Chapter 2: Titanium alloys

Question 1

Properties of titanium Material

Answer 1

Light weight (4.5 g/cm3) (compared to steel)
 High strength to weight ratio
 Good properties at elevated temperature
 Excellent corrosion resistance (forms titanium oxide)
 Good high-temperature strength
 Allotropic transformation (α  β)

Question 2

What is an allotropic material?

Answer 2

An allotropic material is a material that has 2 crystal structure

Question 3

Why is Beta alloys prefered compared to alpha alloys?

Answer 3

More manageable properties
•Lower packing density
•Stronger
•Can be transformed (β→α)
•Lamellar, acicular or needlelike (martensite)

Question 3

Why is Beta prefered compare to alpha structure

Answer 3

More manageable properties
•Lower packing density
•Stronger
•Can be transformed (β→α)
•Lamellar, acicular or needlelike (martensite)

Question 4

Behaviour of near alpha titanium alloys

Answer 4

Not heat treatable and not weldable

Medium strength, good creep strength and good corrosion resistance

Question 5

Behaviour of Alpha + Beta titanium alloys

Answer 5

Heat treatable, good forming properties

High strength and good creep strength

Question 6

Behaviour of Beta titanium alloys

Answer 6

Low ductility
High strength
Very heat treatable and readily formable

Question 7

List two near alpha stabilizers

Answer 7

Tin and Zirconium

Question 8

What effects and properties does near alpha stabilizers offers?

Answer 8

It does not affect the transformation temperature

and it has excellent creep resistance at high temperatures

Question 9

List all the alpha stabilizers

Answer 9

1. Aluminium
2. Nitrogen
3. Carbon
4. Oxygen

Question 10

What effects does alpha stabilizers offers?

Answer 10

It increases the transformation temperature

Question 11

List all the Beta stabilizers

Answer 11

1. Vanadium
2. Tantalum
3. molybdenum
4. Niobium

Question 12

What effects does Beta stabilizers offers?

Answer 12

It decreases the transformation temperature

Also it becomes present at room temperatures

Question 13

How do you improve the properties of alpha alloys

Answer 13

1. By work hardening

2. Applying solid solution by adding tin and aluminium

Question 14

What does the imaginary line represent?

Answer 14

fast cooling

Question 15

What happens when Beta alloys are rapidly cooled

Answer 15

There will be a form of needle-like alpha grains

Question 16

What happens when Beta alloys are slowly cooled

Answer 16

There will be a form of plate-like alpha grains

Question 17

List the properties of Alpha + Beta titanium alloys

Answer 17

Excellent	high	temperature creep strength	
– Excellent ductility	
– Excellent toughness	
– High tensile	strength	
– Good fatigue resistance

Question 18

How are alpha + Beta made?

Answer 18

Proper balance of α and β stabilisers

Question 19

Why add alpha and Beta alloys to form Alpha + Beta alloys

Answer 19

Alpha is to increase the transformation temperature of a low alloy content and Beta is to increase the strength

Question 20

Apha + Beta alloys features a superplastic behaviour which allows them to

Answer 20

-It is deformed to high strain ( 1000%) without the formation of unstable
neck
 Complex shapes can be produced out of a single piece with fine details and
close tolerance
 Eliminate secondary operation
 Weight and materials saving because of formability of materials
 Little residual stress occurs in the formed component because grains are not deformed.

Question 21

What are the conditions for the superplastic form

Answer 21

Temperature > 0.5 Tm
Slow strain rate ~10-3
Grains (60 – 75% α) must be equiaxed and small

Question 22

Why are the grains must be small and equiaxed for superplastic forming

Answer 22

equiaxed allow easy grain switching and small size reduce diffusion distance, greater grain boundary sliding

Question 23

What are the advantages of superplastic forming

Answer 23

Complex shapes in a single process
• Reduces weight and cost
• Shorter production lead times
• Elimination of machinery operations

Question 24

List 4 ways to control the microstructures of titanium alloys

Answer 24

Slow cooling -Diffusion and rearrangement (from bcc to hcp), platelike (Widmanstatten) structure formed Faster cooling -Transformed product (hcp α) has more needle-like “interwoven” form (basketweave) Very fast cooling -Diffusionless reaction giving extremely fine needles called martensite Highly alloyed -Reaction suppressed giving metastable β – Can be age-hardened to very high strength

Question 25

Explain why alpha titanium alloy cannot be heat treated

Answer 25

Alpha titanium alloys consist mainly of the α phase • Alpha titanium is non heat treatable at the temperature below the allotropic transformation temperature • During fast cooling, needle-like α forms while during slow cooling, plate-like α forms • The microstructure remains as α titanium single phase and stable throughout • The properties do not respond to heat treatment processes • Thus alpha titanium alloy cannot be heat treated.

Question 26

Discuss the relationship between grain size, amount of | α, lamellar α+β and the various properties

Answer 26

Grain size-reduction improves proof strength, UTS and resistance to fatigue crack initiation The specific amount of α optimises fatigue response and creep behaviour Lamellar α+β promotes toughness, creep strength and fatigue crack resistance

Question 27

Discuss superplastic formation (SPF) of a titanium alloy

Answer 27

It is deformed to high strain ( 1000%) without formation of unstable neck  Complex shapes can be produced out of a single piece with fine details and close tolerance  Eliminate secondary operation  Weight and materials saving because of formability of materials  Little residual stress occurs in the formed component because grains are not deformed.

Question 28

Compare and contrast alpha phase to Beta phase

Answer 28

ALPHA -Directional properties • High packing density • Can be difficult to work BETA ``` More manageable properties • Lower package density • Stronger • Can be transformed (heat treatable) • Different appearance: Lamellar, acicular or needlelike (martensite) ```