David Dye Department of Materials, Imperial College Royal School of Mines, Prince Consort Road,...

© Imperial College London

David Dye

Department of Materials, Imperial College

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

+44 (207) 594-6811, [email protected]

Engineering Alloys (307) Lecture 7Titanium Alloys I


Outline

• 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


Ti Primary Production – Kroll Process

• 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

– 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


Subsequent Processing

harvey fig p11


Casting

• Use skull melting (EBHCR) instead of VIM/VAR/ESR for final melting stage in triple melting process


Ti Allotropes, Phase Diagram

• Pure Ti:– L→β (bcc) @ 1660 C

– β→α (hcp) @ 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


Alloying: Pure α alloys

• 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 = 231 + 10.5 – cold work– martensite reaction exists, of little

benefit (not heat-treatable)

• Uses: chiefly corrosion resistance– chemical plant– heat exchangers– cladding

d

harvey fig p13

Table of CP Ti


Microstructures – near α alloys

• 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 h.t. @ >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


Properties – near-α alloys

• 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


Ti Creep Rates


α+β alloys: Microstructures

• 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

– forge @ 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)


Ti-6-4: heat treat


Ti-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


Defects

• 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


Review: Titanium I (L7)

α-Ti Alloys

near-α microstructure

α/β microstructure

Casting PhaseDiagram

David Dye Department of Materials, Imperial College Royal School of Mines, Prince Consort Road,...

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