Chapter 10-1 Titanium and Its Alloystriangle.kaist.ac.kr/lectures/MS371/2019 spring/Chap 10-1... · 2019-05-02 · Specific strength vs. temperature /MS371/ Structure and Properties

/MS371/ Structure and Properties of Engineering Alloys

Chapter 10-1

Titanium and Its Alloys

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

Introduction (refer to Guggenheim Museum, Bilbao)

• Titanium named after Titans, the powerful sons of

the Gaia (earth) & Uranus (sky) in Greek mythology

• Titanium, 4th abundant metal on earth crust

(~ 0.86%) after Al, Fe and Mg

• Not found in its free, pure metal form in nature but

as : ilmenite (FeTiO3) and rutile (TiO2)

• Having similar strength as but with a weight

nearly of steel

Ilmenite (FeTiO3) Rutile (TiO2)


Advantages of Ti alloys

TITANIUM

High

corrosive

resistance

Low

specific

gravity

High

specific

strength

Non

magnetic

property

Bio

compatible

materialDensity of selected metals

Specific strength vs. temperature


Extraction of Ti sponge

• Titanium ore – rutile (TiO2) is converted into titanium sponge by

1) Passing Cl2 gas to charge the ore, resulting in colorless titanium

tetrachloride TiCl4

2) TiCl4 is purified by fractional distillation

3) TiCl4 is reacted with either Mg or Na under an inert (Ar) atmosphere to

obtain titanium while Mg or Na is recycled

TiO2 + 2Cl2 + C → TiCl4 + CO2

TiCl4(g) + 2Mg(l) → Ti(s) + 2MgCl2(l)

Titanium sponge


Preparation of Ti ingots

• Vacuum arc refining (VAR)

– Sponge and alloying elements to be blended together and then

hydraulically pressed to produce blocks (briquette)

– The briquettes to be welded together to produce first melt electrode or

‘stick’

– The electrode is double or triple melted in VAR furnace to produce sound

ingot


Physical properties of Ti

• Experiences transformation (hcp → bcc) at 882.5oC

• Highly reactive with O, N, C and H

• Difficult to extract → expensive

• Used mainly in wrought forms for advanced applications where

is not critical: aerospace industry

• High strength and toughness

Crystal structure

Atomic diameter

Density (g.cm-3)

Melting point (oC)

HCP (<882.5oC)

BCC (>882.5oC)

0.320 (nm)

4.54

1668

22 HCP,BCC

TiTitanium

47.87


Alloying system of Ti alloys

α phase

HCP structure

β phase

BCC structure

Allotropic

transformation

882.5oC

α system

β system β system

Alloying elements

• α stabilizers– Al, O, N, C

• β stabilizers– isomorphous: Mo, V,

W, Nb, Ta

– eutectoid: Fe, Cr, Cu,

Ni, Co, Mn

• Neutrual– Zr, Si, Sn


Classification of Ti alloys

• Commercially pure (CP) titanium α and near-α titanium alloys

– Generally non-heat treatable and weldable

– Medium strength, good creep strength & good corrosion resistance

• α-β titanium alloys

– Heat treatable, good forming properties

– Medium to high strength, good creep strength

• β titanium alloys

– Heat treatable and readily formable

– Very high strength, low ductility

Different crystal structures and properties allow manipulation of heat

treatments to produce different types of alloy microstructures to suit the

required mechanical properties.


Basic principle of heat treatment

• Strength of annealed alloys increases

gradually and with increasing alloy

contents.

• Quenching from the β phase field gives a

transformation with improved

strength (depending on comp).

• For low alloyed Ti, rapid quenching from

the β phase field gives strength

at Mf.

• For high alloyed Ti, rapid quenching from

β phase field gives lowest strength but

after , the maximum strength is

obtained.

Heat treatment is mainly applied to α-β and β titanium alloys due to

the α-β transformation (typically in the β isomorphous Ti alloy group).

Heat treatment diagram of β

isomorphous titanium alloys


Commercially pure (CP) Ti alloys

• Purity of titanium : 99.0~99.5%, HCP structure

• Main elements in unalloyed titanium are and interstitial elements such as

O, N, C, H.

• Small addition of 0.2% Pd to commercially pure titanium

→ excellent resistance

• Application: petroleum-processing industry, airframes,

heat exchangers, chemicals, marine, surgical, implants

HCP α phase structureHCP α phase structure with βspheroidal particles due to

0.3% Fe as impurity

Hot-rolled structure


Mechanical properties of CP Ti alloys

• O, N, C content to determine the grade and strength

• Interstitial effect

Oxygen equivalent %Oequiv = %O + 2.0(% N) + 0.67(% C)


α Ti alloy

• 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 Al equivalent to prevent

embrittlement due to ordering.

• 5~6% Al can lead to a finely dispersed,

ordered phase (α2, Ti3Al), which is coherent

to lattice → deleterious ductility

• Small addition of Sn and Zr → stabilize the α

phase and give strength

α stabilizers are more in the α phase and

raise the β temperature.

Phase diagram of α

stabilized Ti alloy

Aluminum equivalent


Microstructure of α Ti alloy

Ti-5Al-2.5Sn alloy in sheet form Homogeneous α2 precipitation on

dislocations in aged Ti-8%Al with

1780 ppm of O

• Sn is added to improve ductility.

• Spheroidal 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


Hardened α Ti alloys

• Hardened structure

– α-phase stabilizer is sufficiently high

→ α-phase stabilizers can also

precipitate intermetallic compounds

– β-phase stabilizer does not exceed C1

→ hardening will produce a martensitic

phase of the α’ type.

Heat treatment

Microstructure change

Hardened structure

Structures formed during hardening of

titanium alloys containing a β-phase

stabilizer from 882oC


Properties of α Ti alloy

• Moderate strength

• Strength to depend on O and Al contents (Al < 5~6 %)

• Al also reduces its density

• Good oxidation resistance and strength at 315~593C

• Readily weldable

• Applications

- Aircraft engine compressor blades, sheet-metal parts

- High pressure cryogenic vessels at -253oC (20K)

Documents

Chapter 10-1 Titanium and Its Alloystriangle.kaist.ac.kr/lectures/MS371/2019 spring/Chap 10-1... · 2019-05-02 · Specific strength vs. temperature /MS371/ Structure and Properties