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The Role of Refractory Metals in the Development of Structural

Intermetallic Compounds

R. V. Ramanujan a

a Materials Science Division, Bhabha Atomic Research Centre, Mumbai, India

To cite this Article Ramanujan, R. V.(2001) 'The Role of Refractory Metals in the Development of Structural Intermetallic

Compounds', Mineral Processing and Extractive Metallurgy Review, 22: 2, 615 — 631

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The Role of Refractory Metals

in the Development of Structural

lntermetallic Compounds

R.V. RAMANUJAN

Materials Science Division, Bhabha Atomic Research Centre,

Trombay, Mumbai - 400 085, India

The role of refractory metals in high temperature applications is discussed in the context

of structural intermetallic compounds. The use of refractory metals in the elemental form

as well as its use as alloying additions is first discussed. A catalogue of refractory metal

based structural intermetallia is then itemised. Two examples. a niobium-based struc-

tural intermetallic compound and the role of the refractory metal W in improving the

thermal stability of TiAl-based intcrmetallics are then discussed.

Keyworh: Refractory metals; lntermetallic compounds; Titanium aluminides

In one definition, the tenn refractory metal refers to the elements Mo,

W, Nb and Ta. Rhenium, which possesses a h.c.p. crystal structure

and V which is a b.c.c. crystal are often included [I]. All these metals

have a high melting point, vanadium has the lowest melting point of

2175 K and W has the highest at 3680 K. These refractory elements,

by virtue of their melting points are attractive materials for high

temperature applications, but these elements generally oxidize at

temperatures less than half their melting point. Therefore, in the

elemental form, these elements have been used either at room tem-

perature or at high temperatures in a controlled environment where

they do not oxidize appreciably. Because of these limitations of

the elemental form, considerable effort has been devoted to the use

of refractory metals either as alloying additions or in the form of


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616 R. V. RAMANUJAN

intermetallic compounds [2,3]. We consider first the use of refractory

metals in high temperatures applications, followed by their use in

alloys and intermetallic compounds.

REFRACTORY METALS AND ALLOYS

In addition to their poor oxidation resistance, refractory elements like

Mo and W are brittle at room temperatures and hence have to be processed

at high temperatures. However, because of their many advantages,

there are many high temperature applications, such as the a.lloy

Nb-1Zr used in alloy tubing for containment of liquid metals such as

lithium and cesium. Ta and Ta clad steel process equipment is used in

high temperature sulphuric acid service. Mo and W are used in solid

propellant rockets with a flame temperature of approximately 3000 K

and W is used in rockets with a flame temp of ~3800 K. In several

radiation shields and heat shields, W alloys and Ta are used. In rocket

nozzles and structures for re-entry refractory metals play a vita1 role.

In process industries, high temperature and corrosive environments

are common and hence refractory metals and alloys are extensively

used. Tantalum, for example, is used in a number of components such

as heating and cooling coils, heat exchangers, condensers, crucibles

and glass processing equipment. In the area of special equipment,

mention may be made of the use of W, Mo and Ta in heating elements,

susceptors, extrusion dies and thermocouples (W, W - Re alloys).

In the alloy form the Nb base alloy C 103 is used for rocket

components requiring moderate strength at temperatures between

1 100°C and 1370°C. Nb- l Zr has been used for nuclear applications

because of low absorption cross section for thermal neutrons,

corrosion resistance and resistance to radiation. As mentioned earlier,

it can be used for liquid metal systems operating at temperatures

around 1000°C. The alloy Nb-55 Ti is used for fasteners in aircraft.

lntermetallics

In the above discussion, we have considered high temperature uses of

refractory metals and refractory metal based alloys. Refractory metals

are also used as strengthening elements in other solvents, e.g., the use


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THE ROLE OF REFRACTORY METALS 617

of Nb as a strengthening element in Ni-base superalloys. Nb forms

the intermetallic Ni3 Nb which improves the strength of some catego-

ries of superalloys. The use of refractory metal interrnetallics as

precipitates led to the study of the feasibility of the use of monolithic

refractory metal intermetallics [4- 121. The main constraint has been

that such intermetallics are generally brittle at low temperatures and

show low fracture toughness. Multiphase intermetallic systems can be

used to overcome these problems by distributing a ductile refractory

phase in a brittle intermetallic system. Dissipation of plastic energy

by the ductile phase increases the fracture toughness while the inter-

metallic matrix shows high temperature strength and creep resistance.

