Light Alloys- Effects of Alloying Elements, Alpha and Near-Alpha Alloys Flashcards
Structure of α and β phases of Ti
α is hcp
β is bcc
Beta transus temperature at 883C separates β from α+β regions on phase diagram.
Alpha stabilisers and effect on phase diagram
O, Al, N, C
O and Al most important
Increase beta transus (steeper positive gradient on phase diagram)
Beta stabilisers and effect on phase diagram
V, Mo, Nb, Fe, Si, Cu, Cr, Mn
V most important
Decrease beta transus (more negative gradient). Eutectoid temperature can also arise when using eutectoid formers (like Fe) so beta transus and lower diagonal line end on this temperature. Another line forms v with beta transus on eutectoid temperature
Neutral alloying elements
So and Zr
No effect on beta transus
Effect of substitutional additions
Like Al, Sn and Zr. Increase UTS by 35 to 70MPa per weight% added. Less effect than interstitials
Effect of interstitial additions
Like O, N and C. Increasing O content of air from 0.1 to 0.3 wt% raises the UTS from 500 to 800MPa and reduces elongation to fracture from 40% to 4%. Effects in order of:
N>O>C
What is oxygen equivalence?
OE=2[N]+[O]+2/3[C] in wt%
Used for Vickers hardness (kg/mm2):
=75+310(OE)
What is the addition of α-stabilisers limited by?
Formation of a finely dispersed ordered Ti3Al phase (α2). This α sub 2 phase has D0 sub 19 (Ni3Sn-type) structure and is coherent with α phase so is embrittling.
What is aluminium equivalence?
Gives the limiting addition level of α stabilisers.
AE=[Al]+1/3[Sn]+1/6[Zr]+10[2N+O+C] in wt%
Should be less than 9wt% to avoid α2 formation
Describe the pseudo binary phase diagram
Temperature vs α stabilisers going left or β stabilisers going right. Slightly lower steep concave curve down separates α from α+β. Slightly higher shallower concave curve down separates α+β from β. In between curves is the Ms/Mf temperature curve
What does relation between the curves on the pseudo binary phase diagram mean when you add more β stabilisers?
The separation between the curves gets greater as you go right and decrease temperature. Adding more β stabilisers results in a broader α+β region
Confusion about martensitic start and finish temperatures
They are so close they basically follow the same line. Can get deformation induced martensite below Md but his does not mean you are between Ms and Mf
Where are alpha alloys in the phase diagram?
Very thin rectangle at left of diagram. Mostly within the α region for most temperatures except highest ones. Commercially pure titanium
Features of alpha alloys
Corrosion resistant, weldable. Non heat treatable as their composition is such that β cannot be retained at RT in any form, stable or metastable
Where are alpha alloys solution treated?
In the β field and then quench martensite formed is α
Methods of strengthening alphas alloys
Solid solution strengthening from oxygen.
Grain refinement where yield strength follows Hall-Petch equation.
Cold work
What does ELI stand for in names of Ti alloys?
Extra low interstitial content alloy
Where are near alpha alloys on the phase diagram?
Similarly thin rectangle next to alpha alloys. Encloses most of the boundary between α and α+β.
What are near alpha alloys good for?
Creep resistance and fatigue resistance
Where is the typical hot working range for near alpha alloys and how is this effected by β stabilisers?
Hot working range between 50-100% β. This has a corresponding temperature range. This T range is widened by β stabilisers. If any part of a component goes outside of this temperature range the whole composent gets scrapped. Wider T range is therefore easier to process. 829 has hot working range about 20C but 834 has about 50C.
Why do near alpha alloys still not want too much β phase?
Want majority to be the high T resistant α phase as diffusion is roughly 100x faster in the β phase which leads to poorer creep resistance
Major use for near alpha alloys
Disks and blades in compressor section. Disks are at lower T and must not suffer from fatigue. Where in contact with disks, blades are at lower T and are fatigue limited. Outer parts of blades are hotter and are creep limited. So need balance between creep and fatigue resistance
Fatigue performance and creep performance vs % β phase at the hot working temperature
Fatigue starts high, slow decline until close to β transus (vertical line) then faster decline continuing beyond beta transus.
Creep starts low, increases nearly linearly, becomes more concave as you go right. Crosses fatigue performance just below β transus.
What is secondary alpha?
Aka transformed β. Is alpha phase but was β phase during hot working
Balance of grain size for fatigue and creep performance in near alpha alloys
Small grains (less β during hot working) good for fatigue performance but bad for creep as lots of GBs so GB sliding easier. Large grains therefore good for creep resistance as fewer GBs but cracks easily propagate along GBs for large grains reducing fatigue performance. Large grains can also have lots of dislocations building up near GBs which cause cause fatigue crack initiation. This could be due to processing in single phase β region leading to large prior β grain size. Balance achieved by forging just below the β transus.
Why is creep resistance good in near alpha alloys?
Si pins dislocations inhibiting dislocation creep.
Large prior beta grain size inhibits GB sliding.
Slow diffusion in alpha structure.
Excellent creep resistance up to 600C
Why can near alpha alloys not be used above 600C?
Above 600C Ti loses corrosion and oxidation resistance as it starts to dissolve its own oxide on the surface. Leaves bare metal surface which takes in more oxygen making it more brittle.
