chapter 10-2 titanium and its alloys - kaisttriangle.kaist.ac.kr/eng/lectures/ms371/2019...

/MS371/ Structure and Properties of Engineering Alloys

Chapter 10-2

Titanium and Its Alloys

/MS371/ Structure and Properties of Engineering Alloys/MS371/ Structure and Properties of Engineering Alloys

Near-α Ti Alloys

• Small amounts of stabilizers (Mo,V) are added, giving a microstructure of β

phase in the α phase structure

→ improved performance and efficiency

• Sn and Zr are added to compensate Al contents while maintaining strength

and ductility

• Show greater creep strength than fully α Ti alloy up to 400oC

• Ti-8Al-1Mo-1V and Ti-6Al-2Sn-4Zr-Mo alloys are the most commonly used for

aerospace applications, i.e., airframe and engine parts

Duplex annealed Ti-8Al-1Mo-1V Forged compressor disc made

from near-α alloy IMI 685


Heat Treatment in Near-α Ti Alloys

• Heat-treated from α+β phase field

– Alloys should contain high amount

of α stabilizers without severe loss

of ductility

– Small amounts of Mo or V (beta

stabilizers) are added to promote

the response to heat-treatment

– The alloy is heated up to obtain

equal amount of α and β phases Pseudo-binary diagram for Ti-8%Al

with Mo and V addition

IMI679 Air-cooled

from α+β phase field,

having white primary

α phase and

Widmanstätten α

– Air-cooling gives equi-axed primary

α phase and Widmanstätten α

formed by nucleation and growth

from the β phase in next figure

– Faster cooling transforms β into

martensitic α’ which gives higher

strength


Pseudo-binary diagram for Ti-8%Al


• Heat-treated from β phase field

– Quenching from the β phase

field produces laths of

martensitic α’, which are

delineated by thin films of β

phase

– Aging causes precipitation of fine

α phase dispersion


(a) Near α Ti (IMI 685) oil-quenched

(b) quenched from β phase field and aged

at 850oC

(a) (b)


Pseudo-binary diagram for Ti-8%Al


• Heat-treated from β phase field

– Air-cooling from the β phase field

gives a basket weave structure

of Widmanstätten α phase

delineated by β phase, fig (c)


(c) Near α Ti (IMI 685) air-cooled

from the β phase field

(c)



Increasing cooling rate

Effects of cooling rate from the beta phase field on lamellar microstructure

in Ti 6242 alloy

• Effects of cooling rate from β phase field in lamellar

microstructure

(a) 1oC/min (c) 8000oC/min(b) 100oC/min


Properties of Near-α Ti Alloys

• Moderately high strength at RT and relatively good ductility (~15%)

• High toughness and good creep strength at high temperatures

• Good weldability

• Good resistance to salt-water environment

• Application

– Airframe and

jet engine parts


α-β Ti Alloys

• α-β titanium alloys contain both α and β

• α stabilizers are used to give with 4~6% β stabilizers to allow the β

phase to retain at RT after quenching from β or α+β phase field

• Improved strength and in comparison to α-Ti alloys

• Microstructure depends on chemical composition, processing history and

heat treatments, i.e., annealing, quenching and tempering

• Heat treatment can be done in corporation with thermo-mechanical

processes to achieve desired microstructure/properties

• Ti-6Al-4V (IMI 318) is the most widely commercially used

Forged Ti-6Al-4V blades


Heat treatment of α-β Ti Alloys

• Furnace cooling from the β and

α+β phase field

• Air cooling from the β and α+β

phase field

• Quenching from β and α+β phase

fields.