An example, considered in more detail later, is the 2 phase Nb-Nb5

Si3 system which offers a balance of low temperature toughness, high

temperature strength and creep resistance in the 1000°C to 1600°C

temperature range.

EXAMPLES OF NIOBIUM BASED

INTERMETALLICS

4 has attractive superconducting properties. It is also being

considered for high temperature structural applications, in spite of its

low temperature brittleness, because of its high stability and strength

at high temperatures. As in the case of the Nb-Nb5 Si3 system, the

two phase Nb-Nb3Al system offer the possibility of increasing the

ductility. The ductile to brittle transition temperature (DBIT) in-

creases as the volume fraction of the Nb-rich phase increases, and a

DBTT of room temperature has been achieved for a Nb-16 at % A1

alloy. The Nb phase toughens the alloy by crack binding, plastic

stretching and interfacial bonding.

Nb-A1 alloys with A1 contents in the range 25 to 33 at % contain

the a phase Nb,Al, which is also being considered for high tem-

perature applications.

The Nb-Nb3Al system can be thought of as discontinuously

reinforced in-siru composites; and Nb3A1 has been used in inter-

metallic matrix composites which are reinforced by continuous

ceramic fibres. Laminated intermetallic matrix composites have

been prepared by high rate sputtering. The addition of Ti to Nb3 Al


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618 R. V. RAMANUJAN

leads to a phase with the B2 structure, which is related to Ti3 AI

base (super cu2) intermetallics. Nb3 Si is another niobium based inter-

metallic which is a high temperature line compound with a tetragonal

crystal structure. Nb-Si in-situ composites containing thesis Nb3 Si

phase are considered for high temperature applications.

LAVESPHASE

The Laves phase with the A B2 composition in the binary case form a

large group of intermetallics. An example is Mo (Co, Si), which is

used in Tribaloys. Tribaloys are used in wear and corrosion resistant

applications and are Co-Mo-Cr-Si alloys containing large volume

fraction of Mo(Co, Si)*, in a coarse distribution, inside a Co-rich

matrix.

The Laves phase based refractory intermetallics Ta Fe, and Nb Fez

have been considered as strengthening precipitates in high temperature

ferritic Fe-base alloys. Another family of Laves phase intermetallics

are the Ti-Nb-(Ti, Nb) Cr2 alloys which have demonstrated good

strength and acceptable room temperature toughness. In this case

also, the intermetallic acts as a strengthening phase. Similarly, the

Nb-Cr alloy system is promising for high temperature use because

of the formation of the Nb Cr2 intermktallic. Besides such two

phase metallic/intermetallic systems, Laves phase intermetallics can

also be treated as matrices for intermetallic matrix composites with

strengthening dispersoids and fibres.

The ternary A1 containing Laves phases NblNi, All2, Ta(Ni, All2

and Ta(Fe,Al), have been considered for high temperature use

because of the high melting point, high strength and good oxidation

resistance. Two phase intermetallic systems with decreased brittleness

can also be formed e.g., Ni AI-Nb (Ni, A1)2, Ni Al-Ta(Ni, Al)z; Ni

Al-(Nb, Ta) Ni Ai and Fe A1 -Ta ~e Al.

Beryllides

The reactive and refractory metal beryllides, especially the Be-rich

phases with Ti, Zr, Hf, Nb, Ta or Mo have attractive high temperature

properties because of their low densities, high melting point, high


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THE ROLE OF REFRACTORY METALS 619

strength and oxidation resistance. Typical compositions are M Bel3

for M = Zr or Hf, M Bell for M = Ti, Nb, Ta or Mo and MBe,, for

M =Ti, Zr, Hf, Nb or Ta. As an example, the improvement in the

mechanical properties of Nb Be12 may be cited, this improvement is

achieved by combining it with other metallic or intermetallic mate-

rials, e.g., Be, Nb?, Be17 or Nb Be3. Similarly, Nb Bel2, Ti Bel2, Zr Bel3,

Nb2 Be,, or Ta2 Be17 have been suggested as the second phase in

intermetallics based on Fe Al.

Silicides

A candidate for high temperature applications is Nbs Si3 with a

melting point of 2484°C. To alleviate the brittleness, two phase Nb-

NbS Si3 have been prepared with Nb rich particles in the Nb5 Si3

matrix.

