Titanium and it’s alloys

Titanium and it’s alloys
By
Dudekula Jamla

History
 Titanium is named after the Titans,
the powerful sons of the earth in
Greek mythology.
 The element, Titanium, was
discovered by William Gregor in
1791.
Titan Reverend William Gregor

Introduction
 Titanium is the ninth abundant element on earth crust, makes up about (~ 0.57%).
 it will Not found in its free, pure metal form in nature but as oxides, i.e., ilmenite (FeTiO3),
sphene (CaTiSiO5) and rutile (TiO2).
 The top producing countries of these ores are Australia, South Africa, and Canada.
 Have similar strength as steel but with a weight nearly half of steel.
 Titanium has a density of 4420 kg/m3, Young's modulus of 120 GPa, and tensile strength of 1000
Mpa.

Properties of Titanium
 Titanium has high boiling and melting points.
 Titanium possesses a very low density and high strength.
 It has high tensile strength to density ratio.
 titanium is well known for its high resistance to corrosion.
 Highly react with oxygen, nitrogen, carbon and hydrogen.
 Difficult to extract(Expensive).
 High strength and toughness.

Classification of Titanium Alloys
 Different crystal structures and properties allow manipulation of heat treatments to produce
different types of alloy microstructures to suit the required mechanical properties.
 There are Three types of titanium alloys available.
 Commercially pure (CP) titanium alpha and near alpha(α)titanium alloys
-Generally non-heat treatable and weldable.
- Medium strength, good creep strength, good corrosion resistance.
 Alpha-beta titanium alloys (α-β)
-Heat treatable, good forming properties
- Medium to high strength, good creep strength.
 Beta titanium alloys (β)
- Heat treatable and readily formable.
- Very high strength, low ductility.

Alloying system of titanium Alloys
 Alloying Elements are
 Alpha stabilizers
• Al(Aluminum), O(Oxygen), N(Nitrogen)
 Beta stabilizers
• Mo(Molybdenum), V(Vanadium), W(Tungsten), Nb(Niobium), Ta(Tantalum)
 Neutral stabilizers
• Zr(Zirconium), Si(Silicon), Sn(Tin)

Allotropic Transformation
α phase HCP structure β phase BCC structure
882.3 C

Basic types of phase diagrams for titanium alloys

Basic principal of heat treatment
 Strength of annealed alloys increases gradually and linearly
with increasing alloy contents.
 Quenching from the β phase field gives a martensitic
transformation with improved strength (depending on
composition).
 For lowly alloyed Ti, rapid quenching from the β phase field
gives maximum strength at Mf.
 For highly alloyed Ti, rapid quenching from β phase field
gives lowest strength but after ageing, the maximum
strength is obtained.

1.Commercially pure (CP) titanium and alpha/near alpha alloys
 Microstructure contains HCP α phase
and can be divided into
i. Commercially pure titanium alloys
ii. Alpha titanium alloys
iii. Near alpha titanium alloys
 Characteristics
 Non-heat treatable
 Weldable
 Medium strength
 Good notch toughness
 Good creep resistance at high
temperature.

Microstructure of commercially pure (CP) titanium alloys
 Purity 99.0-99.5%, HCP structure.
 Main elements in unalloyed titanium
are Fe and interstitial elements such
as C, O, N, H.
 O content determines the grade and
strength.
 C, N, H present as impurities.
 Oxygen Equivalent
%Oequiv = %O + 2.0%N + 0.67%C
HCP α phase structure HCP α phase structure with β spheroidal
particles due to 0.3% Fe as impurity
Hot-rolled structure
Phase diagram of α stabilized Ti
alloy.

Properties and typical applications of commercially
pure (CP) titanium alloys
 Properties
 Lower strength, depending on
contents of O, N.
 Corrosion resistance to nitric acid,
moist chlorine.
 0.2% Pd addition improves corrosion
resistance in HCl, H2SO4, H3PO4.
 Less expensive.
 Applications
 Airframes,
 heat exchangers,
 chemicals,
 marine,
 surgical implants.

