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 9 Titanium II: β-Ti Alloys

© Imperial College London Page 2 Review: Titanium I (L7) α-Ti Alloys near-α microstructure α/β microstructure Casting Phase Diagram

© Imperial College London Page 3 Synopsis SPF Fan Blades: An Aero use of CP Ti β-Ti Alloy Design – reminder of α & β stabilisers Strength and Selection of β-Ti alloys Deformation Processing, subgrains and microstructure control LCB in the auto industry β-flecks ω phase and the β→ω transition. Properties of ω-containing alloys Final thoughts: P/M Ti alloys?

© Imperial College London Page 4 Last gasp on α-Ti: SPF fan blades Hollow-core SPF fan blades: flagship R-R development Superplastically form at strain rates of ~10 -4 – and ~900 C near-CP α-Ti alloy (easier diffusion) Essentially a grain-boundary sliding creep process

© Imperial College London Page 5 Fan Blade Technology + 4% efficiency Clappered Wide-chord fan

© Imperial College London Page 6 Wide-chord Fan Technology Honeycomb construction 1st generation: nd generation: 1995 DB/SPF construction

© Imperial College London Page 7 Fan Section

© Imperial College London Page 8 Swept Fans

© Imperial College London Page 9 β-Ti Alloy design Hard to completely stabilize β w.r.t. hexagonal phases α stabilisers: O, Al (N,C) β stabilisers: V,Mo,Nb,Si,Fe neutral: Sn, Zr Strengthen near-β alloys by –solid solution – Fe,Nb,V –Hall-Petch –cold work Uses: highly formable –Landing gear –Auto bodies

© Imperial College London Page 10 Β-Ti Alloys: Survey Strength and Selection of β-Ti alloys Ti – 10 V – 2Fe – 3Al Ti – 3Al – 8V – 6Cr – 4Mo – 4Zr Ti – 15V – 3Cr – 3Al – 3Sn Landing Gear Springs (Beta C) Springs Ti – 4.5Fe – 6.8Mo – 1.5AlLow Cost Beta (LCB) Development of Beta C σyσy Eσy/ρσy/ρ

© Imperial College London Page 11 Spring Applications

© Imperial College London Page 12 Microstructure of Ti Deformation Processing, subgrains and microstructure control

© Imperial College London Page 13 Deformation Processing Rough forge in B condition – softer Age to A+B condition, develop grain boundary α Finish forge in A+B condition – break down and dynamically recover β grains, forming subgrains, avoid β grain growth Cool – may precipitate some intragranular lath-α

© Imperial College London Page 14 Forging Summary

© Imperial College London Page 15 Low Cost Beta & the Auto Industry LCB in the auto industry Uses: VW Lupo FSI (sport version) suspension arms (~£10k) Derived from Beta-C, an aerospace alloy used on Boeing cargo bay door springs Beta alloys expensive partially because of the pure alloys contained About 60 percent lower cost in finished-wire form than Beta-C Use of an iron-molybdenum (ferromoly) master alloy. Directly derived from the ore -> saves cost over aerospace alloys.

© Imperial College London Page 16 β-flecks Major problem in β-Ti alloys is the production of β-flecks, region of high β content due to solute segregation in the melt of the VIM billet Similar to freckles in Ni alloys Mechanisms unclear

© Imperial College London Page 17 ω phase

© Imperial College London Page 18 ω phase – Orientation Relationship Transformation of beta -> omega

© Imperial College London Page 19 ω phase ω phase result of low stiffness in directions and presence of a soft phonon mode

© Imperial College London Page 20 ω phase – TEM patterns ω phase gives daughter peaks in the bcc diffraction pattern, plus diffuse scattering

© Imperial College London Page 21 ω phase - Consequences Present as ~10nm inclusions (like gamma prime in nickel?) Generally felt to be embrittling Observed in most beta alloys as an alternative to alpha MAY be possible to use as the basis for superelasticity in Ti alloys

© Imperial College London Page 22 P/M Ti alloys? Have been produced: would be useful in e.g. controlling grain size in pure β forging BUT: problem of avoiding oxide layer on powder particles and consequent TiO 2 and α inclusions

© Imperial College London Page 23 SPF Fan Blades: An Aero use of CP Ti Β-Ti Alloy Design – reminder of α & β stabilisers Strength and Selection of β-Ti alloys Deformation Processing, subgrains and microstructure control LCB in the auto industry β-flecks ω phase and the β→ω transition. Properties of ω-containing alloys Final thoughts: P/M Ti alloys? Synopsis