Dual steels, TRIP steels, Maraging Steels, Smart materials etc

1. MODERN METALLIC MATERIALS
DUAL STEELS
The desire to produce high strength steels with formability greater than micro
alloyed steel led the development of DP steels in the 1970s.
Dual-phase steel (DP steel) is a high-strength steel that has a ferritic-
martensitic microstructure. DP steels are produced from low or medium carbon
steels that are quenched from a temperature above A1 but below A3 determined
from continuous cooling transformation diagram. After cold rolling, dual phase
sheet steels are continuously heated to the temperature region at which the
structure is part austenite and part ferrite. Temperature in the range from 1360
to 1400oF (730 to 760o C) is typical. The steel is then cooled at a rate sufficient
to cause the austenite to transform to martensite or lower bainite. This results
in a microstructure consisting of a soft ferrite matrix containing islands of
martensite as the secondary phase (martensite increases the tensile strength).
For achieving these microstructures, DP steels typically contain 0.06–0.15 wt.%
C and 1.5-3% Mn (the former strengthens the martensite, and the latter causes
solid solution strengthening in ferrite, while both stabilize the austenite), Cr &
Mo (to retard pearlite or bainite formation), Si (to promote ferrite
transformation), V and Nb (for precipitation strengthening and microstructure
refinement).
Dual phase steels usually contain some alloy additions such as manganese or
silicon, but the strengthening effect employed in these steels is the formation of
martensite or bainite in the ferrite matrix.
Fig. Microstructure of Dual phase steel
DP steels have high ultimate tensile strength (UTS, enabled by the martensite)
combined with low initial yielding stress (provided by the ferrite phase), high
early-stage strain hardening and macroscopically homogeneous plastic flow.
These features render DP steels ideal materials for automotive-related sheet
forming operations.

2. Their advantages are as follows:
 Low yield strength.
 Low yield to tensile strength ratio (yield strength /t ensile strength = 0.5).
 High initial strain hardening rates.
 Good uniform elongation.
 A high strain rate sensitivity (the faster it is crushed the more energy it
absorbs.
 Good fatigue resistance.
Due to these properties DP steels are often used for automotive body panels,
wheels, and bumpers. In addition, given their high energy absorption capacity
and fatigue strength, cold rolled Dual Phase Steels are particularly well suited
for automotive structural and safety parts such as longitudinal beams, cross
members and reinforcements.
MICRO ALLOYED STEEL:
Micro alloyed steel is a type of alloy steel that contains small amounts of
alloying elements (0.05 to 0.15%), including niobium, vanadium, titanium,
molybdenum, zirconium, boron, and rare-earth metals. They are used to refine
the grain microstructure or facilitate precipitation hardening.
In terms of performance and cost, micro alloyed steels are between a carbon
steel and a low alloy steel. Their yield strength is between 275 and 750 MPa (40
and 110ksi) without heat treatment. Weldability is good, and can even be
improved by reducing carbon content while maintaining strength. Fatigue life
and wear resistance are superior to similar heat-treated steels. The
disadvantages are that ductility and toughness are not as good as quenched
and tempered (Q&T) steels. They must also be heated hot enough for all of the
alloys to be in solution; after forming, the material must be quickly cooled to
540 to 600 °C (1,004 to 1,112 °F).
Cold-worked micro alloyed steels do not require as much cold working to
achieve the same strength as other carbon steel; this also leads to greater
ductility. Hot-worked micro alloyed steels can be used from the air-cooled state.
If controlled cooling is used, the material can produce mechanical properties
similar to Q&T steels. Machinability is better than Q&T steels because of their
more uniform hardness and their ferrite-pearlite microstructure.
Because micro alloyed steels are not quenched and tempered, they are not
susceptible to quench cracking, nor do they need to be straightened or stress
relieved. However, because of this, they are through-hardened and do not have
a softer and tougher core like quench and tempered steels.

3. HIGH STRENGTH LOW ALLOY (HSLA) STEELS
High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed
to provide better mechanical properties and/or greater resistance to
atmospheric corrosion than conventional carbon steels. They are not considered
to be alloy steels in the normal sense because they are designed to meet specific
mechanical properties rather than a chemical composition (HSLA steels have
yield strengths greater than 275 MPa, or 40 ksi). The chemical composition of
specific HSLA steel may vary for different product thicknesses to meet
mechanical property requirements. HSLA steels are low carbon, formable steels
possessing high strength than conventional steels.
The HSLA steels in sheet or plate form have low carbon content (0.05 to
0.25% C) in order to produce adequate formability and weldability, and they
have manganese content up to 2.0%. Small quantities of copper, nickel,
niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare
earth elements, or zirconium are used in various combinations. Copper,
titanium, vanadium, and niobium are added for strengthening purposes.
These elements are intended to alter the microstructure of carbon steels,
which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of
alloy carbides in an almost pure ferrite matrix. This eliminates the toughness-
reducing effect of a pearlitic volume fraction yet maintains and increases the
material's strength by refining the grain size, which in the case of ferrite
increases yield strength by 50% for every halving of the mean grain diameter.
HSLA steels are also more resistant to rust than most carbon steels because of
their lack of pearlite – the fine layers of ferrite (almost pure iron) and cementite
in pearlite. HSLA steels usually have densities of around 7800 kg/m³.
HSLA steels Possess:
 High Strength to weight ratio
 Improved low temperature toughness
 Fatigue resistance
 High temperature creep resistance
 Atmosphere corrosion resistance
 Improved notch toughness
 Weldability
 Formability
HSLA steels can be divided into six categories:
 Weathering steels, which contain small amounts of alloying elements
such as nickel, copper and phosphorus for improved atmospheric
corrosion resistance and solid-solution strengthening.
 Microalloyed ferrite-pearlite steels, which contain very small
(generally, less than 0.10%) additions of strong carbide or carbonitride

