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Engineering Alloys (307) Lecture 7 Titanium Alloys I

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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

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