Disilicides

Mo Si2 has found extensive use as heating elements in high

temperature furnaces upto 1700°C and is being considered in both

the monolithic and composite forms as a structural material.

In composite form Mo Si2-Sic and Mo Si2-Tic have attracted

interest.

Refractory Metal Use in Ti3 Al

The intermetallic Ti3 Al has been widely studied for structural

applications at high temperatures. Nb which substitutes for Ti, is very

important with respect to mechanical behaviour. Nb increases the

ductility and Ti3 Al based alloys of engineering significance contain 10

to 30 at % Nb. Small amounts of Nb lead to the activation of more

slip systems in Ti3 A1 which has an ordered hexagonal DOl9 structure.

Larger amounts of Nb result in the formation of 0-Ti in the

disordered state, B2 in the ordered state and/or the orthorhombic 0

phase. Alloying with Nb improves all the mechanical properties with

the exception of creep resistance, and the effect increases with

increasing Nb. Other elements that improve the strength are Cr and

the refractory metals Ta and Mo. Unlike Nb, Mo increases the creep


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620 R. V. RAMANUJAN

resistance. Two phase intermetallic systems can be prepared by

alloying Ti3 Al with Si to produce the intermetallic Ti5 Si3 as a

strengthening phase. Nb can be used to improve the mechanical

properties of this two phase system. Nb, Ta and Mo also improve

the oxidation resistance. Examples of Ti3 A1 alloys which are of

engineering importance and containing refractory metals are Ti-24

Al-1 lNb, Ti-25A1-10 Nb-3V- 1 Mo, Ti-25 A1-I 7 Nb- 1 Mo and Ti-23.5

AI-24 Nb.

Refractory Metal Use in Ti At

The intermetallic two phase system TiAl + Ti, A1 has been the subject

of extensive R & D efforts in order to replace Ni based superalloys in

a wide variety of high temperature applications [13]. TiAl has been

alloyed with the refractory metals Nb, Ta, Mo and W which act as Ti

substituents. Additions of alloying elements in the range of I to 3 at %

are used to improve the mechanical properties of the TiAl + Ti3 Al

system. Among the refractory metals, V increases the ductility and

produces solid solution strengthening. Nb, Ta and W produce solid

solution strengthening and improved 'oxidation resistance but they

decrease ductility. Alloying of Ti A1 with larger amounts of Nb

can lead to further ordering of the Llo structure. Typical composi-

tions of the two phase system are Ti-48 A1-2 Mn-2Nb and Ti48AI-

2Cr-2 Nb.

Other Refractory Metal Based

lntermetallics [4]

m: Al, V has several potential applications in nuclear reactor

technology.

Al, Nb: A13 Nb is a line compound and is a candidate phase for high

temperature structural materials. A13 Nb can be used as one

of the phases in an intermetallic multiphase system by

combining Al, Nb and another more ductile phase e.g., Ni

A1 + Al, Nb. A13 Nb is also being considered as an oxidation

resistant coating for ~b-based alloys.

A13 Ta: A13 Ta has a higher melting point ('1627"~) than A13 Nb and

is being considered for composite materials development.


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THE ROLE OF REFRACTORY METALS 62 1

NI,V: This intermetallic is being considered for applications in

high temperature plasma reactors such as the Tokamak

configuration.

& w: Ni, Nb, as mentioned earlier, is used extensively as a

strengthening phase in some Ni-base superalloys.

Trialurninides

The Ai3 Nb-Ni system is attractive because of its combination of

high melting point and low density.

Ni - Mo System

Ni4 Mo is of special interest, since Ni-Mo alloys with about 20%

Mo and other additions have been of industrial significance for their

high strength and good corrosion resistance.

Having considered the above comprehensive list of intermetallics

based on refractory metals we consider two examples in detail, one of

the development of a Nb based intermetallic and secondly of the use

of the refractory metal W to improve the thermal stability of the

intermetallic system Ti Al+Ti3AI.

Example 1: The Development of Niobium

Based lntermetallics

Niobium based intermetallics are intended to support engineering

stresses at high service temperatures [6, 10, 121. In one method, the

crystal structures which could yield high temperature structural inter-

metallics were identified. These are (a) geometrically close packed

derivatives, (b) body centred cubic derivatives and (c) complex struc-

tures. In category (a) are Ti Al alloys and transition metal trialu-

minides. In category (b) is the intermetallic Ni Al. In category (c)

are the compounds with melting point more than 1400°C and crystal

structures such as the Laves, a and Nowotny phases found in many

refractory metal, aluminide and silicide systems. Consider within

category (c) in which refractory metals are included: There are 4


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622 R. V. RAMANUJAN

possibilities; all of which contain refractory metals.