4. forming elements such as niobium, vanadium, and/or titanium for
precipitation strengthening, grain refinement, and possibly
transformation temperature control.
 As-rolled pearlitic steels, which may include carbon-manganese steels
but which may also have small additions of other alloying elements to
enhance strength, toughness, formability, and weldability.
 Acicular ferrite (low-carbon bainite) steels, which are low-carbon (less
than 0.05%C) steels with an excellent combination of high yield strengths,
(as high as 690MPa, or 100ksi) weldability, formability, and good
toughness.
 Dual-phase steels, which have a microstructure of martensite dispersed
in a ferritic matrix and provide a good combination of ductility and high
tensile strength.
 Inclusion-shape-controlled steels, which provide improved ductility and
through-thickness toughness by the small additions of calcium,
zirconium, or titanium, or perhaps rare earth elements so that the shape
of the sulfide inclusions is changed from elongated stringers to small,
dispersed, almost spherical globules.
APPLICATIONS OF HSLA STEELS include oil and gas pipelines, heavy-duty
highway and off-road vehicles, construction and farm machinery, industrial
equipment, storage tanks, mine and railroad cars, barges and dredges,
snowmobiles, lawn mowers, and passenger car components. Bridges, offshore
structures, power transmission towers, light poles, and building beams and
panels are additional uses of these steels.
The choice of specific high-strength steel depends on a number of application
requirements including thickness reduction, corrosion resistance, formability,
and weldability. For many applications, the most important factor in the steel
selection process is the favorable strength-to-weight ratio of HSLA steels
compared with conventional low-carbon steels. This characteristic of HSLA
steels has lead to their increased use in automobile components.
STRENGTHENING MECHANISMS:
The micro-alloyed steels used nowadays are obtained by means of a
suitable combination of chemical composition and thermo-mechanical
treatment parameters, with the aim to achieve the proper balance between
strength, toughness, ductility and formability. These properties depend upon
micro structural features, while weldability is generally accepted as being
composition dependent.
The yield strength of steel can be increased by one or more of several
strengthening mechanisms. These include:

5. a. Dislocation strengthening: the resistance to dislocation movement
due to the obstacles presented to other dislocations. The limitations
of this mechanism are due to the saturation of the structure with
dislocations.
b. Grain-boundary strengthening: grain-boundary hindering of
dislocation movement. Grains can be refined to a very small size,
and a very high strength may be achieved. Below a critical grain
size the toughness may be lowered.
c. Solid-solution strengthening: the resistance to dislocation
movement due to the presence of interstitial or substitutional solute
atoms in a crystal lattice. The limitation of this mechanism is
imposed by the solubility limit in each alloying system.
d. Precipitation strengthening: the resistance to dislocation movement
due to the effect of second-phase particles. Second-phase particles
may be dispersoids (stable particles mechanically added and
sintered) or precipitates (particles formed from a supersaturated
solid solution) which is more present in practice. The limitation of
this mechanism is governed by the influence of the size and shape
on toughness, not on the strengthening.
e. Texture strengthening: the resistance to dislocation movement due
to the presence of a texture-preferred orientation in the structure.
This mechanism does not seem to be as potent as other
strengthening mechanisms, while the development of texture is
difficult in steels with a dispersed second phase.
f. Phase-transformation strengthening: the resistance to dislocation
movement due to the presence of newly formed phases introduced
by a phase transformation that starts simultaneously with a
deformation. This mechanism gives new opportunities and it may
be considered as the main research challenge in the future.

7. STEEL TRANSFORMATION INDUCED PLASTICITY (TRIP) STEEL
TRIP steel is a new class of high-strength steel alloys typically used in
naval and marine applications and in the automotive industry. TRIP stands for
"Transformation induced plasticity," which implies a phase transformation in
the material, typically when a stress is applied. These alloys are known to
possess an outstanding combination of strength and ductility.
Microstructure:
TRIP steels possess a microstructure consisting of austenite with
sufficient thermodynamic instability such that transformation to martensite is
achieved during loading or deformation. It has Triple phase microstructure –
Ferrite, bainite and retained austenite.
Many automotive TRIP steels possess retained austenite within a ferrite
matrix, which may also contain hard phases like bainite and martensite. In the
case of these alloys, the high silicon and carbon content of TRIP steels results in
significant volume fractions of retained austenite in the final microstructure.
TRIP steels use higher quantities of carbon than dual-phase steels to
obtain sufficient carbon content for stabilizing the retained austenite phase to
below ambient temperature. Higher contents of silicon and/or aluminum
accelerate the ferrite/bainite formation. They are also added to avoid formation
of carbide in the bainite region.
For use in naval and marine applications, both martensitic/austenitic
and fully austenitic steels have been of interest due to their exhibited large
uniform elongation, high strength, and high fracture toughness. These
properties are exhibited because of a deformation-induced martensitic
transformation from parent phase (FCC γ austenite) to the product phase (BCC
α' martensite). This transformation is dependent on temperature, applied stress,
composition, strain rate, and deformation history, among others.

