Light Alloys- Effects of Alloying Elements, Alpha and Near-Alpha Alloys

Question 1

Structure of α and β phases of Ti

Answer 1

α is hcp
β is bcc
Beta transus temperature at 883C separates β from α+β regions on phase diagram.

Question 2

Alpha stabilisers and effect on phase diagram

Answer 2

O, Al, N, C
O and Al most important
Increase beta transus (steeper positive gradient on phase diagram)

Question 3

Beta stabilisers and effect on phase diagram

Answer 3

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

Question 4

Neutral alloying elements

Answer 4

So and Zr
No effect on beta transus

Question 5

Effect of substitutional additions

Answer 5

Like Al, Sn and Zr. Increase UTS by 35 to 70MPa per weight% added. Less effect than interstitials

Question 6

Effect of interstitial additions

Answer 6

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

Question 7

What is oxygen equivalence?

Answer 7

OE=2[N]+[O]+2/3[C] in wt%
Used for Vickers hardness (kg/mm2):
=75+310(OE)

Question 8

What is the addition of α-stabilisers limited by?

Answer 8

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.

Question 9

What is aluminium equivalence?

Answer 9

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

Question 10

Describe the pseudo binary phase diagram

Answer 10

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

Question 11

What does relation between the curves on the pseudo binary phase diagram mean when you add more β stabilisers?

Answer 11

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

Question 12

Confusion about martensitic start and finish temperatures

Answer 12

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

Question 13

Where are alpha alloys in the phase diagram?

Answer 13

Very thin rectangle at left of diagram. Mostly within the α region for most temperatures except highest ones. Commercially pure titanium

Question 14

Features of alpha alloys

Answer 14

Corrosion resistant, weldable. Non heat treatable as their composition is such that β cannot be retained at RT in any form, stable or metastable

Question 15

Where are alpha alloys solution treated?

Answer 15

In the β field and then quench martensite formed is α

Question 16

Methods of strengthening alphas alloys

Answer 16

Solid solution strengthening from oxygen.
Grain refinement where yield strength follows Hall-Petch equation.
Cold work

Question 17

What does ELI stand for in names of Ti alloys?

Answer 17

Extra low interstitial content alloy

Question 18

Where are near alpha alloys on the phase diagram?

Answer 18

Similarly thin rectangle next to alpha alloys. Encloses most of the boundary between α and α+β.

Question 19

What are near alpha alloys good for?

Answer 19

Creep resistance and fatigue resistance

Question 20

Where is the typical hot working range for near alpha alloys and how is this effected by β stabilisers?

Answer 20

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.

Question 21

Why do near alpha alloys still not want too much β phase?

Answer 21

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

Question 22

Major use for near alpha alloys

Answer 22

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

Question 23

Fatigue performance and creep performance vs % β phase at the hot working temperature

Answer 23

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.

Question 24

What is secondary alpha?

Answer 24

Aka transformed β. Is alpha phase but was β phase during hot working

Question 25

Balance of grain size for fatigue and creep performance in near alpha alloys

Answer 25

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.

Question 26

Why is creep resistance good in near alpha alloys?

Answer 26

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

Question 27

Why can near alpha alloys not be used above 600C?

Answer 27

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.