david dye department of materials, imperial college royal school of mines, prince consort road,...
TRANSCRIPT
© 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
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Page 2Outline
• 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
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Page 3Ti 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
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Page 4Subsequent Processing
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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→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
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Page 7Alloying: 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
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Table of CP Ti
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Page 8Microstructures – 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
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Page 9
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
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Page 10Ti Creep Rates
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Page 11α+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)
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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 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
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Page 15Review: Titanium I (L7)
α-Ti Alloys
near-α microstructure
α/β microstructure
Casting PhaseDiagram