetallics are very promising
for replacing Ni based superalloys in the range of 1100~1400 ?
application [4]. Nb silicide in situ composites can provide increased
temperature ability and reduced density. Nb-Si-based alloys usually consist of ductile
Nb solid solutions (Nbss) and sti?ening
Nb/Si silicides. In case of these composites, the ductile phase of Nb solid
solution (Nbss) can provide room temperature toughness and high
temperature strength can be provided by hard-brittle intermetallic of  Nb5Si3 (and/or Nb3Si).
However, due to the insufficient balance between high-temperature strength and low-temperature
damage tolerance, it is still one of the major issue for practical purpose. In
case of Nb-Si based alloys, a promising method for improving the mechanical
properties is microstructure control. It involves two types of phase reactions:
eutectic solidification (Eq.1) which is followed by a eutectoid composition
reaction (Eq. 2)

 L ? Nbss + Nb3Si                                                                                                                (1)

Nbss + Nb5Si3                                                                                                          (2)

 The primary Nbss dendrite phase and
the Nb3Si phase are produced by solidification (Eq. 1). Recently,
researchers have focused on mostly developing the ternary Nb-Ti-Si system based
alloys, which are considered of having good combination of properties. However,
these are still very prone to oxidation at high temperatures. Alloying is one
of the most advantageous technique for optimization of integrated properties of
the alloys. For investigation the alloying e?ect
on the microstructure, phase formation and oxidation behavior of the alloys, alloying
Nb-Ti-Si based system with elements such as Cr, Mo, Ge and Sn, etc. has been
taken under consideration [5-6]. It has been reported that the appropriate
content ranges additions of these elements can be advantageous for oxidation
resistance of the Nb-Ti-Si based alloys. Alloying addition of Mo in the alloys helps
to straighten the materials by solid solution hardening, whereas detrimental e?ect
on the oxidation resistance has been observed because of the formation of
porous scale and the evaporation of MoO3. In case of Nb-based or
Nb-Si–based alloys, one of the major disadvantages is their poor oxidation
resistance at high temperature [7].Nb5Si3 undergoes
accelerated pest disintegration in the temperature range of 7000 C
to 10000 C, forming Nb2O5 [8]. Complete
disintegration of the Nb5Si3 has taken place on exposure
at 10000C for 1 to 3 hours [7].For the improvement of oxidation
resistance in case of the binary Nb-Si alloys, research has adopted the
addition of different alloying elements such as Ti, Al, and Cr [9-11]. Rapid
oxidation behavior is experienced by Arc-melted specimens having large number
of micro-cracks and then they fully get transformed into powder after 3 hrs
exposure in the air at 1023K. Grain boundary and pores can enhance oxidation
reaction rate [12].

2. Experimental procedure                                                                        

                Nb silicide based alloys were
prepared by adding 20 wt% Ti and 5 wt% Mo. Arc melting was carried out under
argon atmosphere and then samples were cut by electrical discharge machine (EDM).
The specimens were polished to mirror finish and then cleaned in acetone and
alcohol consequently prior to observation. Microstructures of specimens were
examined by scanning electron microscope (SEM) and elemental analysis was done
by energy dispersive spectroscopy (EDS). Arc-melted sample was exposed at 1100
°C for 12hrs after polishing. X-ray diffraction (XRD) was performed to
characterize the constitutional phases which were present in the oxides scale
and then EDS was carried out for elemental analysis.

3. Results and Discussion

3.1. Microstructure

Typical SEM (BSE) images of the hypoeutectic Nb-Ti-Si-Mo alloy at (a) lower and
(b) higher magnifications.


Nb-Ti-Si-Mo alloy with Si concentration less than that at the eutectic point is
hypoeutectic. Figures 1(a) and (b) show the typical SEM (BSE) images of the
hypoeutectic Nb-Ti-Si-Mo alloy. The microstructure of Nb-Ti-Si alloy comprises
intermetallic dendritic phase Nb5Si3 and eutectic mixture
of Nbss and Nb5Si3. The primary phase in the hypoeutectic
alloy is Nb5Si3.

3.2. Oxidation behavior

       The isothermal oxidation behavior of Nb- Silicide
has been evaluated at 11000C and the characteristics of the oxide
scales are discussed.


3.3. Scale morphology

                Figure (2) depicts the XRD pattern
obtained  from  oxide  scale
of  the investigated alloy showing the peaks
representing Nb2O5, TiO2 and SiO2.


Figure 2:
XRD profile of the oxide formed on the Nb-Si-Ti-Mo based alloy after exposure
at 1100?C for 12 hrs


the given temperature, for the corresponding alloy, the top surfaces and cross
sections of the oxide scales correspond to higher mass gain or oxidation and it
is usually extremely rough with discontinuities and most  probably produced by spallation.

3(a) at lower and (b) at higher magnifications show the SEM images delineating the
top surface of the oxide scale produced on the hypoeutectic alloy because of exposure
at 11000C for 12 hrs. No trace of Mo could be found in the oxide

Figure 3:
SEM images of oxide scale formed on the Nb-Si-Ti-Mo based alloy after exposure
at 1100?C for 12 hrs



Nb2O5 and SiO2 produce a eutectic mixture of
melting point 14490C, where the melting point of Nb2O5
is approximately 15500C. Even if the temperature of  isothermal oxidation experiment is lower in
this study, it is likely that the di?usivity
of oxygen would be higher in the eutectic mixture.

of Ti and Mo enhance oxidation resistance. Ti improves the oxidation resistance
by the formation of protective layer over the surface and Mo helps in oxidation
resistance by increasing the sinterability of oxide scale surface. Voids are
appeared as defects.


4. Conclusions

                Arc- melted specimen has been
observed under SEM and found to be comprised of Nb5Si3
and Nbss phases. The hypoeutectic Nb Silicide based alloy, prepared by arc melting,
is subjected to isothermal temperature at 11000C for 12hrs. Nb2O5,
SiO2 and TiO2 phases have been found from XRD analysis
and these protect the sample from further oxidation.



The author is
very much grateful to Prof. Rahul Mitra, Kasturi Sala (Research Scholar) and
technicians of Metallurgical and Materials Engg. Department,CRF at IIT



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