 Al and O are the main alloying elements, which
provide solid solution strengthening.
 O and N present as impurities give interstitial
hardening.
 The amount of α stabilizers should not exceed 9% in
the aluminum equivalent to prevent embrittlement
due to ordering.
 5-6% Al can lead to a finely dispersed, ordered phase
(α2), which is coherent to lattice.
 Sn and Zr are also added in small amount to stabilize
the α phase and give strength.
 Aluminum equivalent-
%Alequiv = Al + 1/3 Sn + 1/6 Zr +10(O + C + 2N) ≤ 9%
 α stabilizers are more soluble in the α phase and raise the β transus temperature.
Phase diagram of α stabilized Ti alloy
Alpha Titanium Alloys

Alpha Titanium Alloys-Microstructure
 Sn is added to improve ductility.
 Spheroidal β phase is due to 0.3% Fe
as impurity.
 >5-6% Al addition produces
coherent ordered α2 phase
(Ti3Al)embrittlement.
 Co-planar dislocations are produced
early fatigue cracking.
Ti-5%Al-2.5% Sn alloy in sheet form
Homogeneous α2 precipitation on dislocations in
aged Ti 8%Al with 1780 ppm of O

Alpha titanium alloys Properties and Applications
 Properties
 Moderate strength.
 Strength depends on O and Al
contents. (Al <5-6%).
 Al also reduces its density.
 Good oxidation resistance and
strength at 600 to 1100F.
 Readily weldable.
 Applications
 Aircraft engine compressor blades,
sheet-metal parts.
 High pressure cryogenic vessels at -
423 C.

Near-alpha titanium alloys
 Small amounts of β stabilizers (Mo, V) are added, giving a microstructure
of β phase dispersed 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 neat alloy IMI 685

Near alpha titanium alloys
Properties:
 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.
 Applications: Airframe and jet engine parts.

Heat treatment in CP and alpha titanium alloys
Treatment from the β phase field
 Annealing of CP Ti at high temperature gives
a HCP α phase structure, see in fig(a).
 Quenching of CP Ti from the β phase field
change the HCP structure to the hexagonal
martensitic α’ phase with remained β grains,
See in fig (b).
 Air-cooling of CP Ti from the β phase field
produces Widmanstatten α plates, See in fig
(c).
 This transformation contribute to only little
strength.

Heat treatment in near α titanium 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 T to obtain equal amount of α and β
phases.
 Air-cooling gives equiaxed primary α phase and Widmanstatten
α formed by nucleation and growth from the β phase, fig.
 Faster cooling transforms β into martensitic α’ which gives
higher strength.
Pseudo-binary diagram for Ti-8%Al
with Mo and V addition

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.
 Ageing causes precipitation of fine α
phase dispersion.
 Air-cooling from the β phase field gives a
basket weave structure of Widmanstatten
α phase delineated by β phase, see in fig
(c).
(a) Near α Ti (IMI 685)
oil-quenched
(b) quenched from β phase
field and aged at 850oC
(c) Near α Ti (IMI 685) air-cooled from
the β phase field

 Effects of cooling rate from β phase field on lamellar microstructure Ti
6242 alloy
Increasing cooling rate

Alpha-beta titanium alloys
 α+β - Ti alloys are by far the most important group of titanium alloys used in
aircraft.
 These alloys are produced by the addition of a-stabilizers and b-stabilizers to
promote the formation of both α -Ti and b-Ti grains at room temperature.
 The popularity of α+β - Ti alloys stems from their excellent high temperature creep
strength, ductility and toughness (from the α -Ti phase) and high tensile strength
and fatigue resistance (from the b-Ti phase).
 The amounts of α -stabilizers and b-stabilizers are typically in the range of 2–6%
and 6–10%, respectively.

