Ti-Al Intermetalics (1)

7/29/2019 Ti-Al Intermetalics (1)

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BY

Sandeep Akki


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Continents1. Introduction

2. Multi-scale Finite Element Simulation of Failure

2.1 Successive Initiation and Damage Model.3. Finite Element Results

4. Summary and Conclusion

5. References


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Intermetallic compound : A compound of two metals that has a distinctchemical formula on phase diagram, it appears as an intermediate phasethat exist over a very narrow range of compositions.

Different classification of intermetalic compounds are:-

Aluminides, Nitrites, Silicades, Iron aluminides,Beryllides, Nickel aluminides etc.

Titanium Aluminides: our material of interests are Ti3Al (also known as2) & TiAl () both of which exist over a range of compositions, Together

with the stoichemetric compound TiAl3 (75)-(77) .

Properties of these aluminides are Improved levels of stiffness,

Higher values of elastic modulus than normal Ti alloys.

Resistance to oxidation with increase in %age of Al.

Enhanced creep resistance (strong bonding between two atoms atelevated temp.)

However these alloys have limited capacity to undergo slip results in lowvalues of ambient temp. ductility due to presence of impurity elementssuch as Oxygen & hydrogen. Formability at elevated temp is also

restricted. It is these problem that have delayed the introduction ofthese elements.


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1. Ti3Al (2):

Has a outstanding strength ;weightratios when compares withconventional titanium alloys,

although improvements in creepstrength have so far been relativelysmall & the upper limit foroperation in air has been littlechanged.

It undergoes an order/disordertransition within the compositionrange 22-39% Al. to form an ordedDO19 Hexagonal structure.

Low ductility at room temp. isattributed a coplanar mode of slip& the lack of sufficient slip systemsparallel, or inclined to hexagonaldirection of the unit cell.

It does not undergo

deformation by twinning.


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1. Ti3Al (2):

It does not undergo deformation by twinning.

Attempts are being made to ad alloying elements that substitutefor Ti in crystal lattice & improve low temp. Ductility byincreasing the number if active slip systems.

Niobium has become widely used in this regard although

molybdenum in smaller quantities, also behaves in this way &provides the additional advantage of solid solution strengthening& improved creep strength.

Vanadium is also used instead of Niobium because of its lowerdensity although oxidation resistance is reduced.

Examples of alloys of recent developments Ti-24% Al- 11% & Nb,

Ti 25% Al-17% Nb-1% & Mo, Ti-25% Al-10% Nb-3% V-1% Mo,

Densities ranges from 4.2 to 4.7 g cm-3 which compare with 4-5 for

titanium.


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2. TiAl ()

It is a recent development, this compound intrinsically, has even

less room temp ductility than Ti3Al. alloys have lower densities combined with superior values of

elastic modulus , oxidation resistance, thermal stability &plasticity at elevated temperatures.

TiAl has fc tetragonal structure in which Ti & Al atoms form asuccessive layers on the 002 planes.

The composition may extend from 48.5 66% Al as per practicalinterest.

Fracture is predominantly by cleavate at both Low & high temp. It retains its tetragonality up to its melting point of 1460C

They usually have aluminum contents within the range 46-49% ,with 1-4% of other additional elements.

Density ranges from 3.9 to 4.1 gcm-3.


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3. TiAl3

Has DO22 Tetragonal crystal structure

Has highest specific stiffness & oxidation resistance of all thetitanium aluminides.

It exhibits compressive ductility above 620C

It is brittle at lower temp. at which deformation occurs solely by

twinning. To improve its ductility ternary additions of transition metals that

encourage formation of structurally related but more symmetriccubic structure.

Ex: Ti-65Al-10Ni in this Al & Ni atoms occupy the face centeredsites & Ti as cube corners.

Ternary addition of copper, manganese, zinc, iron & chromiumhave been made, but at low temp. they are still brittle & exhibit

cleavage fracture.


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4. Applications of Titanium aluminides.

Aerospace industry

Substantial weight saving would be possible if Ti Al could be usedto replace nickel based super alloys.

Turbine engine components : discs, blades, valves & spacer rings

Contenders for use in structure of hypersonic transatmospheric

aircraft Motor car engines such as cast turbocharger rotors

Cost reduction can be achieved

Automotive engine valves. Etc


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Ti-Al Intermetalics (1)

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