Advances in SPecail Steels : Super alloy ni and ti alloys

Super Alloys
Content:
1) Application
2) Introduction
3) Specific Characteristics of Super Alloy
4) Production of Super Alloys
Introduction to Nickel and its alloys
1) Composition of Ni- Base Super Alloy:
2) Composition–microstructure relationships in nickel
alloys
3) Heat Treatment of Ni-Alloys
Introduction to Ti Alloys
1) Ti –Alloys Composition, Properties and Uses
2) Heat Treatment of Ti- Alloys
1

4
Boeing 777 aircraft engine
• When significant
resistance to
loading under
static, fatigue
and creep
conditions is
required, the
nickel-base
superalloys have
emerged as the
materials of
choice for high
temperature
applications.
• Particularly true
when operating
temperatures are
beyond about
800 ◦C.

• single-crystal superalloys are being used in increasing quantities in the
gas turbine engine.
• creep rupture lives of the single-crystal superalloys about 250 h at 850
◦C/500MPa
• a typical first-generation alloy such as SRR99, to about 2500 h
• the third-generation alloy RR3000. Under more demanding conditions,
for example, 1050 ◦C/150MPa, rupture life has improved fourfold from
250 h to 1000 h.
6

• Fan - The fan is the first component in a turbofan. The large
spinning fan sucks in large quantities of air. Most blades of
the fan are made of titanium.
• Compressor - The compressor is made up of fans with many
blades and attached to a shaft. The compressor squeezes
the air that enters it into progressively smaller areas,
resulting in an increase in the air pressure.
• Combustor - In the combustor the air is mixed with fuel and
then ignited. There are as many as 20 nozzles to spray fuel
into the airstream. The mixture of air and fuel catches fire.
This provides a high temperature, high-energy airflow. The
fuel burns with the oxygen in the compressed air, producing
hot expanding gases. The inside of the combustor is often
made of ceramic materials to provide a heat-resistant
chamber. The heat can reach 2700°.
7

• Turbine - The high-energy airflow coming out of the combustor goes
into the turbine, causing the turbine blades to rotate. The turbines
are linked by a shaft to turn the blades in the compressor and to spin
the intake fan at the front. This rotation takes some energy from the
high-energy flow that is used to drive the fan and the compressor.
The gases produced in the combustion chamber move through the
turbine and spin its blades. The turbines of the jet spin around
thousands of times. They are fixed on shafts which have several sets
of ball-bearing in between them.
• Nozzle - The nozzle is the exhaust duct of the engine. This is the
engine part which actually produces the thrust for the plane. The
energy depleted airflow that passed the turbine, in addition to the
colder air that bypassed the engine core, produces a force when
exiting the nozzle that acts to propel the engine, and therefore the
airplane, forward.
• The combination of the hot air and cold air are expelled and produce
an exhaust, which causes a forward thrust. The nozzle may be
preceded by a mixer, which combines the high temperature air
coming from the engine core with the lower temperature air that was
bypassed in the fan. The mixer helps to make the engine quieter.
8

Introduction
(After World war II)
• The term "super alloy" used to describe a group of alloys
developed for use in
turbo superchargers and aircraft turbine engines that required high
performance at elevated temperatures.
• The range of applications for which super alloys are used has
expanded to many other areas and now includes aircraft and land-
based gas turbines, rocket engines, chemical, and petroleum
plants.
• These alloys have an ability to retain most of their strength even
after long exposure times above 650°C 9
• “Super-alloys are unique high temperature
materials used in gas turbine engines, which
display excellent resistance to mechanical and
chemical degradation”

Super-alloys are based on Group VIIIB elements and
usually consist of various combinations of Fe, Ni, Co, and
Cr, as well as lesser amounts of W, Mo, Ta, Nb, Ti, and Al.
But in general.
The three major classes of Super-alloys are considered:
1. Nickel-based
2. Iron-based
3. Cobalt-based alloys.
• Nickel-based superalloys are used in load-bearing
structures to the highest homologous temperature of any
common alloy system (Tm = 0.9, or 90% of their melting
point).
10