 Of the many types of a+b- Ti alloys, the most important
is Ti–6Al–4V (IMI318), which makes up more than one-
half the sales of titanium in the United States and
Europe, and is the most used titanium alloy in aircraft.
 Ti–6Al–4V is used in both jet engines and airframes; it
accounts for about 60% of the titanium used in jet
engines and up to 80–90% for airframes
 Ti–6Al–4V is the most commonly used titanium alloy in
airframes, and is found in highly-loaded structures such
as wing boxes, stiffeners, spars and skin panels
 The mechanical properties of a+b-Ti alloys are often
between those of a-Ti and b-Ti alloys. For example, the
yield strength of annealed Ti–6Al–4V is about 925
MPa, which is higher than most near-alpha Ti alloys
(~800 MPa) but lower than many b-Ti alloys (1150–
1400 MPa).
Forged Ti-6AI-4V
blades

Composition and applications of α+β titanium alloys

Beta (β) titanium alloys
 Beta-titanium alloys are produced by the addition of b-stabilizing elements
 A large number of alloying elements can be used as b-stabilizers, although only V, Mo, Nb, Fe
and Cr are used in significant amounts (typically10–20 wt %).
 The strength and fatigue resistance of b-Ti alloys is generally higher than the a-Ti alloys.
 The first b-Ti alloy to be used in commercial quantities was Ti–13V–11Cr–3Al, which was used
in the airframe of the SR-71 Blackbird military aircraft.
 The alloy was used in the fuselage frame, wing and body skins, longerons, bulkheads, ribs and
landing gear. Blackbird was designed to fly at Mach 2.5 and, at this speed, the skins are
heated to nearly 300 °C by frictional aerodynamic drag. Aluminum alloys soften at these
temperatures, and therefore titanium alloy was used because of its superior strength at high
temperature
 However, the use of b-Ti alloys is very low; accounting for less than a few percent of all the
titanium used by the aerospace industry owing to their low creep resistance at high
temperature.

 The Boeing 777 uses b-Ti alloys in many components, including landing gear, exhaust
plugs, nozzles, and sections of the nacelle.

β titanium alloys
 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 equiaxed β phase.
 After solution heat treating + quenching it will give very
high strength (up to 1300-1400 MPa).
 Metastable β Ti alloys are hardenable while stable β Ti
alloys are non-hardenable.
Ti-13V-11Cr-3Al alloy solution heat-treated at
788oC/30min and water-quenched
Flow stress for Ti alloys hot-worked at
810 oC

Composition and applications of β titanium alloys

Advantages and Disadvantages of β titanium alloys

α+β And β titanium alloys- Heat Treatment
1. Annealing from the β and α+β phase
field.
2. Air cooling from the β and α+β phase
field.
3. Quenching from β and α+β phase
fields.
4. Tempering of titanium martensite
5. Decomposition of metastable β

1.Annealing from β and α+β phase field
 Annealing from the β phase field (β
annealed) causes a transformation from β
to α microstructure containing lamellar
structure of similar crystal orientation.
 Annealing from the α+β phase field (mill
annealed) produces microstructure
approaching equilibrium equiaxed primary α
phase surrounding with retained β phase.
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)

2.Air cooling from β and α+β phase field
 Air cooling from the β phase field
produces fine acicular α, which is
transformed from the β phase by
nucleation and growth.
 Air cooling from the α+β phase field
provides equiaxed primary α phase in a
matrix of transformed β phase (acicular).
 Air cooling provides intermediate cooling rates
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)

3.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.
Rapid transformation increases dislocation
density
Increase hardness (strength) but not as
high as in steel.
 Following tempering and ageing at elevated
temperature lead to decomposition of martensite.
Ti-6-4 alloy solution-heat-treated at
1066 degree C/30min and water
quenched

Quenching from β phase field(Cont..)
 Martensite of different crystal structure.
 Increasing solute, α’ to α’’
(a) Hexagonal α’ lath (b) hexagonal lenticular α’ (c) orthorhombic α’’
Possible reactions due to quenching from the β
phase field

Quenching from α+β phase field (Cont…)
Below β transus but above Ms
 Microstructure consists of primary α
phase embedded in transformed β
phase (α’ martensite).
Below Ms
 Microstructure consists of primary α
phase and small amount of retained or
untransformed β.
Ti-6-4 alloy solution treated at 954oC and then water
quenched
Ti-6-4 alloy solution treated at 843oC and then
water quenched

4. Tempering of titanium martensite
 Decomposition of martensitic
structure occurs when a quenched
alloy is subject to subsequent
elevated temperature treatments.
Decomposition of martensite
 Decomposition reaction depends
upon ‘martensite crystal structure’
and ‘alloy composition’.

