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David Dye Department of Materials, Imperial College Royal School of Mines, Prince Consort Road, London SW7 2BP, UK +44 (207) , © Imperial College London Engineering Alloys (307) Lecture 7 Titanium Alloys I
© Imperial College London Page 2 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
© Imperial College London Page 3 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 TiCl 4 from TiO 2 and Cl 2 –TiCl Mg → 2 MgCl 2 + 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
© Imperial College London Page 4 Subsequent Processing harvey fig p11
© Imperial College London Page 5 Casting Use skull melting (EBHCR) instead of VIM/VAR/ESR for final melting stage in triple melting process
© Imperial College London Page 6 Ti Allotropes, Phase Diagram Pure Ti: –L→β 1660 C –β→α 883 C ρ=4.7 g/cc highly protective TiO 2 film Diffusion in α 100x slower than in β –origin of better α creep resistance
© Imperial College London Page 7 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 = –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
© Imperial College London Page 8 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 >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
© Imperial College London 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
© Imperial College London Page 10 Ti Creep Rates
© Imperial College London Page 11 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 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)
© Imperial College London Page 12 Ti-6-4: heat treat
© Imperial College London Page 13 Ti-6-4: properties N.B. Must avoid Ti 3 Al formation –via Al equivalent: Al+0.33 Sn Zr + 10 (O+C+2N) < 9 wt% ppt hardening + grain size
© Imperial College London Page 14 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
© Imperial College London Page 15 Review: Titanium I (L7) α-Ti Alloys near-α microstructure α/β microstructure Casting Phase Diagram
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