1. Ductile systems These include the ,O phase compositions based on

the Nb-Ti Al system and ductile wire reinforced versions of these'

alloys. Figure 1 shows the alloy density versus service temperature

for selected materials. Note the position of Nb-Ti Al-Cr and

Nb/Nb5 Si3 base systems. Thus a new class of refractory metal

base system can emerge.

2. Ductile-Brittle systems A ductile matrix of Nb-Ti-A1 with

reinforcement of brittle particles or fibres shows promise. This

category includes dispersion strengthened systems.

3. Brittle-Ductile systems This includes systems between interme-

tallic phases and refractory metal solid solutions, e.g., Nb-Ti- Al,

Nb-Ti-Si and Nb-Cr. Another example is Mo Siz with ductile

reinforcements of Nb, W, Mo or Ta.

4. Brittle-Brittle systems An example is Mo Si2 with ceramic

particulate and/or fibre reinforcements of Sic, All O3 and

Zr 02.

A

%

8

< 6-

CII

-

2.

.- -

r '-

I

P

Fe. Ni. Co - Bdse

- INbl/Nb3Si3- Base

Ti-Base

Al-Base

Nb-Ti-Ai-Cr-Base

NiAl-Single Crystals

Ti-,Al-Base

TiAl-Base

0 1 I

200

I

LOO

I I I I I

600 800 1000 1200 1400

Service Temperalure. OC

FIGURE 1 Density versus servicc temperature for selected alloys. Note the service

temperature of the refractory metal based systems (after Dimiduk er al.).


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THE ROLE OF REFRACTORY METALS 623

In ductile systems and within the Nb-Ti-A1 system, two or three

phase alloys may be formed with the (Nb, Ti), Al, a (Nb, Ti), A1 or

the orthorhombic Ti2 Nb A1 intermetaHics.

A plot of yield strength vls temperature for a Nb -Ti-Al- Cr alloy

and a Nb- 10 Si alloy is shown in Figure 2. This figure shows that the

service temperature of Ni-base alloys can be exceeded by both Nb-

Ti-Al-Cr and Nb- 10 Si. Creep data for several refractory metal

based systems is shown in Figure 3. These systems include Mo-41 Re,

Nb+NbS Si3, Nb-Ti-Cr- Al, W-3 Re/Mo Si2, Nb and Mo Siz. Both

the stress to rupture vls Larson-Miller parameters and minimum creep

rates v/s stress data show the promise of these systems.

In brittle-ductile systems, the main contenders are Nb/Nb5 Si3 in

situ composites and Mo Si2/X, where X is a refractory metal phase,

such as W, Ta, Nb, Mo and alloys of these metals such as W- Re. In

such systems, environmental resistance is a key issue and Figure 4

0 LOO

Nb-Ti-Al-Cr Alloy '\,

\ \

Temperature. OC

Crystal PWlCBO

FIGURE 2 Yield strength for Nb-TiAl alloy, Nb-10 Si in-siru composite and two

superalloys. (after Dimiduk er 01.).


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-

R. V. RAMANUJAN

l a )

I I I

1000 - -

0

Q FeAlr\

I

-

-

Ma-41Re

vi

U) ThOrW

al

L

VI

Fe, Al

Beta

10

15 2 0 2 5 3 0 3 5 LO L 5

P = T(OKIIIO~ t I~I + 20 1 ~ 1 0 ~

\

l b l

lca I " " I I

12OO0C

lrl - d

-

7 r)

0)

d

* lo-6 - w -

1u7 - -

a A

nl

B 1% Creep

lfa -in 50 Hours

10-~ - -

lfn 1

10

I I I

5 0 100 200

Stress. IMPaI

I I l l I

FIGURE 3 Creep properties for selected systems (a) stress to rupture v/s Larson-Miller

parameter and (b) minimum creep rate v/s stress at IZOWC (after Dimiduk er al.).

shows the result of several studies designed to produce an environ-

mentally stabIe system. The data from Figure 4 shows that while

enormous progress has been made compared to commercial Nb alloys,

-

-


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THE ROLE OF REFRACTORY METALS 625

