SHAPE MEMORY ALLOYS
Fundamentals and applications
RICCARDO CASATI
MILANO, FEBRUARY 2022
Shape memory alloys are functional materials
that show two peculiar properties:
Video Shape Memory Effect
• Shape Memory Effect (SME)
• Pseudoelasticity/Superelasticity
Both effect are based on a martensitic transformation
Video Superelasticity
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• The SMA functional properties are due to a particular martensitic
transformation (MT), i.e. shear-dominant diffusionless solid-state phase
transformation occurring by nucleation and growth of the martensitic
phase from a parent austenitic phase.
• MT is associated with an inelastic deformation of the crystal lattice with
no diffusive process involved. Parent and Martensite phases coexist during
the phase transformation (first order transition) and as a result there
exists an invariant plane, which separates the parent and product phases.
• This transformation is crystallographically reversible.
• Since the crystal lattice of the martensitic phase has lower symmetry than
that of the parent austenitic phase, several variants of martensite can be
formed from the same parent phase crystal.
• Martensite can be Thermally Induced (TIM) or Stress Induced (SIM)
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Diffusive transformations
• Formation of a phase with a new chemical composition
• They occur thanks to the transport of atoms over relative distances
• Diffusion of atoms takes time to take place (time dependent);
• They can be suppressed by quenching.
• They are defined as isothermal because they can proceed over time at a constant temperature.
Displacive transformations
• They do not modify the composition of the parent phase, but only the crystalline structure
• Atoms rearrange themselves in short-range cooperative motion
• Since no atomic migration is necessary, displacive transformations progress almost independently of
time, with the speed of the interface between two phases able to move at the speed of sound.
• They are referred to as athermic transformations, since they do not progress at a constant temperature
and the amount of the new phase present depends only on temperature, not time.
• Martensitic transformations are displacive transformations. The term martensite refers to the stable
phase at low temperature and austenite refers to the stable parent phase at high temperature.
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Cooling from austenite:
• Martensite start
• Martensite finish
Heating from martensite:
• Austenite start
• Austenite finish
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It takes place through two distinct contributions:
• Bain deformation (lattice deformation) which consists of all the atomic movements that serve to
produce the new crystalline structure. Very small atomic shifts are required to advance the interface
• Lattice-invariant shear: accommodation
of the shape variation that allows the
new martensitic phase to stay within the
austenitic matrix that surrounds it.
This accommodation can occur through the phenomenon
of sliding dislocations (slip) as in steels or through the
formation of twinning as occurs in many SMAs. During
martensitic transformation the crystalline lattice
undergoes a shearing parallel to a particular
crystallographic plane. The different potential orientations
in which such shearing can be produced are named
variants
Slip
Twinning
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Twinning causes the formation of a new variant and plays a key
role in the martensitic transformation, in the deformation of
martensite, in the shape memory effect and in superelasticity.
• On the twinning plane, atoms are part of the two adjacent
lattice regions, no dislocations are required, and the energy
accumulated by the boundary is low.
Twinning in a BCC structure
• Twin boundaries are very mobile, small movements are
enough to make a row of atoms move and advance the twin
boundary.
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• The martensite formed by cooling the austenite has a
twinned structure, forming several variants. Twin
boundaries can move easily (low energy, high mobility)
and certain martensite variants can grow or shrink with
small system energy changes. The growing of most
favorable variants instead of others when a load is
applied is named DETWINNING, it is possible to change
the shape of a Nitinol artifact by spending little energy.
• If the deformation is sufficiently high, a martensitic grain
consisting of multiple variants can transform into a grain
consisting in a first approximation of a single variant.
• Detwinning is exploited for SME
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Austenite
(Shape I)
Austenite
Detwinned-M
Twinned-M
Detwinned-M
(Shape II)
1.
Austenite (shape I)
2.
Cooling T< Mf (TIM)
3.
Loading – Detwinned Martensite
4.
Unloading – recovery of elastic deformation (shape II)
Twinned-M
(Shape I)
1. Heating – recovery of shape I
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.
• The unique ability of martensite
to accommodate the change in
shape through the growth of
some variants allows it to
accommodate the deformations
and reduce the internal energy of
the system.
LOAD
UNLOAD
• At temperatures where the chemical energy of austenite is less than that of martensite,
the total energy of the martensitic system could be less if a stress is applied. Hence the
formation of stress-induced martensite (SIM) is energetically favored.
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Temperature is constant (>Af)
A→SIM
1 Elastic deformation of Austenite
2 Forward P-M transformation
SIM→A
3 Unloading of Martensite
4 Reverse M-P transformation
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Nitinol
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• The term Nitinol refers to a class of alloys with a
quasi-equiatomic composition of Nickel and
Titanium.
• The term derives from the words "NickelTitanium-Naval-Ordnance-Laboratory" referring to
the center where these alloys were initially
studied.
• NiTi is the most widely used Shape Memory Alloy
(SMA) for applications in the industrial and
medical fields.
William J. Buehler
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• The
NiTi
intermetallic
compound can accept an
excess of Ni atoms, in
particular above 600° C
• The number of substitutional
Ni
atoms
affects
the
transformation temperatures
• Excess in Ti leads to the
precipitation of Ti2Ni, while
an excess in Ni causes the
precipitation of Ni3Ti2
• It is also possible to create
other metastable phases by
specific aging treatment
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• Space group of cesium chloride type (CsCl): each of the two types of atoms (Ni and Ti) form a
simple cubic lattice; the two lattices are interpenetrated so that each atom of one type is at
the center of a unit cell of the simple cubic lattice of the other atoms. Each unit cell
therefore contains 1 Ni atom and 1 Ti atom.
• This structure is also defined B2 according to the designation Strukturbericht or Pm3m
according to the Hermann – Mauguin notation.
