Ni-based Superalloy

1. STUDY OF NIMONIC-75 SUPERALLOY
MAINAK SAHA
Roll no:14/MM/65
Department of Metallurgical and Materials Engineering,NIT DURGAPUR,WEST
BENGAL,INDIA
WHAT IS NIMONIC?
Nimonic is a registered trademark ofSpecial Metals Corporation that refers to a family of nickel-
based high-temperature lowcreep superalloys.Nimonic alloys typically consist ofmore than 50%
nickel and 20% chromium with additives such as titanium and aluminium. The main use is in gas
turbine components and extremely high performance reciprocating internal combustion engines.
The Nimonic family of alloys was first developed in the 1940s by research teams at the Wiggin
Works in Hereford, England, in support ofthe development ofthe Whittle jet engine
 COMPOSITION
COURTESY:SPECIAL METALS
 A BRIEF HISTORY:
Working at Inco's Wiggin facility in the United Kingdom, L.B.Pfeil is credited with the
development ofNimonic alloy 80 in 1941,and used in the Power Jets W.2B.Four years later

2. Nimonic alloy 80A followed, an alloy widely used in engine valves today. Progressively stronger
alloys were subsequently developed: Nimonic alloy 90 (1945), Nimonic alloy 100 (1955), and
Nimonic alloys 105 (1960) and 115 (1964 - ProfJohn Gittus FREng. DSc. D Tech 1986.)
 PROPERTIESAND USES
Due to its ability to withstand very high temperatures, Nimonic is ideal for use in aircraft
parts and gas turbine components such as turbine blades and exhaust nozzles on jet
engines, for instance, where the pressure and heat are extreme. It is available in different
grades, including Nimonic 75, Nimonic 80A, and Nimonic 90.[4][5] Nimonic 80a was used for
the turbine blades on the Rolls-Royce Nene and de Havilland Ghost, Nimonic 90 on the
Bristol Proteus, and Nimonic 105 on the Rolls-Royce Spey aviation gas turbines. Nimonic
263 was used in the combustion chambers of the Bristol Olympus used on the Concorde
supersonic airliner.
Nimonic 75 has been certified by the European Union as a standard creep reference
material.alloy 75 (UNS N06075/W.Nr. 2.4951&2.4630) is an80/20 nickel-chromium
alloywith controlledadditions oftitanium andcarbon. First introducedinthe 1940sfor
turbine blades intheprototype Whittlejet engines, it is now mostlyusedfor sheet
applications calling for oxidationandscaling resistance coupledwithmedium strength
at high
operating temperatures. It is still usedingas turbine engineering andalso for industrial
thermal processing, furnace componentsandheat-treatment equipment. It is readily
fabricatedand welded.
 METALLOGRAPHY
NIMONIC alloy75 has a stable, austenitic, solidsolutionatall temperaturesbelow the
solidus and, apart from recrystallizationand graingrowth, is unaffectedby heat
treatment. Typically, three phases are visible:
1) Oxide or silicate inclusions that arise from de-oxidation ofthe moltenmetal, visible
inthe polishedconditionas small, darkinclusions frequentlypresent as stringers
orientedinthe principle workingdirection.
2) Intergranularlyoccurring primarycarbides, nitrides or carbonitrides ofthe general
type M(CN) where M is usuallytitanium. They are visible inthe polished
conditionas substantiallyequiaxed, roughlyspherical o rcuboidinclusions whosecolor
varies from whiteto purple for the carbide, to yellow for the nitride.
3) Chromium-richgrainboundarycarbides, usuallyofthe
type M23C6. The carbides willdissolve intothe matrix withheat, but at a temperature
much higher thanthe normal annealing temperature.
Suitable etchesfor NIMONIC alloy75 are:
1) A 4% aqueous solutionofsulfuric acidused electrolyticallyat approximately4V.

3. 2) Fry’s Reagent (4 g cupric chloridedissolvedin20 cm3
concentratedhydrochloric acidand 20 cm3water).
HEAT TREATMENT
For bar:30-60 minutesat 1050°C (1920°F) followedby air cooling.
For sheet 5-10minutes at 1050°C (1920°F) followedby air coolingor, before welding, 5-
10 min/1050°C(1920°F)/AC + an optional 10min/1050°C(1920°F)/AC
COLD WORKING
The goodductilityand malleabilityofannealedNIMONICalloy75 make it amenable to
many methods ofcold deformation. The alloyissubject to rapidstrainhardening.
Because ofthis increasedstrength, largereductions canbe made without rupture.
WELDING
Material shouldbe inthe annealedcondition beforewelding althoughasmall amount
of
cold workistolerable. The amount of permissiblecoldworkvaries withthe
component designbut simplebending and rollingoperations neednot involve re-
annealing prior to welding. The heat-affected zone producedinNIMONIC alloy75
does
not introduce weld decayand post-weldheat- treatment isnot, normally, necessary. If