8. Metallurgical properties:
During plastic deformation and straining, the retained austenite phase is
transformed into martensite. Thus increasing the strength by the phenomenon
of strain hardening. This transformation allows for enhanced strength and
ductility.
High strain hardening capacity and high mechanical strength lend these
steels excellent energy absorption capacity. TRIP steels also exhibit a strong
bake hardening effect.
Bake hardening is an increase in strength observed when work hardening
during part formation is followed by a thermal cycle such as paint-baking.
Research to date has not shown much experimental evidence of the TRIP-
effect enhancing ductility, since most of the austenite disappears in the first 5%
of plastic strain, a regime where the steel has adequate ductility already. Many
experiments show that TRIP steels are in fact simply a more complex dual-
phase steel.
The amount of carbon determines the strain level at which the retained
austenite begins to transform to martensite. At lower carbon levels, the retained
austenite begins to transform almost immediately upon deformation, increasing
the work hardening rate and formability during the stamping process. At higher
carbon contents, the retained austenite is more stable and begins to transform
only at strain levels beyond those produced during forming.
Composition of TRIP steel:
 Hypo eutectoid iron carbon alloys.
 0.1 – 0.4 % carbon by weight.
 Alloying elements to prevent high carbon cementite phase, which raises
the carbon concentration of the austenite phase.
 Silicon and Aluminium are two common alloying elements.
 Titanium, Niobium, Vanadium, etc., are other alloying elements can be
added to improve the strength of the alloy.
 Good formability and drawability.
PROCESSING METHOD
 Intercritical annealing is used to obtain the correct phase distribution.
 Temperature above Eutectoid – material is composed of a solid austenite
phase and solid ferrite phase.
 The material is then isothermally cooled at a temperature of
approximately 400 degrees Celsius, in order to allow the austenite to form
a banitic ferrite phase.
 In a typical steel alloy, the excess carbon would form a high carbon
cementite phase. However, the silicon and aluminium prevent the
formation of cementite.
 The result of the intercritical annealing process is a material composed
primarily of ferrite, and bainite. Formed from the austenite phase during

9. intercritical annealing, as well as dispersed retained austenite, and
martensite phases.
ECONOMICS OF TRIP STEELS:
 Usually, steel is used for its strength, formability and low cost relative to
other metals.
 Titanium, magnesium and aluminum offer significant weight savings in
automobile components.
 Lower abundance, high production cost and high machining cost.
 TRIP steels don not face any of these difficulties, because they are low
alloy steel.
 TRIP steels can be produced for the same price as other high strength
steel.
APPLICATIONS:
As a result of their high energy absorption capacity and fatigue strength,
TRIP steels are particularly well suited for automotive structural and safety
parts such as cross members, longitudinal beams, B-pillar reinforcements, sills
and bumper reinforcements. The TRIP effect can also be utilized in forming
operations, where improvements to ductility enable greater bend angles and
more aggressive forming operations without cracking.
The most common TRIP range of steels comprises 2 cold rolled grades in
both uncoated and coated formats (TRIP 690 and TRIP 780) and one hot rolled
grade (TRIP 780), identified by their minimum ultimate tensile strength
expressed in MPa.
TRIP steels are well suited to armor applications, where increases in
uniform ductility (and therefore ballistic energy absorption) can improve
protection against projectiles and ballistic threats while maintaining or reducing
plate thicknesses.
MARAGING STEEL
Maraging steels are steels (iron alloys) that are known for possessing
superior strength and toughness without losing malleability, although they
cannot hold a good cutting edge. Aging refers to the extended heat-treatment
process. These steels are a special class of low-carbon ultra-high-strength steels
that derive their strength not from carbon, but from precipitation of
intermetallic compounds. The term ‘maraging’ is derived from the strengthening
mechanism, which is transforming the alloy to martensite with subsequent age
hardening. (Martensite + Ageing).
The principal alloying element is 15 to 25 wt.% nickel. Secondary alloying
elements, which include cobalt, molybdenum and titanium, are added to
produce intermetallic precipitates. Since ductile Fe-Ni martensites are formed
upon cooling, cracks are non-existent or negligible. These steels can be nitrided
to improve case hardness. C is considered as an impurity element in these steel
and it should be kept below 0.03%.

10. The common, non-stainless grades contain 17–19 wt.% nickel, 8–12 wt.%
cobalt, 3–5 wt.% molybdenum and 0.2–1.6 wt.% titanium. Addition of
chromium produces stainless grades resistant to corrosion. This also indirectly
increases hardenability as they require less nickel; high-chromium, high-nickel
steels are generally austenitic and unable to transform to martensite when heat
treated, while lower-nickel steels can transform to martensite. Alternative
variants of Ni-reduced maraging steels are based on alloys of Fe and Mn plus
minor additions of Al, Ni and Ti where compositions between Fe-9wt.% Mn to
Fe-15wt.% Mn have been used.
The Mn has a similar effect as Ni, i.e. it stabilizes the austenite phase.
Hence, depending on their Mn content, Fe-Mn maraging steels can be fully
martensitic after quenching them from the high temperature austenite phase or
they can contain retained austenite. The latter effect enables the design of
maraging-TRIP steels where TRIP stands for Transformation-Induced-Plasticity.
Properties:
Due to the low carbon content maraging steels have good machinability.
Prior to aging, they may also be cold rolled to as much as 90% without cracking.
Maraging steels offer good weldability, but must be aged afterward to restore the
original properties to the heat affected zone.
When heat-treated the alloy has very little dimensional change, so it is
often machined to its final dimensions. Due to the high alloy content maraging
steels have a high hardenability. Since ductile Fe-Ni martensites are formed
upon cooling, cracks are non-existent or negligible. The steels can be nitrided to
increase case hardness and polished to a fine surface finish.
Non-stainless varieties of maraging steel are moderately corrosion-
resistant and resist stress corrosion and hydrogen embrittlement. Corrosion-
resistance can be increased by cadmium plating or phosphating.

