Light Alloys- Titanium Alpha+Beta, Near Beta and Beta Alloys Flashcards
Where are alpha+beta alloys on the phase diagram?
Quite a wide region in the middle. Starts right of the boundary between α and α+β and ends about where Ms/Mf line ends
General features of alpha+beta alloys
Superplastic
Good combination of mechanical properties
Still mostly α at RT
Ti-6Al-4V forgings
Is an alpha+beta alloy. Used for bulkheads in mid fuselage. After forging, the Ti shapes are extensively machined. Forgings weigh about 1500kg before machining and just over 140kg afterwards
Microstructure of alpha+beta alloys from super β transus forge and furnace cool or slow air cool
Furnace cool: slowest, α at prior β GBs, lamellae α and solute enriched retained β separating the intragranular α plates within prior β grains.
Slow air cool: less GB ανand larger number of variants of lamellar α colonies within each β grain
Microstructure of alpha+beta alloys from super β transus forge and medium air cool or rapid air cool
Medium air cool: get a basketweave structure of lamellar α.
Rapid air cool: get Widmanstatten array of α plates in prior β grains, looks like lots of thick needles, similar to basketweave but more contrast between phases.
Microstructure of alpha+beta alloys from super β transus forge and water quench
Fully martensitic structure (lots of fine needles). Still α phase
Microstructure of alpha+beta alloys from sub β transus forging (high in α+β) and air cool
Bimodal structure (looks like islands of one phase in another). Primary α and transformed β (fine secondary lamellar α and solute enriched retained β within prior β grains)
Microstructure of alpha+beta alloys from sub β transus forging or superplastic forming and air cool
This is at lower T than the sub transus forging (high in α+β).
Get equiaxed duplex structure of primary α and fine transformed structure in prior β grains
Where are near β alloys on the phase diagram?
Near beta is when the Ms/Mf temperature is very close to RT. The region is thin and just to the right of the Ms/Mf curve.
General features of near β alloys
Good hardenability (precipitation hardening).
Can get omega phases (ωath and ωiso) that are very embrittling.
Why do near beta alloys contain some Al?
This is an alpha stabiliser and inhibits ωath formation on quenching
Describe the omega phases in near beta alloys
ωath is athermal omega phase. Is hexagonal structure like alpha but very different to alpha and very embrittling.
ωiso is isothermal omega phase. Same crystal structure as ωath but different composition (depleted in V and Fe for Ti-10V-2Fe-3Al). Still embrittling
What happens when you process near β alloys above the β transus and what is the balance for cooling rate?
α forms along GBs which reduces ductility. Need cooling rate to be fast enough to avoid this GB alpha but not too fast to induce large quenching stresses which cause deformational stress-induced martensite. However an intermediate ratio results in formation of some ω phases.
How are near beta alloys actually processed?
Process just below the beta transus to give small volume fraction of primary α to restrict β grain growth. The β phase becomes more enriched in V and Fe (for Ti-10V-2Fe-3Al) so more resistant to forming ω or stress-induced martensite. Cool slowly to get some transformation of β to α but this grows on pre-existing α so no coating of α on prior GBs.
How does precipitation hardening of near beta alloy To-10V-2Fe-3Al work?
Precipitation harden the quenched material. Avoid formation of ωath. ωiso forms, allow it to grow then heat further to transform it into αiso to give fine precipitate dispersion. Age harden between 450C and 550C to precipitate secondary alpha