4. equipment is requiredfor serviceincontact withcausticsoda, fluosilicates, or some
mercurysalts, astress-relieving treatment couldbe desirable.NIMONIC alloy75 canbe
weldedbythe MMA, TIG, MIG and submerged-arc processes. Matchedcomposition
welding
consumables are availablefrom Special MetalsWelding Products Co. The alloyis
also amenable to electronbeam, oxy-acetylene andresistance welding, and to
brazing and soldering.
PHYSICAL AND MECHANICAL PROPERTIES:
Some physical properties for NIMONIC alloy75 are giveninTables . Thermal
expansiondata inTable were determinedonfullyheat-treatedmaterial. The data are
subject to a variationofapproximately±5% according to processing variables. The
dynamic modulus data inTable 4 were obtainedfrom fullyheat-treatedcylindrical
specimens vibrated inthe flexural mode. The recommendedheat treatmentsare
quotedin the Heat Treatment sectionofthis bulletin. Creep-rupture properties are
shownbyLarson-MillerpresentationinFigure For all these tests, bar material was
heat-treatedfor30-60 minutesat 1050°C (1920°F) andair-cooled, and sheet material
was heat-treatedfor 5-10minutes at 1050°C (1920°F) andair-cooled.
COURTESY:SPECIAL METALS

6. RESULTS OF STRESS-RELAXATION TEST

7. CORROSION RESISTANCE
Oxidationresistance dataare presentedinTables 5 and 6, and in Figure 4. For the
continuous heating tests the specimenswere oxidizedinstill air andthe weight loss was
determinedafter descaling electrolyticallyinNaOH.
*Cooled to room temperature after every 24 hrs

8. No of cycles of operation
MECHANISMS OF STRENGTHENING

9. Courtesy:HKDH BHADESHIA,CAMBRIDGE UNIVERSITY
The Ni-Al-Ti ternary phase diagrams show the γ and γ' phase field. For a given chemical
composition, the fraction of γ' decreases as the temperature is increased. This phenomenon
is used in order to dissolve the γ' at a sufficiently high temperature (a solution treatment)
followed by ageing at a lower temperature in order to generate a uniform and fine
dispersion of strengthening precipitates.
The γ-phase is a solid solution with a cubic-F lattice and a random distribution of the
different species of atoms. Cubic-F is short for face-centred cubic.
By contrast, γ' has a cubic-P (primitive cubic) lattice in which the nickel atoms are at the
face-centres and the aluminium or titanium atoms at the cube corners. This atomic
arrangement has the chemical formula Ni3Al, Ni3Ti or Ni3(Al,Ti). However, as can be seen
from the (γ+γ')/γ' phase boundary on the ternary sections of the Ni, Al, Ti phase diagram,
the phase is not strictly stoichiometric. There may exist an excess of vacancies on one of the
sublattices which leads to deviations from stoichiometry; alternatively, some of the nickel
atoms might occupy the Al sites and vice-versa. In addition to aluminium and titanium,
niobium, hafnium and tantalum partition preferentially into γ'.
The γ phase forms the matrix in which the γ' precipitates. Since both the phases have a
cubic lattice with similar lattice parameters, the γ' precipitates in a cube-cube orientation
relationship with the γ. This means that its cell edges are exactly parallel to corresponding
edges of the γ phase. Furthermore, because their lattice parameters are similar, the γ' is
coherent with the γ when the precipitate size is small. Dislocations in the γ nevertheless find
it difficult to penetrate γ', partly because the γ' is an atomically ordered phase. The order
interferes with dislocation motion and hence strengthens the alloy.
The small misfit between the γ and γ' lattices is important for two reasons. Firstly, when
combined with the cube-cube orientation relationship, it ensures a low γ/γ' interfacial
energy. The ordinary mechanism of precipitate coarsening is driven entirely by the
minimisation of total interfacial energy. A coherent or semi-coherent interface therefore
makes the microstructure stable, a property which is useful for elevatedtemperature
applications.
The magnitude and sign of the misfit also influences the development of microstructure
under the influence of a stress at elevatedtemperatures. The misfit is said to be positive
when the γ' has a larger lattice parameter than γ. The misfit can be controlled by altering
the chemical composition, particularly the aluminium to titanium ratio. A negative misfit
stimulates the formation of rafts of γ', essentially layers of the phase in a direction normal
to the applied stress. This can help reduce the creep rate if the mechanism involves the
climb of dislocations across the precipitate rafts.
The transmission electron micrographs shown below illustrate the large fraction of γ',
typically in excess of 0.6, in turbine blades designed for aeroengines, where the metal
experiences temperatures in excess of 1000oC. Only a small fraction (0.2) of γ' is needed

