1. Engineering Alloys (307) Lecture 7 Titanium Alloys I
© Imperial College London

2. Outline Ti primary production CP Ti and applications
© Imperial College London
Ti primary production
CP Ti and applications
α-Ti alloying, alloy design
near-α alloy microstructures, forging and heat treatment
α/β alloys, Ti-6Al-4V
defects

3. Ti Primary Production – Kroll Process
© Imperial College London
Ti common in Earth’s crust
Energy to separate ~125 MWhr/tonne (£4/kg just in power)
Batch process over 5 days:
Produce TiCl4 from TiO2 and Cl2
TiCl 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

4. Subsequent Processing
© Imperial College London
harvey fig p11

5. Casting
© Imperial College London
Use skull melting (EBHCR) instead of VIM/VAR/ESR for final melting stage in triple melting process

6. Ti Allotropes, Phase Diagram
© Imperial College London
Pure Ti:
L→β 1660 C
β→α 883 C
ρ=4.7 g/cc
highly protective TiO2 film
Diffusion in α 100x slower than in β
origin of better α creep resistance

7. Alloying: Pure α alloys
© Imperial College London
α stabilisers: O, Al (N,C)
β stabilisers: V,Mo,Nb,Si,Fe
neutral: Sn, Zr
Strengthen pure α alloys by
solid solution – O, Al, Sn
Hall-Petch – σ =
cold work
martensite reaction exists, of little benefit (not heat-treatable)
Uses: chiefly corrosion resistance
chemical plant
heat exchangers
cladding
harvey fig p13
Table of CP Ti

8. Microstructures – near α alloys
© Imperial College London
α stabilisers – raise α/β transus
β stabilisers to widen α/β field and allow hot working
heat – treatable
~10% primary (grain boundary) α during >900C
oil quench – intragranular α’ plates + retained β
age at ~625C to form α, spheroidise β and stress relieve
Then >>90% α
Lightly deformed (~5%) Ti-834

9. Properties – near-α alloys
© Imperial College London
Refined grain size
stronger
better fatigue resistance
Predominantly α – few good slip systems
good creep resistance
Si segregates to dislocation cores – inhibit glide/climb further

10. Ti Creep Rates
© Imperial College London

11. α+β alloys: Microstructures
© Imperial College London
Contain significant β stabilisers to enable β to be retained to RT
Classic Ti alloy: Ti-6Al-4V
>50% of all Ti used
Classically
1065 C all β
955C – acicular α on grain boundaries to inhibit β coarsening
Air cool – produce α lamellae colonies formed in prior β grains (minimise strain), w/ β in between (think pearlite)

12. Ti-6-4: heat treat
© Imperial College London

13. Ti-6-4: properties N.B. Must avoid Ti3Al formation
© Imperial College London
N.B. Must avoid Ti3Al formation
via Al equivalent: Al+0.33 Sn Zr + 10 (O+C+2N) < 9 wt%
ppt hardening
+ grain size

14. Defects
© Imperial College London
Major α-related problem is the production of α-rich regions due to oxygen (+N) embrittlement – the entrapment of O-rich particles during melting
Called α case
Also a problem in welding – often Ti is welded in an Ar-filled cavity to avoid this
β alloys suffer from β-rich regions from solute segregation (β flecks), and/or from embrittling ω phase, a diffusionless way to transform from β-bcc to a hexagonal phase.
more in lecture on β alloys

15. Review: Titanium I (L7) α-Ti Alloys near-α microstructure
© Imperial College London
near-α
microstructure
α/β microstructure
α-Ti Alloys
Casting
Phase
Diagram