[1]SAYANTANI (~1100?C) which has come to or

                                                                                                                                                                         

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                                               

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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.

 

Keywords:
Microstructure,
XRD, SEM, Oxidation

1. Introduction

The
jet engine 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 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 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 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 1449?C, wherethe melting 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.

 

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 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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