Engineering Alloys (307) Lecture 7 Titanium Alloys I David Dye Department of Materials, Imperial College Royal School of Mines, Prince Consort Road, London SW7 2BP, UK +44 (207) 594-6811, [email protected] Imperial College London Outline Page 2 Imperial College London
Ti primary production CP Ti and applications -Ti alloying, alloy designTi alloying, alloy design near-Ti alloying, alloy design alloy microstructures, forging and heat treatment / alloys, Ti-Ti alloying, alloy design6Al-Ti alloying, alloy design4V defects Ti Primary Production Kroll Process Page 3 Imperial College London Ti common in Earths 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-Ti alloying, alloy design8t) from reactor cost 5/kg Chlorides corrosive, nasty World annual capacity ~100,000 t, demand ~60,000t ($500m -Ti alloying, alloy design small) Need a cheaper process that is direct FFC (Cambridge) and others Subsequent Processing
Page 4 Imperial College London harvey fig p11 Casting Page 5 Imperial College London Use skull melting (EBHCR) instead of VIM/VAR/ESR for final melting stage in triple melting process Ti Allotropes, Phase Diagram Page 6 Imperial College London
Pure Ti: L (bcc) @ 1660 C (hcp) @ 883 C =4.7 g/cc highly protective TiO2 film Diffusion in 100x slower than in origin of better creep resistance
Alloying: Pure alloys Page 7 Imperial College London stabilisers: O, Al (N,C) stabilisers: V,Mo,Nb,Si,Fe neutral: Sn, Zr Strengthen pure alloys by
solid solution O, Al, Sn Hall-Ti alloying, alloy designPetch = 231 + 10.5 d cold work martensite reaction exists, of little benefit (not heat-Ti alloying, alloy designtreatable) Uses: chiefly corrosion resistance chemical plant heat exchangers cladding harvey fig p13 Table of CP Ti
Microstructures near alloys Page 8 Imperial College London stabilisers raise / transus stabilisers to widen / field and allow hot working heat treatable ~10% primary (grain boundary) during h.t. @ >900C oil quench intragranular plates + retained
age at ~625C to form , spheroidise and stress relieve Then >>90% Lightly deformed (~5%) Ti-834 Properties near- alloys Page 9 Imperial College London Refined grain size stronger better fatigue resistance
Predominantly few good slip systems good creep resistance Si segregates to dislocation cores inhibit glide/climb further Ti Creep Rates Page 10 Imperial College London + alloys: Microstructures Page 11 Imperial College London
Contain significant stabilisers to enable to be retained to RT Classic Ti alloy: Ti-Ti alloying, alloy design6Al-Ti alloying, alloy design4V >50% of all Ti used Classically 1065 C all forge @ 955C acicular on grain boundaries to inhibit coarsening Air cool produce lamellae colonies formed in prior grains (minimise strain), w/ in between (think pearlite)
Ti-6-4: heat treat Page 12 Imperial College London Ti-6-4: properties Page 13 Imperial College London 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 Page 14 Imperial College London Major -Ti alloying, alloy designrelated problem is the production of -Ti alloying, alloy designrich regions due to oxygen (+N) embrittlement the entrapment of O-Ti alloying, alloy designrich particles during melting Called case Also a problem in welding often Ti is welded in an Ar-Ti alloying, alloy designfilled cavity to avoid this
alloys suffer from -Ti alloying, alloy designrich regions from solute segregation ( flecks), and/or from embrittling phase, a diffusionless way to transform from -Ti alloying, alloy designbcc to a hexagonal phase. more in lecture on alloys Review: Titanium I (L7) Page 15 Imperial College London near-Ti Alloys microstructure / microstructure Casting
-Ti AlloysTi Alloys Phase Diagram