10. when the alloy is designed for service at relatively low temperatures (750oC) and where
welding is used for fabrication.
Crystal structure of γ
Transmission
electron micrograph
Transmission electron micrograph showing a small fraction of
spheroidal γ' prime particles in a γ matrix. Ni-20Cr-2.3Al-2.1Ti-

11. showing a large
fraction of cuboidal
γ' particles in a γ
matrix. Ni-9.7Al-
1.7Ti-17.1Cr-6.3Co-
2.3W at%. Hillier,
Ph.D. Thesis,
University of
Cambridge, 1984.
5Fe-0.07C-0.005 B wt%. Also illustrated are M23C6 carbide
particles at the grain boundary running diagonally from bottom left
to top right.
Strength versus Temperature
The strength of most metals decreases as the temperature is
increased, simply because assistance from thermal activation makes
it easierfor dislocations to surmount obstacles. However, nickel
based superalloys containing γ', which essentially is an intermetallic
compound based on the formula Ni3(Al,Ti), are particularly
resistant to temperature.
Ordinary slip in both γ and γ' occurs on the {111}<110>. If slip was
confined to these planes at all temperatures then the strength would
decrease as the temperature is raised. However, there is a tendency
for dislocations in γ' to cross-slip on to the {100} planes where they
have a lower anti-phase domain boundary energy. This is because
the energy decreases with temperature. Situations arise where the
extended dislocation is then partly on the close-packed plane and
partly on the cube plane. Such a dislocation becomes locked,
leading to an increase in strength. The strength only decreases
beyond about 600oC whence the thermal activation is sufficiently
violent to allow the dislocations to overcome the obstacles.
To summarise, it is the presence of γ' which is responsible for the
fact that the strength of nickel based superalloys is relatively
insensitive to temperature.
The yield strength of
a particular
superalloy containing
only about 20% of γ'.
The points are
measured and the
curve is a theoretical
prediction. Notice
how the strength is at
first insensitive to
temperature.

12. When greater strength is required
at lower temperatures (e.g. turbine
discs), alloys can be strengthened
using another phase known as γ''.
This phase occurs in nickel
superalloys with significant
additions of niobium (Inconel 718)
or vanadium; the composition of the
γ'' is then Ni3Nb or Ni3V. The
particles of γ'' are in the form of
discs with (001)γ''||{001}γ and
[100]γ''||<100>γ
The crystal structure of γ'' is based
on a body-centred tetragonal lattice
with an ordered arrangement of
nickel and niobium atoms.
Strengthening occurs therefore by
both a coherency hardening and
order hardening mechanism. The
lattice parameters of γ'' are
approximately a=0.362 nm and
c=0.741 nm
X-ray Powder Diffraction
A comparison of the diffraction patterns indicated below reveals
many more peaks from the γ'. The additional reflections are quite
weak in intensity. They arise because the γ' lattice is primitive
cubic, which means that planes such as {100} give rise to diffracted
intensity, whereas the reflections from the corresponding {100}
planes of γ have zero intensity (destructive interference with the
{200} planes). The additional reflections from the γ' prime are
termed superlattice reflections and are weak because they depend
on the difference in scattering power between the Ni and Al atoms.

13. X-ray
diffracti
on
pattern
from γ,
for a
particula
r set of
diffracti
on
conditio
ns.
X-ray
diffracti
on
pattern
from γ',
for a
particula
r set of
diffracti
on
conditio
ns.
Electron Diffraction
The figures below show a superimposed electron diffraction pattern
from γ, γ' and M23C6 carbide. The γ and γ' phases have their cubic-
lattice edges perfectly aligned.