Specific characteristics of Super Alloy
(718 –Ni Base)
1. Good ductility is evidenced at 649°C to 760°C.
2. Yield & tensile strength, creep, and rupture strength properties are
exceedingly high at temperatures up to 705°C.
3. The unique property of slow aging response permits heating and
cooling during annealing without the danger of cracking. Problems
associated with welding of age hard-enable alloys are eliminated
with Alloy 718.
4. Fracture toughness tests have been conducted on the alloy in
forms other than tubing at temperatures from –195°C to 538°C
with excellent values reported.
11

Production of Super Alloy – (Summary)
The following is a summary of processes for the manufacture of typical super
alloys:
1. Vacuum Induction Melting:
• Vacuum induction melting is used as the standard melting practice for the
preparation of superalloy stock.
• Raw metallic materials including scrap are charged into a refractory crucible
and the crucible is maintained under a vacuum during melting of the charge.
• Typically, more than 30 elements are refined or removed from the superalloy
melt during VIM processing. A final step involves the transferring of the liquid
metal from the crucible into a pouring system, and the casting of it into
molds under a partial pressure of argon.
12

2. Investment Casting:
• The investment casting or 'lost-wax' process is used for the
production of superalloy components of complex shape, e.g. turbine
blading or nozzle guide vanes.
• A wax model of the casting is prepared by injecting molten wax into a
metallic 'master' mold. These are arranged in clusters connected by
wax replicas of runners and risers; this enables several blades to be
produced in a single casting.
• Next, an investment shell is produced. Finally, the mold is baked to
build up its strength. After preheating and degassing, the mold is
ready to receive the molten superalloy, which is poured under
vacuum.
• After solidification is complete, the investment shell is removed and
the internal ceramic core leached out by chemical means, using a
high pressure autoclave.
13

3. Secondary Melting (Vacuum Arc Remelting / Electroslag Remelting )
• For many applications, a secondary melting process needs to be applied to
increase the chemical homogeneity of the superalloy material and to reduce
the level of inclusions. (Normally Super alloys double or tripled VAR)
• After VIM processing, it would be normal for the cast ingot to possess a
significant solidification pipe and extensive segregation. Removing the pipe
reduces productivity and the segregation can lead to cracking and fissuring
during subsequent thermal-mechanical working.
• Furthermore, VIM can leave non-metallic (ceramic) inclusions present in the
material which can be harmful for fatigue properties.Application of the
secondary melting practices can reduce the problems associated with these
effects. Vacuum arc remelting (VAR) or electroslag remelting (ESR) are used
for this purpose, sometimes in combination with each other.
14

4. Powder Metallurgy:
• P/M is an expensive processing route but it is used for producing
heavily alloyed alloys with acceptable chemical homogeneity.
• The need for powder metallurgy (P/M) first arises for the production
of some high-integrity superalloy components such as turbine discs.
15

5. Heat Treatment:
• The term "heat treatment" when applied to superalloys may mean
many different processes, including
1. stress relieve annealing,
2. in process or full annealing,
3. solution treating,
4. Precipitation hardening.
• In-process annealing may be used after welding to relieve stress or in
between severe forming operations.
• Full annealing is used to obtain a fully recrystallized, soft and ductile
structure.
• Solution treating is done to dissolve second phases so that the
additional solute is available for precipitation hardening.
• Precipitation hardening (also called age hardening) is used to bring
out strengthening phases and to control carbides and the
topologically close packed phases.
6. Applications of coatings also involve exposures to elevated
temperatures.
16

An Introduction to Nickel and its alloys
• Nickel is the fifth most abundant element on earth.
• The atomic number is 28, weight is 58.71
• The crystal structure is face-centred cubic (FCC), from
ambient conditions to the melting point, 1455 ◦C, which
represents an absolute limit for the temperature capability
of the nickel-based superalloys.
• The density under ambient conditions is 8907 kg/m3. Thus,
compared with other metals used for aerospace
applications, for example, Ti (4508 kg/m3) and Al (2698
kg/m3), Ni is rather dense.
17