11. Heat treatment cycle:
Steels are solution treated at 820oC, to absorb all precipitates or alloying
elements & produce uniform austenitic structure. Upon cooling in
air/quenching; a Fe-Ni BCC Martensite is formed instead of ordinary Tetragonal
Martensite (Fe-C).
This is of lath-like BCC form, softer & tougher than ordinary martensite but
heavily dislocated martensite (means high dislocation density = high energy =
means favorable site for precipitation).
Upon ageing at 480oC for 3 or more hours, coherent precipitate of intermetallic
compound (Mo, Ti & Al) Ni3 are formed.
The main function of “Co” seems to produce more sites for the nucleation of
(Mo, Ti & Al)Ni3 precipitates. Or Co reduces the solubility of Ti, Mo & Al in the
matrix as a result, these increases the volume fraction of rich precipitate.
Applications
Maraging steel's strength and malleability in the pre-aged stage allows it
to be formed into thinner rocket and missile skins than other steels, reducing
weight for a given strength. Maraging steels have very stable properties and,
even after overaging due to excessive temperature, only soften slightly. These
alloys retain their properties at mildly elevated operating temperatures and have
maximum service temperatures of over 400 °C (752 °F).
They are suitable for engine components, such as crankshafts and gears,
and the firing pins of automatic weapons that cycle from hot to cool repeatedly
while under substantial load. Their uniform expansion and easy machinability
before aging make maraging steel useful in high-wear components of assembly
lines and dies. Other ultra-high-strength steels, such as AerMet alloys, are not
as machinable because of their carbide content.
It is also used in surgical components and hypodermic syringes, but is
not suitable for scalpel blades because the lack of carbon prevents it from
holding a good cutting edge.
Maraging steel is used in aircraft, with applications including landing
gear, helicopter undercarriages, slat tracks and rocket motor cases –
applications which require high strength-to-weight material. Maraging steel
offers an unusual combination of high tensile strength and high fracture
toughness. Most high-strength steels have low toughness, and the higher their
strength the lower their toughness. The rare combination of high strength and
toughness found with maraging steel makes it well suited for safety-critical
aircraft structures that require high strength and damage tolerance.
Stainless maraging steel is used in bicycle frames and golf club heads.
In the sport of fencing, blades used in competitions run under the
auspices of the Fédération Internationale d'Escrime are usually made with
maraging steel. Maraging blades are superior for foil and épée because crack

12. propagation in maraging steel is 10 times slower than in carbon steel, resulting
in less blade breakage and fewer injuries.
Maraging steel production, import and export by certain states, such as
the United States, is closely monitored by international authorities because it is
particularly suited for use in gas centrifuges for uranium enrichment; lack of
maraging steel significantly hampers this process.
Types of Maraging Steels: Well known grades 18%, 20% & 25%
INTERMETALLICS
 Most of the alloy systems do not show complete solid solubility. When the
amount of solute element is more than the limit of solid solubility, a second
phase also appears apart from the primary solid solution. The second phase
which forms is an intermediate phase.
 An intermetallic compound can be defined as an ordered alloy phase formed
between two metallic elements, where an alloy phase is ordered if two or
more sub-lattices are required to describe its atomic structure. The ordered
structure exhibits superior elevated-temperature properties because of the
long-range ordered super lattice, which reduces dislocation mobility and
diffusion processes at elevated temperatures.
 It is a phase formed at intermediate composition between the two primary
components (pure metals).
 The crystal structure of the intermediate phase is different from the both
primary components. Some of these intermediate phases have a fixed
composition and are called Intermetallic compounds.

13.  An intermetallic (also called an intermetallic compound, intermetallic alloy,
ordered intermetallic alloy, and a long-range-ordered alloy) is a type of
metallic alloy that forms a solid-state compound exhibiting defined
stoichiometry and ordered crystal structure.
 Intermetallics are similar to alloys, but the bonding between the different
types of atoms is partly ionic, leading to different properties than traditional
alloys.
 In general, the larger the electro negativity difference between the host atom
and the impurity, the greater the tendency to form compounds and the less
solubility there is. So, elements with similar electro negativities tend to form
alloy, whereas elements with large electro negativity difference tend to have
more ionic bonds.
 An intermetallic compound contains two or more metallic elements,
producing a new phase with its own composition, crystal structure and
properties. Intermetallic compounds are almost always very hard and brittle.
 Intermetallics are similar to ceramic materials in terms of their mechanical
properties. Often dispersion-strengthened alloys contain an intermetallic
compound as the dispersed phase.
 The following conditions are to be satisfied for the formation of an
intermetallic compound:
 It has to be formed by at least two different elements
 It must have a different crystal structure from the one observed in the
pure components
 The component element should be having opposite electrochemical
nature.
 Generally, Intermetallic compounds are formed when one metal having
chemical properties which are strongly metallic and the other metal having
chemical properties which are weakly metallic. For example, Magnesium (Mg)
is strong metal whereas as lead (Pb) or Tin (Sn) is a weak metal. They the
different crystal structure and have opposite electrochemical nature. So they
can combine together to form Mg2Sn or Mg2Pb. Some other examples of
intermetallic compounds are Mg3 Sb2, Fe3C, Al6Mn … etc.
 Intermetallics are classified into two types
 Stoichiometric Intermetallics: They have fixed composition. They are
represented in the phase diagram by a vertical line.
 Examples:
 Au2Pb in Au-Pb system
 AlSb in Al-Sb system
 Fe3C in steels
 Mg2Pb in Mg-Pb system
 Non-stoichiometric intermetallics have a range of compositions and are
sometimes called intermediate solid solutions.
 Examples:
 phase in Mo-Rh system

14.  Β phase in brass
 CuAl2 in Al-Cu system
 Mg2Al3 in Al-Mg system
Properties of Some Intermetallics:
Intermetallic
Compound
Crystal Structure Melting
Temperature
(oC)
Density
(g/cm3)
Young’s
Modulus
FeAl Ordered BCC 1250-1400 5.6 263
NiAl Ordered FCC
(B2)*
1640 5.9 206
Ni3Al Ordered FCC
(L12)*
1390 7.5 337
TiAl Ordered
tetragonal (L10)
1460 3.8 94
Ti3Al Ordered HCP 1600 4.2 210
MoSi2 Tetragonal 2020 6.31 430
* B2 – Binary compound structure having 1:1 stoichiometry; * L1 – Alloys

