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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, [email protected]
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