Composition of Ni- Base Super Alloy:
• The compositions of most important nickel-based superalloys is given
in the table.
• One can see that the number of alloying elements is often greater
than ten and consequently, if judged in this way, the superalloys are
amongst the most complex of materials engineered by man.
• Nickel 50.0-55.0%
• Chromium 17.0-21.0%
• Niobium + Tantalum
4.75-5.50%
• Molybdenum 2.80-3.30%
• Aluminum 0.20-0.80%
• Titanium 0.65-1.15%
• Carbon 0.08% max
• Silicon 0.350% max
• Manganese 0.350% max
• Sulfur 0.015% max
• Copper 0.300% max
• Phosphorus 0.015% max
• Cobalt 1.00% max.
• Iron Balance
Alloy 718
18

• Certain superalloys, such as IN718 and IN706, contain significant
proportions of iron, and should be referred to as nickel–iron superalloys.
• the behaviour of each alloying element and its influence on the phase
stability depends strongly upon its position within the periodic table as
shown in fig 1.
• Fig.1:
Categories of
elements
important to
the constitution
of the nickel-
based
superalloys, and
their relative
positions in the
periodic table.
Composition–microstructure relationships in nickel alloys

• A first class of elements includes nickel, cobalt, iron, chromium,
ruthenium, molybdenum, rhenium and tungsten; these prefer to
partition to the austenitic γ and thereby stabilize it. These elements
have atomic radii not very different from that of nickel.
• A second group of elements, aluminum, titanium, niobium and
tantalum, have greater atomic radii and these promote the formation
of ordered phases such as the compound Ni3(Al, Ta, Ti), known as γ’
• Boron, carbon and zirconium constitute a third class that tend to
segregate to the grain boundaries of the γ phase, on account of their
atomic sizes, which are very different from that of nickel. Carbide and
boride phases can also be promoted. Chromium, molybdenum,
tungsten, niobium, tantalum and titanium are particularly strong
carbide formers; chromium and molybdenum promote the formation
of borides.
20

• The microstructure of a typical superalloy consists therefore of different
phases, drawn from the following list.
(i) The Gamma phase, denoted γ . This exhibits the FCC structure, and in
nearly all cases it forms a continuous, matrix phase in which the other
phases reside. It contains significant concentrations of elements such as
cobalt, chromium, molybdenum, ruthenium and rhenium, where these are
present, since these prefer to reside in this phase.
(ii) The Gamma Prime precipitate, denoted γ’ .This forms as a precipitate
phase, which is often coherent with the γ -matrix, and rich in elements such
as aluminium, titanium and tantalum. In nickel–iron superalloys and those
rich in niobium, a related ordered phase, γ’’ , is preferred instead of γ’
(iii) Carbides and borides. Carbon, often present at concentrations up to 0.2
wt%, combines with reactive elements such as titanium, tantalum and
hafnium to form MC carbides. During processing or service, these can
decompose to other species, such asM23C6 and M6C,which prefer to reside
on the γ –grain boundaries, andwhich are rich in chromium, molybdenum
and tungsten. Boron can combine with elements such as chromium or
molybdenum to form borides which reside on the γ –grain boundaries.
21

Heat treatment of Ni-Alloys
• Following are the treatments:
1. Annealing:
• Produce recrystallized grain structure.
• To soften the work hardened Ni- alloy.
• Normally done from 700 to 1200°C (Depending upon alloy)
2. Stress Relieving
• given to work hardened and non-age hardened Ni-alloys.
• For reduce or remove residual stresses.
• Normally done from 430 to 870°C.
3. Stress Equalizing
• Low temperature HT process.
• Given to Ni-alloys to balance stresses in cold worked alloys without
affecting mechanical properties.
22