5. Decomposition of metastable β
 Retained β obtained after quenching decomposes
when subjected to ageing at elevated temperatures.
developing high tensile strength. The metastable β is
transformed
 The metastable β is transformed into equilibrium α
phase at high ageing temperatures due to difficulty in
nucleating HCP α phase on BCC β matrix.
o Medium alloy content-(100- 5000C) β β+ω β+α
o Concentrated alloys 200- 500 o C β β+β1 β+α >
500 0C β β+α
Possible reactions
 ω phase formation
 β phase separation
 Equilibrium α phase formation
β isomorphous alloy phase diagram

Decomposition of metastable β(Cont…)
 ω phase- embrittlement
• Appears as very fine dispersion particles after metastable β is isothermally
aged at 100-500 o C.
• Avoided by controlling ageing conditions, temp (475 oC), composition.
 β phase separation -not significantly important
• 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 not significantly
important.
 Equilibrium α phase formation -strength
• Equilibrium α phase can form directly from β phase or indirectly from ω or
β1. Laths of Widmanstatten α
• Finely dispersed α particles
Dense dispersion of cuboids of ω
phase

Heat treatment scheme for β titanium alloys

Casting of titanium alloys
 Titanium castings contribute to small amount of titanium
products recently used. There are several methods as
follows;
I. Conventional casting
II. Investment casting
III. Vacuum casting
 Titanium castings are normally near net shape products
with minimized metal waste, which can occur during
mechanical processing and machining.
Casting processes

Conventional casting
 Rammed graphite is used as the
mould rather than sand due to its
minimal tendency to react with
molten titanium.
 Produce intricate shapes with good
surface finish condition.
Rammed graphite casting made from
CP Ti grade 2

Investment casting
 This process begins with
1) Duplicating a wax part from engineering drawing of the
specific part.
2) Dipping in ceramic slurry until a shell is formed.
3)The wax is then melted out and the fired shell is filled
with molten metal to form a part near to the net shape of
the drawing.
 Used for structural applications requiring metallurgical
integrity and sports applications such as golf heads

Vacuum die casting
 Vacuum is applied for die casting to
reduce gas entrapment during metal
injection and to decrease porosity in
the casting
 Reduce porosity in the castings
 Provide high quality parts

Properties of titanium alloys
 Material strength, creep resistance and fatigue properties are the
main properties usually required for applications of titanium
alloys.
 Titanium alloys provide superior specific yield strength (high
strength to weight ratio) than other alloys.
Approaches to modify the properties of
titanium alloys
Specific proof stress of various
materials

Strength and toughness of titanium alloys

Strength and toughness of titanium alloys
 Tensile strength of different Ti alloys at a range of temperatures

Microstructure and tensile properties of titanium alloys
 Bimodal microstructure is more resistant to
fracture due to equaixed α phase -giving
higher strength.
 Equiaxed α phase is also more resistant to
nucleation of voids- higher ductility.