14. Alloy Compositions
Commercial superalloys contain more than just Ni, Al and Ti.
Chromium and aluminium are essential for oxidation resistance

15. small quantities of yttrium help the oxide scale to cohere to the
substrate. Polycrystalline superalloys contain grain boundary
strengthening elements such as boron and zirconium, which
segregate to the boundaries. The resulting reduction in grain
boundary energy is associatedwith better creep strength and
ductility when the mechanism of failure involves grain decohesion.
There are also the carbide formers (C, Cr, Mo, W, C, Nb, Ta, Ti
and Hf). The carbides tend to precipitate at grain boundaries and
hence reduce the tendency for grain boundary sliding.
Elements such as cobalt, iron, chromium, niobium, tantalum,
molybdenum, tungsten, vanadium, titanium and aluminium are
also solid-solution strengtheners, both in γ and γ'.
There are, naturally, limits to the concentrations that can be added
without inducing precipitation. It is particularly important to avoid
certain embrittling phases such as Laves and Sigma. There are no
simple rules governing the critical concentrations; it is best to
calculate or measure the appropriate part of a phase diagram.
Alloying
element
effects in
nickel based
superalloys.
The "M" in
M23C6
stands for a
mixture of
metal atoms.
Click on
chart to
enlarge.

16. Nominal chemical compositions, wt%. MA/ODS ≡ mechanically
alloyed, oxide dispersion-strengthened.
PM ≡ powder metallurgical origin. The alloy names are
proprietary. SX ≡ single crystal.
The single-crystal superalloys are often classified into first, second
and third generation alloys. The second and third generations
contain about 3 wt% and 6wt% of rhenium respectively. Rhenium
is a very expensive addition but leads to an improvement in the
creep strength. It is argued that some of the enhanced resistance to
creep comes from the promotion of rafting by rhenium, which
partitions into the γ and makes the lattice misfit more negative.
Atomic resolution experiments have shown that the Re occurs as
clusters in the γ phase. It is also claimed that rhenium reduces the

17. overall diffusion rate in nickel based superalloys.
The properties of superalloys deteriorate if certain phases known as
the topologically close-packed (TCP) phases precipitate. In these
phases, some of the atoms are arranged as in nickel, where the
close-packed planes are stacked in the sequence ...ABCABC..
However, although this sequence is maintained in the TCP phases,
the atoms are not close-packed, hence the adjective 'topologically'.
TCP phases include σ and μ. Such phases are not only intrinsically
brittle but their precipitation also depletes the matrix from valuable
elements which are added for different purposes. The addition of
rhenium promotes TCP formation, so alloys containing these
solutes must have their Cr, Co, W or Mo concentrations reduced to
compensate. It is generally not practical to remove all these
elements, but the chromium concentration in the new generation
superalloys is much reduced. Chromium does protect against
oxidation, but oxidation can also be prevented by coating the
blades.
ORDER STRENGTHENING OF NIMONIC 75
This is caused due to dissociation of gamma prime ppts on
dislocations leading to formation of APB(anti –phase boundaries)
between Ni-Ni and Al-Al or Ti-Ti sublattices.
Microstructure and Heat Treatment
To optimise properties (often of a
coating--metal system), nickel based
superalloys are, after solution
treatment, heat treated at two
different temperatures within the
γ/γ' phase field. The higher
temperature heat treatment
precipitates coarser particles of γ'.
The second lower temperature heat
treatment leads to further
precipitation, as expected from the
phase diagram. This latter
precipitation leads to a finer,
secondary dispersion of γ'. The net
result is a bimodal distribution of γ',
as illustrated in this figure (courtesy
R. J. Mitchell).

18. Oxide Dispersion Strengthened Superalloys
Oxide dispersion strengthened superalloys can be produced starting
from alloy powders and yttrium oxide, using the mechanical
alloying process. The yttria becomes finely dispersed in the final
product. It is also a very stable oxide, making the material
particularly suitable for elevatedtemperature applications.
However, mechanical alloying is a very difficult process so such
alloys have limited applications. A transmission electron
micrograph showing the oxide dispersion in a mechanically-alloyed
nickel based superalloy is shown below.
ODS
alloy
MA6000
Applications of nickel based superalloys
Turbine Blades

19. A major use of nickel based superalloys is in the manufacture of
aeroengine turbine blades. A single-crystal blade is free from γ/γ
grain boundaries. Boundaries are easy diffusion paths and
therefore reduce the resistance of the material to creep
deformation. The directionally solidified columnar grain structure
has many γ grains, but the boundaries are mostly parallel to the
major stress axis; the performance of such blades is not as good as
the single-crystal blades. However, they are much better than the
blade with the equiaxed grain structure which has the worst creep
life.
One big advantage of the single-crystal alloys over conventionally
cast polycrystalline superalloys is that many of the grain boundary
strengthening solutes are removed. This results in an increase in the
incipient melting temperature (i.e., localised melting due to
chemical segregation). The single-crystal alloys can therefore be
heat treated to at temperatures in the range 1240-1330°C, allowing
the dissolution of coarse γ' which is a remanent of the solidification
process. Subsequent heat treatment can therefore be used to
achieve a controlled and fine-scale precipitation of γ'. The primary
reason why the first generation of single-crystal superalloys could
be used at higher temperatures than the directionally solidified
ones, was because of the ability to heat-treat the alloys at a higher
temperature rather than any advantage due to the removal of grain
boundaries. A higher heat-treatment temperature allows all the γ'
to be taken into solution and then by aging, to precipitate in a finer
form.
Superalloy blades are used in aeroengines and gas turbines in
regions where the temperature is in excess of about 400oC, with
titanium blades in the colder regions. This is because there is a
danger of titanium igniting in special circumstances if its
temperature exceeds 400oC.

