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David Dye

Department of Materials, Imperial College

Royal School of Mines, Prince Consort Road, London SW7 2BP, UK

+44 (207) 594-6811, david.dye@imperial.ac.uk

Imperial College London

Engineering Alloys (307) Lecture 7

Titanium Alloys I

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Page 2Outline

Ti primary production

CP Ti and applications -Ti alloying, alloy design

near- alloy microstructures, forging and heat treatment

/ alloys, Ti-6Al-4V

defects

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Page 3Ti Primary Production Kroll Process

Ti common in Earths crust

Energy to separate ~125 MWhr/tonne (4/kg just in power) Batch process over 5 days:

Produce TiCl4 from TiO2 and Cl2

TiCl4 + 2 Mg 2 MgCl2 + Ti

chip out Ti sponge (5-8t) from reactor

cost 5/kg

Chlorides corrosive, nasty

World annual capacity ~100,000 t, demand ~60,000t ($500m - small)

Need a cheaper process that is direct FFC (Cambridge) and others

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Page 4Subsequent Processing

harvey fig p11

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Page 5Casting

Use skull melting (EBHCR) instead of VIM/VAR/ESR for final melting

stage in triple melting process

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Page 6Ti Allotropes, Phase Diagram

Pure Ti:

L (bcc) @ 1660 C (hcp) @ 883 C

=4.7 g/cc

highly protective TiO2film

Diffusion in 100xslower than in

origin of better creep resistance

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Page 7Alloying: Pure alloys

stabilisers: O, Al (N,C)

stabilisers: V,Mo,Nb,Si,Fe neutral: Sn, Zr

Strengthen pure alloys by

solid solution O, Al, Sn

Hall-Petch = 231 + 10.5

cold work

martensite reaction exists, oflittle benefit (not heat-treatable)

Uses: chiefly corrosionresistance

chemical plant

heat exchangers

cladding

d

harvey fig p13

Table of CP Ti

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Page 8Microstructures near alloys

stabilisers raise /

transus stabilisers to widen /

field and allow hot working

heat treatable

~10% primary (grain

boundary) during h.t. @>900C

oil quench intragranular plates + retained

age at ~625C to form ,spheroidise and stressrelieve

Then >>90%

Lightly deformed (~5%) Ti-834

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Page 9

Properties

near- alloys

Refined grain size stronger

better fatigue resistance

Predominantly few good slip systems

good creep resistance

Si segregates to dislocation cores inhibit glide/climb further

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Page 10Ti Creep Rates

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Page 11+ alloys: Microstructures

Contain significant stabilisers to enable to be retained to RT

Classic Ti alloy: Ti-6Al-4V >50% of all Ti used

Classically

1065 C all

forge @ 955C acicular

on grain boundaries toinhibit coarsening

Air cool produce lamellae colonies formedin prior grains (minimise

strain), w/ in between(think pearlite)

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Page 12Ti-6-4: heat treat

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Page 13Ti-6-4: properties

N.B. Must avoid Ti3Al formation via Al equivalent: Al+0.33 Sn + 0.16 Zr + 10 (O+C+2N) < 9 wt%

ppt hardening

+ grain size

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Page 14Defects

Major -related problem is the production of -rich regions due to

oxygen (+N) embrittlement the entrapment of O-rich particlesduring melting

Called case

Also a problem in welding often Ti is welded in an Ar-filled cavityto avoid this

alloys suffer from -rich regions from solute segregation (flecks), and/or from embrittling phase, a diffusionless way totransform from -bcc to a hexagonal phase.

more in lecture on alloys

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Page 15Review: Titanium I (L7)

-Ti Alloys

near-

microstructure

/ microstructure

Casting Phase

Diagram