15. Properties & Applications:
 Molybdenum disilicide (MoSi2)
 This material is used for making heating elements for high
temperature furnaces
 At high temperature (1000 to 1600oC), MoSi2 shows outstanding
oxidation resistance.
 At low temperature (500oC and below), MoSi2 is brittle and
shows catastrophic oxidation known as pesting.
 Copper Aluminide (CuAl2)
 Precipitation of the non-stoichiometric intermetallic copper
aluminide (CuAl2) causes strengthening in a number of
important aluminium alloys.
 Precipitation hardening – by forming θ (CuAl2) phase in α matrix,
gives high strength and toughness.
 Properties:
 High strength (2119: σTS 505-520 MPa).
 Good creep strength at high temp.
 High toughness at cryogenic temp.
 Good machinability.
 Applications:
 Fuel Tanks (2119)
 Pistons, rivets for aircraft constructions (2024-T4):
Al2CuMg
 Al-Mg-Si Alloys (Mg2Si)
 Mg and Si are added in balanced amount to form Mg2Si.
 Mg + Si (0.8-1.2%); Mg + Si (>1.4%)
 Properties:
 Medium strength structural alloys (most widely used
6063-T6, σy 215 MPa, σTS 245 MPa).
 Readily extruded.
 Color anodized.
 Applications:
 Car bodies, Electric trains (6009)
 Structural components (6061)
 Satellite dish (6005)
 Large water pipes (6063)
 Aircraft, Automotive (6013 – T6, T8)
 Platinum silicide (PtSi2)
 Intermetallics based on silicon (e.g., platinum silicide) play a
useful role in microelectronics.
 Niobium family intermetallics
 Certain intermetallics such as NbTi, Nb3Sn, NbZr, Nb3Al and
Nb3Ge are used as superconductors.

16.  β ‘ Brasses (α + CuZn):
Type Color % Cu % Zn Density
g/cm3
MP
oC
Tensile
strength
Psi
Uses
Common Yellow 67 33 8.42 940 70K
Lamp
Fixtures,
Bead
chain
Muntz
metal
Yellow 60 40 8.39 904 70K
Nuts
and
Bolts
 TiAl and Ni3Al (Titanium & Nickel based superalloys):
 Properties:
 TiAl and Ni3Al posses good combinations of high-
temperature mechanical properties and oxidation
resistance up to approximately 650 - 960oC
 Good Toughness and corrosion resistance.
 Applications:
 Aircrafts, space vehicles, rocket engines.
 Industrial gas turbines (In 738LC).
 Nuclear reactors, submarines.
 Steam power plants, petrochemical equipment.
 Combustion engine exhaust valves
Nickel and Titanium Aluminides
Ni Aluminide
 Nickel aluminide (Ni3Al) is an intermetallic alloy of nickel and aluminum with
properties similar to both a ceramic and a metal.
 There are three materials called nickel aluminide:
 NiAl, CAS number 12003-78-0 (see also Raney nickel)
 NiAl3, CAS number 12004-71-6
 Ni3Al, tri-nickel aluminide
 Nickel aluminide is used as a strengthening constituent in high-temperature
nickel-base superalloys; however, unalloyed nickel aluminide has a tendency
to exhibit brittle fracture and low ductility at ambient temperatures.
 In 2005, the most abrasion-resistant material was reportedly created by
embedding diamonds in a matrix of nickel aluminide.
 Nickel aluminide (NiAl) is available commercially in powder form in various
mesh sizes from granular material to an average particle size of a few
micrometers. Nickel aluminide is fairly strong at room temperature with
modulus of transverse rupture values ranging from 204 to 952 MPa,
depending on fabrication methods, while at 1000°C the corresponding values

17. are approximately half the room temperature strength. Creep resistance at
these temperatures is extremely poor. Thermal shock resistance is quite
good.
 Nickel aluminide can be formed by hot pressing or by cold pressing and
sintering, but the former method produces by far the best results.
 Excellent oxidation resistance and fairly good strength make this material of
interest for turbine blading or other combustion chamber applications. It has
impact resistance that is better than most ceramics, intermetallic
compounds, and some cermets. NiAl is also resistant to attack by molten
glass and red and white fuming nitric acid, which suggests possible uses in
the glass-processing industry or in some high-temperature chemical
processes.
 Example: An alloy of Ni3Al, known as IC-221M, is made up of nickel
aluminide combined with several other metals including chromium,
molybdenum, zirconium and boron. Adding boron increases the ductility of
the alloy by positively altering the grain boundary chemistry and promoting
grain refinement. The Hall-Petch parameters for this material were σo = 163
MPa and ky = 8.2 MPaˑcm1/2. Boron increases the hardness of bulk Ni3Al by a
similar mechanism.
This alloy is extremely strong for its weight, five times stronger than common
SAE 304 stainless steel. Unlike most alloys, IC-221M increases in strength
from room temperature up to 800 °C.
The alloy is very resistant to heat and corrosion, and finds use in heat-
treating furnaces and other applications where its longer life span and
reduced corrosion give it an advantage over stainless steel.
Properties
 Density = 7.16 g/cm3
 Yield Strength = 855 MPa
 Hardness = HRC 12
 Thermal Conductivity Ni3Al = 28.85 (W/m.K)
 Thermal Conductivity NiAl = 76 (W/m.K)
 Melting Point Ni3Al = 1668 K
 Melting Point NiAl = 1955 K
 Thermal expansion coefficient = 12.5 (10−6/K−1)
 Bonding = covalent/metallic
 Electrical resistivity = 32.59 (10−8Ωm)
Ti Aluminide
Titanium aluminide, TiAl, is an intermetallic chemical compound. It is
lightweight and resistant to oxidation and heat, however it suffers from low
ductility. The density of gamma TiAl is about 4.0 g/cm³. It finds use in several
applications including automobiles and aircraft. The development of TiAl based
alloys began about 1970; however the alloys have only been used in these