20. Single
crystal
Directiona
lly
solidified
columnar
grains
Equiaxed
polycrystall
ine
Engine materials (source:
Michael Cervenka)
Turbine Discs
Turbine blades are attached to a disc which in turn is connected to
the turbine shaft. The properties required for an aeroengine discs
are different from that of a turbine, because the metal experiences a
lower temperature. The discs must resist fracture by fatigue. Discs
are usually cast and then forged into shape. They are
polycrystalline.
One difficulty is that
cast alloys have a
large columnar grain
structure and contain
significant chemical
segregation; the latter
is not completely
eliminated in the final
product. This can lead
to scatter in
mechanical properties.
One way to overcome
this is to begin with
fine, clean powder

21. which is then
consolidated. The
powder is made by
atomisation in an inert
gas; the extent of
chemical segregation
cannot exceedthe size
of the powder. After
atomisation, Some
discs are made from
powder which is hot-
isostatically pressed,
extruded and then
forged into the
required shape. The
process is difficult
because of the need to
avoid undesired
particles introduced,
for example, from the
refractories used in
the atomisation
process, or impurities
picked up during
solidification. Such
particles initiate
fatigue; the failure of
an aeroengine turbine
disc can be
catastrophic.
Powder metallurgical aeroengine disc.
Image provided by M. Hardy of Rolls-
Royce.
Turbochargers
An internal combustion engine generally uses a stoichiometric ratio
of air to fuel. A turbocharger is a device to force more air into the
engine, allowing a correspondingly greater quantity of fuel to be
burned in each stroke. This boosts the power output of the engine.
The turbocharger consists of two components, a turbine which is
driven by exhaust gases from the engine. This in turn drives an air
pump which forces more air into the engine. The typical rate of spin
is 100-150,000 rotations per minute. Because the turbocharger is

22. driven by exhaust gasses, it gets very hot and needs to be oxidation
resistant and strong.
Turbocharger of nickel based
superalloy Inconel 713C, Ni-2Nb-
12.5Cr-4.2Mo-0.8Ti-6.1Al-0.12C-
0.012B-0.1Zr wt%.
Melt Processing
The superalloys contain reactive elements such as aluminium and
titanium. It is necessary therefore to melt the alloys under vaccum,
with the added advantage that detrimental trace elements are
removed by evaporation. Vaccum induction melting is commonly
used because the inductive stirring encourages homogenisation and
helps expose more of the liquid to the melt-vaccum interface. This
in turn optimises the removal of undesirable gases and volatile
impurities.
Many alloys are then vaccum arc
remelted in order to achieve a higher
purity and better solidification
microstructure. The ingot is made an
electrode (a). An arc burns in the
vacuum, thereby heating the front end of
the electrode. Droplets are formed which
then trickle through the vacuum and
become purified. The molten metal is
contained by a water-cooled copper
mould. There is a liquid pool (b) where
further purification occurs by the
floatation of solid impurities. The
solidified metal (c) has a desirable
directional-microstructure.
The diagram for electroslag refining looks similar to that for

23. vacuum arc remelting, except that the melt pool is covered by a 10
cm thick layer of slag (lime, alumina and flourite). The ingot is
again an electrode in contact with the slag. The slag has a high
electrical resistivity and hence melts, the temperature being in
excess of the melting point of the metal electrode. The tip of the
electrode melts, allowing metal to trickle through the slag into the
liquid sump at the bottom. This refines the alloy.
It is common for alloys destined for critical applications to go
through two or more of these melting processes.
Casting of Blades
Nickel based superalloy blades are generally made using an
investment casting process. A wax model is made, around which a
ceramic is poured to make the mould. The wax is removed from the
solid ceramic and molten metal poured in to fill the mould. The
actual process is more complicated because of the intricate shape of
the blade, with its cooling channels and other feature.