David Dye Department of Materials, Imperial College Royal School of Mines, Prince Consort Road,...

© Imperial College London

David Dye

Department of Materials, Imperial College

Royal School of Mines, Prince Consort Road, London SW7 2BP, UK

+44 (207) 594-6811, [email protected]

Engineering Alloys (307) Lecture 9Titanium II: β-Ti Alloys


Review: Titanium I (L7)

α-Ti Alloys

near-α microstructure

α/β microstructure

Casting PhaseDiagram


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?


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 – 10-5 and ~900 C

• near-CP α-Ti alloy (easier diffusion)

• Essentially a grain-boundary sliding creep process


Fan Blade Technology

+ 4% efficiency+ 4% efficiencyClappered Wide-chord fan


Wide-chord Fan Technology

Honeycombconstruction

1st generation:1984

2nd generation:1995

DB/SPFconstruction


Fan Section


Swept Fans


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


Β-Ti Alloys: Survey


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 – 1.5Al Low Cost Beta (LCB)

Development of Beta C

970-1170

825

780-1050

950-1400

105

88

70-103

110

210-250

171

163-219

250-290

σy E σy/ρ


Spring Applications


Microstructure of Ti-10-2-3



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


Forging Summary


Low Cost Beta & 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.


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


ω phase


ω phase – Orientation Relationship

• Transformation of <111> beta -> <0001> omega


ω phase

• ω phase result of low stiffness in <111> directions and presence of

a soft phonon mode


ω phase – TEM patterns

• ω phase gives daughter peaks in the bcc diffraction pattern, plus

diffuse scattering


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


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 TiO2 and α inclusions


• SPF Fan Blades: An Aero use of CP Ti

• Β-Ti Alloy Design – reminder of α & β stabilisers




• β-flecks

• ω phase and the β→ω transition. Properties of ω-containing alloys

• Final thoughts: P/M Ti alloys?

Synopsis

David Dye Department of Materials, Imperial College Royal School of Mines, Prince Consort Road,...

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