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Engineering Alloys (307) Lecture 7 Titanium Alloys I© Imperial College London
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
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
Subsequent Processing© Imperial College London harvey fig p11
Casting © Imperial College London Use skull melting (EBHCR) instead of VIM/VAR/ESR for final melting stage in triple melting process
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
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
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
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
Ti Creep Rates © Imperial College London
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)
Ti-6-4: heat treat © Imperial College London
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
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
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
Titanium Melts at 1670°C Density 4.51 g cm -3 80% of Ti used in aerospace Use restricted to < 400°C.
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