Microstructure and fracture toughness properties of
titanium alloys
 More torturous path in coarse lamellar microstructure leads to higher energy
dissipation during fracture higher toughness.
Crack paths in the centre of fracture toughness Ti-6Al-4V specimens (a)
coarse lamellar and (b) fine lamellar

Fatigue properties of titanium alloys
 Smaller equiaxed α grains are more beneficial to fatigue
strength.
 Crack nucleates within the lamellar region more easily than
in the equiaxed α phase region.
 But crack propagation is more difficult in the lamellar
structure.
Crack initiation at lamellar
region in bi-modal
Crack propagation paths in (a) lamellar and (b) bi-modal structures

Fatigue properties of titanium alloys
 Moderate tensile strength.
 β annealed has superior fatigue along platelets of α.
 α+β annealed is more fatigue resistance due to slower crack
propagation rate to fatigue crack initiation. better low cycle fatigue
(high stress).
 Duplex structure of 30% equiaxed α and α platelets provides high
temperature applications.
Properties of annealed Ti-6Al-4V (forging)
FCG curves for β and α+β annealed
conditions
Crack propagation path in β annealed Ti-
6-4
LCF of Ti-6Al-4V in α+β annealed
and β annealed

Corrosion of titanium alloys
 When fresh titanium is exposed to environment
containing oxygen, it will develop oxide films
which are…
1. Stable
2. Tenacious
3. Inert
4. Self-healing or re-form
 Good corrosive resistance to salt water and
marine, acids, alkalis, natural waters and
chemicals.

Welding of titanium alloys
 α and α+β titanium alloys are readily
weldable.
 β titanium alloys are not readily
weldable due to high amounts of
alloying element -macro/micro
segregation.
 Types:
I. Tungsten Inert Gas Welding
II. Electron Beam Welding
III. Laser Beam Welding
IV. Friction welding

Tungsten inert gas welding/Tungsten Inert Gas(TIG) welding or welding
 Arc is produced between a non-consumable tungsten electrode and the metals in the presence
of shielding gas (He, Ar).
o Most widely used technique for titanium
welding.
o Require no vacuum
o Lower operating cost
o Provide relatively coarser weld structure
than those obtained from EBW and LBW.
o High heat input -relatively high distortion.

Electron beam welding
o Electron beam is used as a heat
source.
o Vacuum and non-vacuum process
clean.
o Relatively high operating cost and
equipment.
o Multiple or single -pass arc welding
o Low heat input minimum distortion
Electron beam welding Mobile electron beam welding unit

Laser beam welding
o Laser is used as a heat source.
o Correct choice of shielding gas
o Adequate shielding methods
o Pre-cleaning (de-greasing)
o Good joint surface quality
 Advantages of laser beam welding
 High productivity (nearly 10 times faster than
TIG).
 Low heat input and therefore low distortion.
 Ease of automation for repeatability.
 No need for filler wire, thus reducing costs.

Friction welding
o High cost of welding machines.
o Can use to join dissimilar metals.
o Very small distortion.
o Limited to non-round and non-
complex component.
 Friction welding is carried out by moving one part in a linear reciprocating motion to
effect the heat at the joint.
Friction welding process
Weld structure obtained Friction from
friction welding

Defects in titanium welding
 Titanium and titanium alloys are highly reactive to oxygen, therefore care must be
taken for titanium welding. Should be carried out in vacuum or appropriate
shielding gas such as Ar or He.
Macroscopic pore observed in TIG
welding of beta titanium alloy

 The main defects occur in titanium welding are;
 Weld metal porosity
Most frequent defects caused by gas bubbles trapped between dendrites during
solidification.
 Embrittlement
Due to oxygen, nitrogen or hydrogen contamination at T> 500 oC. need effective
shielding.
 Contamination cracking
Due to iron contamination - reducing corrosion resistance, separate from steel
fabrication.

Applications of titanium alloys
 Aeroengines
 Automotive and road transport
 Dental alloys
 Electrochemical anodes
 Geothermal plant
 Marine
 Military hardware
 Offshore production tubulars • Airframes
 Condensers
 Desalination plant
 Flue gas desulpherisation
 Nuclear and environmental safety
 Petrochemical refineries
 Architectural
 Cryogenic logging tools
 Food, brewing and pharmaceutical
 Jewellery manufacture
 Metal extraction equipment
 Offshore piping systems
 Pulp and paper
 Heat exchangers
 Medical implants

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