18. applications since about 2000. Ti aluminides has excellent mechanical
properties at high temperature such as tensile strength, Young’s modulus or
creep behaviour. In addition, their low mass density makes them especially
attractive to mobile applications like it is the case in aeronautic or automotive
industries.
Titanium Aluminide (TiAl) is a member of the material group of ordered
intermetallics, which are characterized by unique mechanical properties due to
their long-range ordered crystal structure. TiAl has high strength, good high-
temperature properties and low density. These properties make the material
attractive for aerospace applications, especially as a substitution for nickel-
based.
The lack of room temperature ductility is the main reason why these
intermetallics have to be produced by non-conventional and costly production
routes. This involves powder metallurgy, near-net-shape casting from very high
temperatures, selective laser melting, electron beam melting and hot forging
methods.
Superalloys used in the low pressure turbine in an aircraft engine.
Titanium aluminide has three major intermetallic compounds: gamma TiAl,
alpha 2-Ti3Al and TiAl3. Among the three, gamma TiAl has received the most
interest and applications. Gamma TiAl has excellent mechanical properties and
oxidation and corrosion resistance at elevated temperatures (over 600 degrees
Celsius), which makes it a possible replacement for traditional Ni based
superalloy components in aircraft turbine engines.
TiAl based alloys have a strong potential to increase the thrust-to-weight ratio
in the aircraft engine. This is especially the case with the engine's low-pressure
turbine blades and the high-pressure compressor blades. These are traditionally
made up of Ni-based superalloy, which is nearly twice as dense as TiAl-based
alloys.
General Electric uses gamma TiAl for the low-pressure turbine blades on its
GEnx engine, which powers the Boeing 787 and Boeing 747-8 aircraft. This was
the first large-scale use of this material on a commercial jet engine, when it
entered service in 2011. The TiAl LPT blades are cast by Precision Castparts
Corp. and Avio s.p.a... Machining of the Stage 6 and Stage 7 LPT blades is
performed by Moeller Manufacturing, Aerospace Division, in Wixom, Michigan,
USA. An alternate pathway for production of the gamma TiAl blades for the
GEnx and GE9x engines using Additive Manufacturing is also being explored.

19. SMART MATERIALS
Smart materials are those materials that can significantly alter one or more of
their inherent properties in response to an external stimulus in a controlled
manner.
The several external stimulus to which the SMART Materials are sensitive are:
Stress, Temperature, Moisture, pH, Electric Fields, Magnetic fields, chemicals,
nuclear radiation etc.
The associated changeable physical properties could be shape, stiffness,
viscosity, damping etc.
For example, the shape of the material will change in response to different
temperature or application of electrical charge or presence of electric field.
Smart materials are the basis of many applications, including sensors and
actuators, or artificial muscles, particularly as electroactive polymers (EAPs).
Terms used to describe smart materials include shape memory material
(SMM) and shape memory technology (SMT).
Smartness is generally programmed by
 Material composition
 Special processing
 Modifying micro structure
 Introduction of defects so as to adapt to various levels of stimuli in a
controlled fashion.
Classification of Smart Materials:
 Active Smart Materials: Possess the capacity of modifying their geometric
and material properties under the application of electric, thermal or magnetic
fields, thereby acquiring an inherent capacity to transducer energy.
 Examples: Piezo-electric materials, SMAs, ER fluids, Magneto-
stictive materials.
 Can be used as force transducers and actuators.
 Passive Smart Materials: These materials lack inherent capability to
transducer energy.
 Examples: Fibre optic cable
 Can act as sensors not as actuators or transducers.
Types of Smart Materials:
There are a number of types of smart material, of which are already common.
Some examples are as following:
 Piezoelectric materials are materials that produce a voltage when stress is
applied. Since this effect also applies in a reverse manner, a voltage across
the sample will produce stress within sample. Suitably designed structures
made from these materials can, therefore, be made that bend, expand or
contract when a voltage is applied.
 Materials Used: Quartz, Rochelle salt, Topaz, Bismuth Ferrite etc.
 Shape –memory alloys and shape-memory polymers are materials in which
large deformation can be induced and recovered through temperature
changes or stress changes (pseudo-elasticity). SMAs are metal alloys which

20. can undergo solid-to-solid phase transformation and can recover completely
when heated to a specific temperature. The shape memory effect results due
to martensitic phase change and induced elasticity at higher temperatures
respectively.
 Magnetic shape memory alloys are materials that change their shape in
response to a significant change in the magnetic field.
 Magnetostrictive materials exhibit a change in shape under the influence of
magnetic field and also exhibit a change in their magnetization under the
influence of mechanical stress.
 Photovoltaic materials or optoelectronics convert light to electrical current.
 Ferro fluid
 Chromogenic systems change color in response to electrical, optical or
thermal changes. These include electrochromic materials, which change their
colour or opacity on the application of a voltage (e.g., liquid crystal
displays), thermochromic materials change in colour depending on their
temperature, and photochromic materials, which change colour in response
to light—for example, light-sensitive sunglasses that darken when exposed to
bright sunlight.
 Smart inorganic polymers showing tunable and responsive properties.
 pH-sensitive polymers are materials that change in volume when the pH of
the surrounding medium changes.
 Temperature-responsive polymers are materials which undergo changes
upon temperature.
 Halochromic materials are commonly used materials that change their color
as a result of changing acidity. One suggested application is for paints that
can change color to indicate corrosion in the metal underneath them.
 Artificial cilia (co-polymer film with hair like structures) which changes its
colour and structure in different conditions

21. Applications
 In accelerometers (stabilizing quad rotors etc.)
 Strain sensors
 Emitters and receptors of stress waves
 Active vibration control of stationary/moving structures (helicopter
blades).
 Smart skins for submarines
 Skin like Piezo electric materials sensing temperature and pressure.
SHAPE MEMORY ALLOYS
Metals are characterized by physical qualities as tensile strength,
malleability and conductivity. In the case of shape memory alloys, we can add
the anthropomorphic qualities of memory and trainability. Shape memory alloys
exhibit what is called the shape memory effect. If such alloys are plastically
deformed at one temperature, they will completely recover their original shape
on being raised to a higher temperature. In recovering their shape the alloys can
produce a displacement or a force as a function of temperature. In many alloys
combination of both is possible.
The metals change shape, change position, pull, compress, expand,
bend or turn, with heat as the only activator. Key features of products that
possess this shape memory property include: high force during shape change;
large movement with small temperature change; a high permanent strength;
simple application, because no special tools are required; many possible shapes
and configurations; and easy to use - just heat. Because of these properties
shape memory alloys are helping to solve a wide variety of problems. In one
well–developed application shape memory alloys provide simple and virtually
leak proof couplings for pneumatic or hydraulic lines. The alloys have also been
exploited in mechanical and electromechanical control systems to provide, for

