Abstract

         Nb-silicide
based alloys exhibit prevalent high temperature strength than the commercial
nickel based superalloys. Endeavor has been made to comprehend the idea of the
eutectic and eutectoid reactions in Nb-rich segment of the Nb-Si binary system
to enhance 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 go about as powerful boundary to oxidation resistance at high
temperatures. The top surface has been portrayed by SEM and XRD after
oxidation, which was exposed at 1100 °C in air for 12 hrs. Addition of Mo and
Ti upgrade oxidation resistance.

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Keywords:
Microstructure,
XRD, SEM, Oxidation

1. Introduction

The
stream motor e?ciency
firmly relies on the most extreme temperature in the motors, i.e., the delta
temperature of the high-weight turbine 1. As the hot-end segments
of gas turbine motors, nickel based super combinations have been near its most
extreme temperature constrain (~1100?C) which has come to or surpassed 85% of
its dissolving point. So the improvement of the new materials was required
urgently for higher temperature structural constituents of gas turbine engines.
It
has been appeared from late research that Nb-Si-based amalgams demonstrate
extraordinary potential to defeat the working temperature obstruction of Ni
superalloys and to enhance the proficiency of stream motors 2,3. Numerous
materials analysts have been pulled in by Nb silicide combinations because of
their high liquefying point, relatively bring down thickness and greatly great
high-temperature quality. 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).
Be that as it may, because of the lacking harmony between high-temperature
strength and low-temperature damage tolerance, it is as yet one of the
significant issue for commonsense reason. If there should arise an occurrence
of Nb-Si based compounds, a promising strategy for enhancing the mechanical
properties is microstructure control. It includes two kinds of phase reactions:
eutectic solidification (Eq.1) which is trailed by an eutectoid composition
reaction (Eq. 2)

 L ? Nbss + Nb3Si                                                                                                               (1)

Nb3Si?
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 expansion of
Mo in the alloys rectifies the materials by strong arrangement solidifying,
while inconvenient e?ect
on the oxidation resistance has been watched as a result of the development 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

Figure
1:
Typical SEM (BSE) images of the hypoeutectic Nb-Ti-Si-Mo alloy at (a) lower and
(b) higher magnifications.

 

  The
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

At
the given temperature, for the corresponding alloy, the top surfaces and cross
sections of the oxide scales relate to higher mass pick up or oxidation and it
is normally to a great degree rough with discontinuities and most presumably
created by spallation.

Figure
3(a) at lower and (b) at higher magnifications demonstrate the SEM images
delineating  the top surface of the oxide
scale created on the hypoeutectic alloy because of exposure at 1100?C for 12
hrs. No hint of Mo could be found in the oxide scales.

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

 

The
Nb2O5 and SiO2 produce an eutectic mixture of softening point 14490C, wherethe
melting point of Nb2O5 is approximately 15500C . Regardless of whether the
temperature of isothermal oxidation explore is bring down in this examination,
it is likely that the di?usivity
of oxygen would be higher in the eutectic mixture.

 

Expansion
of Ti and Mo upgrade oxidation resistance. Ti enhances the oxidation resistance
by the arrangement of protective layer over the surface and Mo helps in
oxidation resistance by expanding the sinterability of oxide scale surface.
Voids are showed up as imperfections.

 

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.

 

Acknowledgements

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
Kharagpur.

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