4. Solution Treating and age hardenig
• Some Ni-Alloys solution treatment and age hardening treatment are
summarized below.
23
Alloy Solution
Treatment
Ageing
Nimonic 90
15%Cr.
0.85%Co,2.5%Ti,1.5%Al,0.05%C
8-12 Hrs at 1080-
1180°C, Air
cooling
12-16 Hrs at 700-
850°C, air cooled
Inconel X
15%Cr,
18%Co,6.75%Fe,0.8%Al,2.5%Ti,
0.7%Mn,0.04%C
2-4 Hrs at 1165°C,
Air cooling
24Hrs, at 860°C air
cooled followed by
20 hrs at 740°Cair
cooled.
Hastelloy B
1%Cr,2.5%Co,23-
30%Mo,2.6%V,1.0%Si,1.0%Mn,
0.05%C
1200°C air cooled 24 Hrs at 860°C, air
cooled

Example:
1. Solution treatment and age hardening:
• Nimonic alloys are heated to 1080-1200°C for about
10hrsand subsequently aged at 700-850°C for 10-16 Hrs.
2. Result:
• A super saturated (gamma) γ-solution with FCC lattice
formed. Upon ageing, the supersaturated γ-solid solution
decomposes and fine precipitates of (gamma prime) γ’-
phase, i.e. Ni(Al) and etc. compounds are formed.
Other important features: (Brittleness in Ni Alloy)
• On longer ageing at 850-900°C, the stable (Eta) η-phase
(Ni3Ti) is formed which is of Hexagonal type and may cause
embrittlement to the alloy.
24

Titanium Alloys
Content
• Introduction
• Ti –Alloys Composition, Properties and Uses
• Heat Treatment of Ti- Alloys
25

Introduction:
• Ti (Alloys) gives variety of light weight strong materials with good
fatigue and corrosion resistance.
• Ti alloys is used as substitute of AL alloy in aircraft structure in the
temperature range 200-500 °C.
• Two allotropic form
1) Alpha (α) Ti = HCP up to 882 °C.
2) Beta (β) Ti = BCC, stable above 882 °C.
Effect of Alloying addition:
• Al is α stabilizer, when Al is added the α to β transformation
temperature is raised.
• Cr, Mo, V, Mn and Fe are β stabilizer, when these are added the α
to β transformation temperature is lowered.
• The relative amount of α and β stabilizing elements in Ti-Alloys,
and the Heat Treatment determine whether its microstructure
would be mainly single α or single β or mixture of α –β over the
range of desired temperature. 26

• On the basis of phases present, Ti alloy may be of three (3)
basic types namely, α, β and. α-β
1. α- Alloys show excellent weldability, good strength at
high and low temperature, and stability at moderate
temp: for sufficient long duration.
2. α-β Alloys are two phase alloy at room temperature and
are stronger than α- Alloys .
3. β-Alloys retain their structure at room temp: and can be
age hardened to give high strength.
4. Other is near-α Alloy containing mainly α- stabilizing
element + less than 2% β-Stabilizing element.
• Following Table shows composition, properties and uses of
some heat treatable Ti-Alloys.
27

28
Alloy Type Form UTS
(MPa)
Yield
(MPa)
Elongation Uses
α- Alloys
Ti-5Al-2.5Sn
Sheets, bars &
Forgings
800 760 10 Compressor Blades and
Welded assemblies
Near α- Alloys
Ti-6Al-3Mo-1Zr-Si
Bars & Forgings 1200 6 Compressor Blades and Dies
α-β Alloys
Ti-6Al-4V
Sheets, bars &
Forgings
1200 1060 8 Pressure Vessels, air frame
and engine part
α-β Alloys
Ti-4Al-4Mo-4Sn-Si
Bars & Forgings 1400 1250 10 Air frame structural Forging
β-Alloys
TI-13V-11Cr-3Al
Sheets, bars &
Forgings
920 850 10 Fasteners, rivets, sheet metal
part and tubing.
Table: Ti-Alloys (heat treatable) : Composition, Properties and Uses