22. example, a precise mechanical response to small and repeated temperature
changes. Shape memory alloys are also used in a wide range of medical and
dental applications (healing broken bones, misaligned teeth etc.).
Definition of a Shape Memory Alloy
Shape memory alloys are a unique class of metal alloys that can recover
apparent permanent strains when they are heated above a certain temperature.
i.e., shape-memory alloys (SMA, smart metal, memory metal, memory alloy,
muscle wire, smart alloy) are metal alloys that can be deformed at one
temperature but when heated or cooled, return to their “original” shape. Shape
memory alloy is a class of smart materials.
The shape memory alloys have two stable phases - the high–temperature phase,
called austenite (named after English metallurgist William Chandler Austen)
and the low–temperature phase, called martensite (named after German
metallographer Adolf Martens).
The key characteristic of all shape memory alloys is the occurrence of a
martensitic phase transformation which is a phase change between two solid
phases and involves rearrangement of atoms within the crystal lattice. The
martensitic transformation is associated with an inelastic deformation of the
crystal lattice with no diffusive process involved. The phase transformation
results from a cooperative and collective motion of atoms on distances smaller
than the lattice parameters. Martensite plates can grow at speeds which
approach that of sound in the metal (up to 1100m/s). together with fact, that
martensitic transformation can occur at low temperatures where atomic

23. mobility may be very small, results in the absence of diffusion in the martensitic
transformation within the time scale of transformation.
Principle:
 Upon cooling in the absence of applied load the material transforms from
austenite into twinned martensite. (no observable macroscopic shape change
occurs)
Fig. Different phases of a shape memory alloy

24.  Upon heating the material in the martensitic phase, a reverse phase
transformation takes place and as a result the material transforms to
austenite.
 If mechanical load is applied to the material in the state of twinned
martensite (at low temperature) it is possible to detwin the martensite.
 Upon releasing of the load, the material remains deformed. A subsequent
heating of the material to a temperature above the austenite finish
temperature (Af) will result in reverse phase transformation (martensite to
austenite) and will lead to complete shape recovery.
(Af: temperature at which transformation of martensite to austenite is complete)
 SMA remembers the shape when it has austenitic structure.
 So if we need SMA to remember and regain/recover certain shape, the shape
should be formed when structure is austenite.
 Reheating the material will result in complete shape recovery.
Applications for Shape Memory Alloys
Bioengineering:
Bones: Broken bones can be mended with shape memory alloys. The alloy plate
has a memory transfer temperature that is close to body temperature, and is
attached to both ends of the broken bone. From body heat, the plate wants to
contact and retrain its original shape, therefore exerting a compression force on
the broken bone at the place of fracture. After the bone has healed, the plate
continues exerting force, and aids in strengthening during rehabilitation.
Memory metals also apply to hip replacements, considering the high level of
super-elasticity.
Reinforcement for Arteries and Veins: For clogged vessels, an alloy tube is
crushed and inserted into the clogged veins. The memory metal has a memory
transfer temperature close to body heat, so the memory expands to open the
clogged arteries.

25. Dental wires: used for braces and dental arch wires, memory alloys maintain
their shape since they are at a constant temperature, and because of the super
elasticity of the memory metal, the wires retain their original shape after stress
has been applied and removed.
Anti-scalding protection:
Temperature selection and control system for baths and showers. Memory
metals can be designed to restrict water flow by reacting at different
temperatures, which is important to prevent scalding. Memory metals will also
let the water flow resume when it has cooled down to a certain temperature.
Helicopter blades:
Performance for helicopter blades depend on vibrations; with memory metals in
micro processing control tabs for the trailing ends of the blades, pilots can fly
with increased precision.
Eyeglass Frames:
In certain commercials, eyeglass companies demonstrate eyeglass frames that
can be bent back and forth, and retain their shape. These frames are made from
memory metals as well, and demonstrate super-elasticity.
Fire security and Protection systems:
Lines that carry highly flammable and toxic fluids and gases must have a great
amount of control to prevent catastrophic events. Systems can be programmed
with memory metals to immediately shut down in the presence of increased
heat. This can greatly decrease devastating problems in industries that involve
petrochemicals, semiconductors, pharmaceuticals, and large oil and gas boilers.
Tubes, Wires, and Ribbons:
For many applications that deal with a heated fluid flowing through tubes, or
wire and ribbon applications where it is crucial for the alloys to maintain their
shape in the midst of a heated environment, memory metals are ideal.
Golf Clubs: A new line of golf putters and wedges has been developed using
SMAs. Shape memory alloys are inserted into the golf clubs. These inserts are
super elastic, which keep the ball on the clubface longer. As the ball comes into
contact with the clubface, the insert experiences a change in metallurgical
structure. The elasticity increases the spin on the ball, and gives the ball more
"bite" as it hits the green.
METALLIC GLASS
Metallic glasses or amorphous metals are novel engineering alloys in which the
structure is not crystalline (as it is in most metals) but rather is disordered,
with the atoms occupying more-or-less random positions in the structure. In
this sense, metallic glasses are similar to the more familiar oxide glasses such
as the soda-lime glasses used for windows and bottles.
From a practical point of view, the amorphous structure of metallic glasses gives
them two important properties. First, like other kinds of glasses they experience
a glass transition into a super-cooled liquid state upon heating. In this state the