• Tempering of titanium martensite

• Decomposition of metastable β


Furnace cooling from β or α+β phase field

Annealed from β phase field, showing

transformed β phase or lamellar

(basket weaves) microstructure of

Ti-6Al-4V

Annealed from α+β phase field,

showing equiaxed α grains (light)

with intergranular retained beta (dark)

• Furnace cooling from the β phase field (β annealed, 1066C) causes a

transformation from β to α microstructure containing lamellar structure

of similar crystal orientation

• Furnace cooling from the α+β phase field (mill annealed, 954C)

produces microstructure approaching equilibrium equiaxed primary α

phase surrounded by retained β phase


Air cooling from β and α+β phase field

• Air cooling from the β phase (1066C) field produces fine acicular α,

which is transformed from the β phase by nucleation and growth

• Air cooling from the α+β phase (954C) field provides equiaxed primary

α phase in a matrix of transformed β phase (acicular)

Air-cooled from β phase field giving

transformed β phase (acicular)

Air-cooled from α+β phase field,

showing primary α grains in a

matrix of transformed β (acicular)


Quenching from β phase field

• The alloy experiences martensitic transformation when quenched from the β

phase field passing through Ms

• Martensite α’ consists of individual platelets which are heavily twinned and

have HCP crystal structure

Ti-6Al-4V alloy solution-heat-treated

at 1066oC/30min and water quenched

Rapid transformation increases

dislocation density

Increase hardness (strength)

but not as high as in steel

Note: Following tempering and aging at elevated temperature lead to

decomposition of martensite.


Quenching from α+β phase field

• Microstructure consists of primary

α phase embedded in transformed

β phase (α’ martensite)

Below β transus but above Ms Below Ms

• Microstructure consists of primary

α phase and small amount of

retained or untransformed β

Ti-6Al-4V alloy solution treated at

954oC and then water quenched

Ti-6Al-4V alloy solution treated at

843oC and then water quenched


Decomposition of metastable β

• Retained β obtained after quenching decomposes when subjected to

aging at elevated temp → developing high tensile strength

• The metastable β is transformed to equilibrium α phase at high aging

temp due to difficulty in nucleating HCP α phase on BCC β matrix

* Possible reactions

• ω phase formation

• β phase separation

• Equilibrium α phase formation

β isomorphous alloy phase diagram


Decomposition of metastable β

ω phase → embrittlement

β phase separation → not significantly important

Equilibrium α phase formation → strength

• Appears as very fine dispersion particles after

metastable β is isothermally aged at 100-500oC

• Avoided by controlling aging conditions, temp

(475oC), composition

• β phase separation into two BCC phases β → β (enrich) + β1(depleted)

occurs in high β stabilizer containing alloy to prevent ω formation

• This β phase will slowly transform into equilibrium α phase

• Equilibrium α phase can form directly from β

phase or indirectly from ω or β1

Dense dispersion of

cuboids of ω phase

- Laths of Widmanstätten α

- Finely dispersed α particles

Laths of

Widmanstätten α


β Ti Alloys

• Low ductility in high-strength condition, thus not used much at present

• β titanium alloys possess a BCC crystal structure, which is readily cold-

worked (than HCP α structure) in the β phase field

• Microstructure after quenching contains (metastable) equiaxed β phase

• After solution heat treating + quenching → giving very high strength (up to

1300-1400 MPa)

• Metastable β titanium alloys are hardenable while stable β titanium alloys are

non-hardenable

Ti-13V-11Cr-3Al alloy solution heat-treated

at 788oC/30min and water-quenched,

metastable beta

Flow stress for Ti alloys

hot-worked at 810oC


Heat treatment of β Ti Alloys

• Most β titanium alloys are metastable

and tend to transform into

(1) coarse α plates after heat-treated in

α+β phase field or

(2) α phase precipitation after long-term

aging at elevated tempβ annealed microstructure, β CEZ (Ti-

5Al- 2Sn-2Cr-4Mo-4Zr)- beta rich

Effect of pre-aging on microstructure of heavily stabilized β alloys

Beta 21S (Ti-15Mo-2.6Nb-3Al-0.2Si)

(a) 690oC/8h+650oC/8h (b) 500oC/8h+725oC/24h (c) 725oC/24h.


Properties of β Ti Alloys

* Application of β titanium alloys

chapter 10-2 titanium and its alloys - kaisttriangle.kaist.ac.kr/eng/lectures/ms371/2019...

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