Chapter 2: Titanium alloys Flashcards
Properties of titanium Material
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)
What is an allotropic material?
An allotropic material is a material that has 2 crystal structure
Why is Beta alloys prefered compared to alpha alloys?
More manageable properties •Lower packing density •Stronger •Can be transformed (β→α) •Lamellar, acicular or needlelike (martensite)
Why is Beta prefered compare to alpha structure
More manageable properties •Lower packing density •Stronger •Can be transformed (β→α) •Lamellar, acicular or needlelike (martensite)
Behaviour of near alpha titanium alloys
Not heat treatable and not weldable
Medium strength, good creep strength and good corrosion resistance
Behaviour of Alpha + Beta titanium alloys
Heat treatable, good forming properties
High strength and good creep strength
Behaviour of Beta titanium alloys
Low ductility
High strength
Very heat treatable and readily formable
List two near alpha stabilizers
Tin and Zirconium
What effects and properties does near alpha stabilizers offers?
It does not affect the transformation temperature
and it has excellent creep resistance at high temperatures
List all the alpha stabilizers
- Aluminium
- Nitrogen
- Carbon
- Oxygen
What effects does alpha stabilizers offers?
It increases the transformation temperature
List all the Beta stabilizers
- Vanadium
- Tantalum
- molybdenum
- Niobium
What effects does Beta stabilizers offers?
It decreases the transformation temperature
Also it becomes present at room temperatures
How do you improve the properties of alpha alloys
- By work hardening
2. Applying solid solution by adding tin and aluminium
What does the imaginary line represent?
fast cooling
What happens when Beta alloys are rapidly cooled
There will be a form of needle-like alpha grains
What happens when Beta alloys are slowly cooled
There will be a form of plate-like alpha grains
List the properties of Alpha + Beta titanium alloys
Excellent high temperature creep strength – Excellent ductility – Excellent toughness – High tensile strength – Good fatigue resistance
How are alpha + Beta made?
Proper balance of α and β stabilisers
Why add alpha and Beta alloys to form Alpha + Beta alloys
Alpha is to increase the transformation temperature of a low alloy content and Beta is to increase the strength
Apha + Beta alloys features a superplastic behaviour which allows them to
-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.
What are the conditions for the superplastic form
Temperature > 0.5 Tm
Slow strain rate ~10-3
Grains (60 – 75% α) must be equiaxed and small
Why are the grains must be small and equiaxed for superplastic forming
equiaxed allow easy grain switching and small size reduce diffusion distance, greater grain boundary sliding
What are the advantages of superplastic forming
Complex shapes in a single process
• Reduces weight and cost
• Shorter production lead times
• Elimination of machinery operations
List 4 ways to control the microstructures of titanium alloys
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
Explain why alpha titanium alloy cannot be heat treated
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.
Discuss the relationship between grain size, amount of
α, lamellar α+β and the various properties
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
Discuss superplastic formation (SPF) of a titanium alloy
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.
Compare and contrast alpha phase to Beta phase
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)
