1SAYANTANI
SANTRA, 2 KASTURI SALA, 3 RAHUL MITRA

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

 

Abstract

         Nb-silicide based
alloys exhibit prevalent elevated temperature strength as compare to 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 through microstructural
control. The constitutional phases of the arc-melted Nb-Si-Ti-Mo based alloy
are Nbss and Nb5Si3 and the phases are affirmed 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.

 

Keywords:
Microstructure,
XRD, SEM, Oxidation

1. Introduction

The
jet engines e?ciency
firmly relies on the most extreme temperature in the engines i.e., the inlet
temperature of the high-pressure turbine 1. As the hot-end
segments of gas turbine engines, nickel based super alloys  have been near its most extreme temperature
constrain (~1100?C) which has come to or surpassed 85% of its softening  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 alloys demonstrate extraordinary potential to defeat the
working temperature obstruction of Ni superalloys and to enhance the
proficiency of jet engines 2,3. Numerous materials analysts have been pulled
in by Nb silicide combinations because of their high liquefying point,
relatively lower density and great high-temperature strength. Nb-Si system
ultra-high temperature intermetallics are extremely encouraging for supplanting
Ni based superalloys in the scope of 1100~1400 ? application. 4. Nb silicide in situ
composites provides 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 ambient temperature toughness and hard-brittle intermetallic of  Nb5Si3 (and/or Nb3Si)
is able to provide elevated temperature strength. Be that as it may, because of
the lacking harmony between elevated-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). As of late,
researchers have concentrated on mostly developing the ternary Nb-Ti-Si system
based alloys, which are contemplated of having great mix of properties. In any
case, these are still extremely 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, while
inconvenient e?ect
on the oxidation resistance has been observed 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
experiences accelerated pest disintegration in the temperature scope of 700? C
to 1000? C, producing Nb2O5. 8. Nb5Si3
has performed complete disintegration on exposure at 1000?C for 1 to 3 hours
7.To ameliorate the oxidation resistance in case of the binary Nb-Si alloys,
research has adopted the addition of various alloying components, for example,
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 samples were
examined with the help of scanning electron microscope (SEM) and elemental
analysis was performed by energy dispersive spectroscopy (EDS). Arc-melted
sample was exposed at 1100 °C for 12hrs after polishing. X-ray
diffraction (XRD) was carried out to portray the constitutional phases which
were present  in the oxides scale and
afterward EDS was completed 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) depict the typical SEM (BSE) images of the
hypoeutectic Nb-Ti-Si-Mo alloy. The microstructure of Nb-Ti-Si alloy comprises eutectic
mixture of Nbss and Nb5Si3 and intermetallic dendritic
phase Nb5Si3.Nb5Si3 is the primary
phase in the hypoeutectic alloy.

3.2. Oxidation behavior

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

 

3.3. Scale morphology

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

 

Figure 2:
XRD profile of the oxide scale produced over 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 oxide scale top surface created over 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 produced over 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 1449?C, where the softening point of Nb2O5
is approximately 1550?C. 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.

 

Addition
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 increasing 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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