Heat treatment of Ti-Alloy
• Purpose of heat treatment of Ti-Alloys are:
1. To reduce residual stress developed fabrication operation,
2. To get optimum combination of ductility, machinability, and dimensional
and structural stability,
3. To obtained improved strength and specific Mechanical properties such as
fracture toughness, fatigue strength and creep resistance.
• α and near- α Ti-alloys are subjected to stress relieving and annealing. High
strength can not be obtained in α and near- α Ti-alloys by HT.
• The commercial β-alloys respond to HT. Ageing at elevated temperature
after solution treatment results in the decomposition of β-phase, and hence
strengthening occurs.
• α – β alloy are two phase alloys and are most popular of the three types of
Ti-alloys.
• The following three HT process are adopted for Ti-alloys.
1. Stress Relieving
2. Annealing
3. Solution Treating and ageing.
29

1. Stress Relieving
• Ti and its alloys are stress relieved to minimized the undesirable residual
stress due to cold working, non-uniform hot forging and solidification.
• Components can be cooled from stress relieving temperature either by air
cooling or slow cooling.
• Following table shows Stress relieving temperature and time for Ti and its
alloys.
30
Alloy Temperature Range
(°C)
Time
(hr)
Commercially Pure Ti 480-590 1/4 -4
α or Near α- Alloys
Ti-8Al-1Mo-1V
Ti-6Al-2Cb-1Ta-0.8Mo
590-700
600-650
1/4 -4
1/4 -2
α-β Alloys
Ti-6Al-4V
Ti-3Al-2.5V
Ti-8Mn
485-645
540-650
480-590
1-4
1/2 -2
1/4 -2
β-Alloys
Ti-13V-11Cr-3Al
Ti-10V-2Fe-3Al
710-730
680-700S
1/2 -1/4
1/2 -2

2. Annealing
• For = improves fracture toughness, ductility, dimensional and thermal stability
and creep resistance.
• Different types of annealing is given to Ti-alloys. (i) mill annealing (ii) duplex
annealing (iii) triplex annealing (iv) recrystallization annealing and (v) beta
annealing.
• Following table shows annealing temperature and time for Ti and its alloys.
31
Alloy Temperature Range
(°C)
Time
(hr)
Commercially Pure Ti 650-760 1/10-2
α or Near α- Alloys
Ti-8Al-1Mo-1V
Ti-6Al-2Cb-1Ta-0.8Mo
785
795-900
1-8
1-4
α-β Alloys
Ti-6Al-4V
Ti-3Al-2.5V
710-790
650-760
1-4
1/2-2
β-Alloys
Ti-13V-11Cr-3Al
Ti-15V-3Al-3Cr-3Sn
710-790
790-810
1/6 -1
1/12 -1/4
Cooling
Medium
Air
Air or Furnace
Air
Air or Furnace
Air
Air or Water
Air

32
3. Solution Treating and Ageing
• α-β and β-alloys are solution treated and aged to obtain a wide
range of strength level…..
How strength achieved ?
• Β-phase is unstable at low temperature b/c it is high temp: phase.
• Higher ratio of β is produced by heating an α-β alloys to the
solution treating temperature, upon quenching proportion of the
phases are maintained.
• During subsequent ageing the decomposition of metastable β-
phase takes place….this result improves the strength level.
Quenching conditions ?
• α-βalloys are quenched in water or a 5% brine or caustic soda
solution.
• β alloys are air quenched.

Example:
• Ti-6Al-4V (α-β alloy) is solution treated at 995-970°C for 1Hour followed
by water quench. This is aged b/w 480-590°C for 4-8 Hours.
• This treatment gives maximum tensile strength properties to the
alloy.
• This alloy contains α’ (Ti- Martensite) which has a needle like
structure.
Brittleness in Ti-Alloy (Omega, ω-Phase)
• When β-phase transforms to a metastable transition phase termed as
ω-Phase. (Due to more rapid quenching and fast reheating to ageing
temperature above 430°C)
• This is generally observed in highly β- stabilized α-β alloys.
• This phase introduces brittleness in Ti alloys.
How ω-Phase is to be suppressed?
1. Avoid rapid quenching and fast reheating to ageing temperature
above 430°C.
2. Addition of Al, Mo and Sn in the alloys prevent the formation of ω-
Phase. 33

Advances in SPecail Steels : Super alloy ni and ti alloys

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