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
What strength do you need for the undercarriage of aircraft?
1100MPa
Why is Ti-10V-2Fe-3Al hard to use for thick sections?
This is the strongest available and is a near beta alloy. There is a very different microstructure when processing at 750C compared to 790C. Only 40C causes a big difference. Thick sections will have different temperatures throughout their thickness and so will not have a constant microstructure throughout its thickness
Where are beta alloys on the phase diagram?
Quite wide region to right of near beta alloys. Beta transus ends sort of mid way along it
General features of beta alloys
Cold formable. Omega transformation suppressed.
Soft, ductile, higher density, weldable, age hardening must be after welding, poor high T properties due to over ageing.
How does microstructure influences tensile stress?
Grain size or effective structural unit size (e.g α colonies) determines strength via Hall-Petch relationship. As grain or α colony size decreases, yield strength increases due to greater number of phase boundaries interfering with slip.
Interstitial elements like O, N, C have very potent SS strengthening effect and deleterious effect on ductility.
Age hardening o soft beta phase in near β alloys precipitates fine scale α which is an effective way of hindering slip and increasing strength.
Microstructure influence on fracture toughness
Lenticular morphologies provide a tortuous (twists and turns) path for cracks. Elongated colony α or Widmanstatten α enhance fracture toughness due to this. α/β interfaces are important points of crack deflection.
Why does enhancing fracture toughness not necessarily increase fatigue resistance? What does it do though?
Microstructural features which enhance fracture toughness will increase the critical length at which a fatigue crack becomes unstable leading to rapid growth and fast fracture. May not increase fatigue resistance as it depends on how long it takes the crack to grow to the critical length
The two competing processes for fatigue resistance
Resistance to crack initiation.
Resistance to crack propagation during cyclic loading
What are fatigue testing conditions component specific with respect to?
Material starting condition.
Stress range.
Nature and frequency of loading.
Environment
Fatigue crack initiation resistance
Crack initiation behaviour determined by the activated mode of deformation. Localised slip bands (planes of high shear stress) and mechanical twins are sources of crack initiation sites during cyclic loading. Small grain and α colony sizes suppress slip band formation and so are preferred if crack initiation resistance is the primary design requirement.
Why must materials engineers be aware of the texture of the component?
Adjacent α grains with the same strong crystallographic orientation can act as a large effective grain (macrozone) leading to poor fatigue crack initiation performance
Fatigue crack initiation resistance in near beta alloys
Near beta alloys with high YS (up to 1500MPa) and thus increased resistance to dislocation motion have a high resistance to crack initiation. This is why they are used in large landing gear components where high cycle (>10^4 cycles) fatigue resistance is imperative. Surface operations like shot peening generate near surface residual compressive stresses that suppress crack initiation.
Fatigue crack propagation resistance
Microstructure that enhance fracture toughness resistance will also increase resistance to crack growth due to convoluted crack path provided by a Widmanstatten α morphology. Cracks can propagate through the hcp α phase as well as along α/β boundaries. Crystallographic texture can be engineered during the processing stage to generate enhanced fatigue resistance - crack propagation can be arrested if confronted by α grains that are badly oriented for slip.
How do properties change as you go through from alpha to beta alloys on the phase diagram?
Density increases
Increasing heat treatment response
Higher short-term strength
Lower creep resistance
Increasing strain rate sensitivity
Reduced weldability
Improved fabricability