Wb-TLHf/Cr-Al RO0.C

beta alloys 1100.C

Mb+Nb, Si, ; t100'C

Hi & Co-base 1200aC

superalloys 1100'C

8-66 1 Hb-base alloy 1 1200'C

Goal * 2.5

I01

= 125

0 5 0 100 15 0

Recession. pm/h

Commercial Nb Alloys r250

Nb-Ti-Hf-AI Beta alloy.

m0.c

0 50 .la0 150 ' 200 250 300

Depth of Oxygen Penetration, pm/h

FIGURE 4 Environmental resistance of selected alloys (a) recession rates due to high

temperature static air exposure and (b) oxygen penetration depths for static exposure of

100 h at'stated temperatures (after Dimiduk et 01.).

further work is required before the goal of < 2.5 pm/h recession rate

is achieved.

In summary, it can be stated that a great amount of success has been

achieved in the development of such systems, but further work is still


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626 R. V. RAMANUJAN

required to meet the service demands of the high temperature mare-

rials environment.

Example 2 - Refractory Metal Additions to lntermetallics

The Ti AlfTi, Al two phase intermetallic system has recently attracted

enormous attention for high temperature applications [13 - 151.

The stability of the microstructure of these intermetallics can be

enhanced by the addition of refractory elements. In detailed studies of

the thermal stability of such intermetallic system, several aspects of the

role of the refractory metal W was studied. Figure 5(a) shows the

transmission electron micrograph of (TEM) of a Ti-47A1-40 wppm

B-0.5 W intermetallic revealing the alternating parallel plates of Ti3 A1

and Ti Al. Aging this alloy at 800°C for 1 week showed instability

occurring by edge migration (Fig. 5(b)). After long tern aging at

1200°C it was observed that spheroidization had taken place and

that the original lamellar structure was altered to plates with a small

aspect ratio (Figs. 6(a), (b)). The effect of cold work and sub-surface

machining damage was also investigated and Figure 7 shows the

optical micrograph of the alloy aged at 1000"~. Figure 7(a) shows

that considerable instability has occurred in the bulk of the structure

after aging for 2 days, while the edge (Fig. 7(b)) has also undergone

preferential damage. Aging for 1 week further increases the instability

and the formation of fine .y grains (Fig. 7(c)). The surface of the alloy

after aging for 1 week shows preferential damage (Fig. 7(d)). Crack

formation can also be seen to lead to instabilities, providing clear

evidence of the role of stress in initiating instabilities in the microstructure.

Even in the absence of stress considerable instability can

take place as shown in Figure 8. Figure 8(a) shows the original microstructure,

after aging at 800°C (Fig. 8(b)), 1000°C (Fig. 8(c)) and

1200°C (Fig. 8(d)) for 1 week. It can be clearly seen that simple

thermal aging at 1000°C and 1200°C can produce a large amount of

microstructural instabilities.

An experimental comparison [13,14] of the results with unalloyed

Ti-47 Al showed that the refractory metal W had a very beneficial

effect and that it refined the initial lamellar structure produced by heat

treatment at 1400°C and then stabilized the original lamellar structure

during subsequent aging at 800- 1000°C. Recent modeling studies [15]

*


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630 R. V. RAMANUJAN

2.5 3.0 3.5 4.0 4.5 5.0

aspect ratio [w/t]

FIGURE 9 Modelling results in the same alloy. showing the dominant mechanisms of

instability as a function of aspect ratio and dihedral angle 0. The results are for 800nC,

1000°C and 1200°C.

of this alloy at 800°C, 1000°C and 1200"~ also showed that termination

migration was the main instability mechanism until an aspect

ratio of = 3.7 when cylinderization takes over (Fig. 9). This was

consistent with the experimental results discussed earlier.

Summary

This paper has discussed the use of refractory metals in high tem-

perature structural applications. The use of refractory metals in the

elemental and alloy form were considered. Refractory metal based

intermetallics and intermetallics containing beneficial additions of

refractory metals were then considered. Two examples were discussed,

the first was the development of refractory metal based intermetallics

and secondly the role of the refractory metal W in improving the

stability of the Ti Al+Ti3 A1 intermetallic system.

Acknowledgements

Dr. C. K. Gupta, Director, Materials Group is thanked for his kind

invitation to write this paper and Dr. S. Banerjee, Associate Director,


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THE ROLE OF REFRACTORY METALS 63 1

Materials Group and Head, Materials Science Division is thanked for

his kind encouragement.

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