26. viscosity of the glass can be controlled over a wide range, creating the possibility
for great flexibility in shaping the glass.
Second, the amorphous atomic structure means that metallic glasses do not
have the crystalline defects called dislocations that govern many of the
mechanical properties of more common alloys. The most obvious consequence
of this is that metallic glasses can be much stronger (3-4 times or more) than
their crystalline counterparts. Another is that metallic glasses are somewhat
less stiff than crystalline alloys. The combination of high strength and low
stiffness gives metallic glass very high resilience, which is the ability to store
elastic strain energy and release it.
 Metallic glasses are the newly developed engineering materials.
 Metallic glasses share the properties of both metals and alloys. Most metals
and alloys are crystalline i.e., their atoms arranged in some regular pattern
that extends over a long distance. In contrast, glass is an amorphous, brittle
and transparent solid. Thus, metallic glasses are metal alloys that are
amorphous. That is, they do not have a long range atomic order.
 The major advantages of such glasses are that they are generally
homogeneous in composition, and offer strong and superior corrosion
resistance.
 To have this particular property, the metallic glasses are to be made by
cooling a molten metal so rapidly at a rate of 2 x 106°Cs-1. During this
process of solidification, the atoms do not have enough time or energy to
rearrange for crystal nucleation. Thus, the liquid upon reaching the glass
transition temperature Tg solidifies as a metallic glass. Again, upon heating
metallic glasses show a reversible glass-liquid transition at Tg.
 Metallic glasses are of two types based on their base material used for the
preparation.
1. Metal-metal glasses, Ex: Ni-Nb, Mg-Zn and Cu- Zr
2. Metal- Metalloid glasses. Transition metal like Fe, Co, Ni and
metalloid like B, Si, C and P are used.
Properties:
 The strength of metallic glasses are very high (nearly twice that of stainless
steel) lighter in weight.
 They are ductile, malleable, brittle and opaque. The hardness is very high.
 The toughness is very high, i.e., the fracture resistance is very high (more
than ceramics).
 They have high elasticity. i.e., the yield strength is very high.
 They have high corrosion resistance.
 They do not contain any crystalline defects like point defects, dislocation,
stacking faults etc.
 They are soft magnetic materials. As a result, easy magnetization and
demagnetization is possible.

27.  Magnetically soft metallic glasses have very narrow hysteresis loop as shown
in figure. Thus, they have very low hysteresis energy losses. As a result, easy
magnetization and demagnetization is possible.
 They have high electrical resistivity which leads to a low eddy current loss.
Preparation
Various rapid cooling techniques such as spraying, spinning and laser
deposition are used for the production of metallic glasses.
In Melt spinning technique, there is spinning disc made of copper. In order to
prepare a metallic glass of a particular type a suitable combination of metal-
metal or metal-metalloid alloy in their stoichiometric ratio are taken in a

28. refractory tube having a fine nozzle at its bottom. The nozzle side of the tube is
placed just over the spinning disc.
An induction heater attached to the refractory tube melts the alloy. This melt is
kept above its melting point till it gets transformed into a homogeneous mixture.
An inert gas such as helium is made to flow through the tube containing the
homogeneous mixture. As a result, the melt gets ejected through the nozzle. The
ejected melt is cooled at a faster rate with the help of spinning cooled copper
disc. The ejection rate can be increased by increasing the pressure of the inert
gas. Thus, a glassy alloy ribbon starts getting formed over the spinning disc.
The thickness of the glassy ribbon may be varied by increasing or decreasing
the speed of the spinning disc.
The other techniques used for producing ribbons of metallic glasses include.
1. Twin roller system
In this technique a molten alloy is passed though two rollers rotating
in opposite directions.
2. Melt extraction system
In this technique the fast moving roller sweeps off molten droplet into
a strip from a solid rod.
Applications:
 Metallic glasses are used as transformer core material in high power
transformers. Usage of metallic glasses in transformers is found to improve
the efficiency of power distribution in transformers. These transformers are
used to convert high-voltage current into low-voltage current to be used for
domestic appliances (120 V and 240V).
 Because of their high electrical resistivity and nearly zero temperature
coefficient of resistance, these materials are used in making cryo-
thermometers, magneto-resistance sensors and computer memories.
 As the magnetic properties of the metallic glasses are not affected by
radiation they are used in making containers for nuclear waste disposal.
 These materials are used in the preparation of magnets for fusion reactors
and magnets for levitated trains etc.
 Metallic glasses can also be used for making watch cases to replace Ni and
other metals which can cause allergic reactions.
 The excellent corrosion resistance property makes these materials to be ideal
for cutting and in making surgical instruments. They can be used as a
prosthetic material for implantation in the human body.
 In future, the usage of metallic glasses in the electronic field can yield
stronger, lighter and more easily moulded castings for personal electronics
products.
 Metallic glasses are used in tape recorder as heads, in manufacturing of
springs and standard resistances.

29. QUASI CRYSTALS
NANO CRYSTALLINE MATERIALS
A Nano-crystalline (NC) material is a polycrystalline material with a crystallite
size of only a few nanometers. These materials fill the gap between amorphous
materials without any long range order and conventional coarse-grained
materials. Definitions vary, but nano-crystalline material is commonly defined
as a crystallite (grain) size below 100 nm. Grain sizes from 100–500 nm are
typically considered "ultrafine" grains.
Nano-crystalline materials are single- or multi-phase polycrystalline solids with
a grain size of a few nanometers (1 nm = 10−9), typically less than 100 nm. Since
the grain sizes are so small, a significant volume of the microstructure in nano-
crystalline materials is composed of interfaces, mainly grain boundaries, i.e., a
large volume fraction of the atoms resides in grain boundaries. Consequently,
nano-crystalline materials exhibit properties that are significantly different
from, and often improved over, their conventional coarse-grained polycrystalline
counterparts.