• Ni and Ti therefore occupy well-defined positions in the lattice rather than being randomly
distributed as in common metal alloys → intermetallic compound
Ni
Ti
1/8 atomo
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tetragonal cell
The martensitic phase of NiTi has a
monoclinic structure, defined B19’ by
the designation Strukturbericht or
P21/m.
Like all martensitic phases, the
martensite of NiTi also has less
symmetry than the parent phase (B2).
Martensite B19‘ inherits a structure
ordered from B2
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B2 transforms into B19‘ lattice, which can have 24 different crystallographic orientations (24
variants), due to the lower symmetry compared to the parent phase. Hence, the martensite
variants all have the same crystal structure but different crystallographic orientation.
Simplified 2D example: A simple square forms 4
variants with identical structure but different
orientation
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In NiTi alloys a further martensitic transformation can take place which involves the formation of a phase
with a rhombohedral structure, the so-called R phase (space group P3 or R3-). The rhombohedral
structure is a distorted structure of austenite B2. The rhombohedral distortion consists in stretching the
cubic cell along a diagonal of the cube (on of the [111] directions of the B2) which involves a reduction of
α from 90 ° to 89 °. The direction [001]B2 does not change at the end of the transformation. The
rhombohedral structure has α = β = γ ≠ 90 ° and a = b = c. There are 4 variants of martensite that can
form, one for each diagonal of the cube. As for the B19 ', twins are formed to self-accommodate the
shape change.
89°
Transformation B2→R in a Ti50.3Ni48.2Fe1.5 alloy
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• The A→R→A transformation is characterized by an extremely low hysteresis, and a
limited SME or SE effect.
• Phase R training is stimulated by the addition of particular alloying elements, residual
plastic deformation, aging treatment, formation of coherent precipitates such as Ni4Ti3,
thermomechanical cycling
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• The austenitic B2 lattice of NiTi allows an
excess of Ni at high temperatures.
Tang et al. (1999)
Frenzel et al. (2010)
• If the material is quickly cooled (quenched)
from high temperatures, the Ni remains
trapped although not in equilibrium
configuration at ambient temperatures.
• Ni in excess causes a drastic reduction in
temperatures. An excess in Ti causes the
formation of second Ti2Ni phases without
impacting the transformation temperatures.
• Ni reduces material ductility.
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PTT diagram (Nishida et al.)
• Aging leads to the formation of Ni-rich phases.
Precipitation of such phases enrich the matrix in
Ti. Thus, transformation temperatures increase
• Alloys with more than 51at.% Ni are brittle if not
aged
TEM image of Ti3Ni4 precipitates
N.B. Ti11Ni14 è stato poi indicizzato in lavora successivi come Ti3Ni4
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90% of applications containing SMA alloys use binary NiTi with a Ni content between
49 and 51% at. without further alloying elements. Interstitial atoms such as carbon and
oxygen are present in very low quantities (few hundreds of ppm) and are considered as
impurities to be eliminated. For particular applications, substitutional elements are
used (NiTiX eg. NiTiFe, NiTiNb, NiTiCr, NiTiCu, NiTiCo, NiTiV, NiTiPt and NiTiHf
functional properties) to control the functional properties:
•
Increase the hysteresis (Nb)
•
Reduce hysteresis (Cu)
•
Increase transformation temperature (Pt, Pd, Hf, Zr)
•
Reduce transformation temperature (or increase the stiffness of B2) (Fe, Co, V, Cr, Mn, Al, Ta)
•
Increase radiopacity (RE)
•
Increase mechanical strength, corrosion resistance or a creep
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NiTi is very reactive with oxygen :
• Formation of a surface oxide TiO2 (Rutile)
• Formation of oxides within the metal matrix such as
Ti4Ni2OX
• Interstitial oxygen in NiTi lattice
Oxygen cannot be avoided (present in raw materials, especially in
Titanium, 200-500 ppm).
Oxygen has a higher solubility in Ti2Ni than in NiTi. The Ti2Ni
precipitates can absorb oxygen until they reach the Ti4Ni2O
composition.
Ti4Ni2OX compounds (0 <X <1) are isostructural.
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Ti4Ni2OX compounds (0 <X <1) are insoluble inclusions in the matrix and inert to heat treatment.
They can affect the fatigue resistance of the material. They are also present in alloys rich in Ni
(superelastic). These Ti-rich inclusions modify the stoichiometry of the alloy, thus lowering the
transformation temperatures
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Carbon does not dissolve in NiTi, it forms carbides with Titanium
(TiC). C is usually not present in the starting materials (Ti and Ni),
it is introduced into the alloy only if the alloy is melted in a
graphite crucible. Ti and Ni absorb C in the liquid phase. About
half of the nitinol is produced in copper crucibles, so it does not
contain C.
TiC particles are generally micrometric in size and have geometric shapes,
with sharp edges
They can affect the fatigue resistance of the material.
To form TiC, the matrix is depleted in Ti, so the alloy composition needs to
be slightly corrected.
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• The two most popular melting technologies for the
production of NiTi ingots are VIM (vacuum-induction
melting) and VAR (vacuum-arc remelting). Starting
material are Ni and Ti with high purity.
• In VIM ovens, Ni and Ti bars are placed in a graphite
crucible which is in turn inserted into a vacuum
chamber. The crucible is heated by magnetic induction
• Advantage: the molten metal is mixed by magnetic
stirring → homogeneous chemical composition and
functional properties
• Disadvantage: carbon impurities (300-700 ppm)
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Micrographs: Carbon inclusion in wire
produced from VIM ingots
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• Ni and Ti weighed in the right ratio and pressed together
to form the consumable electrode of the VAR (Vacuum arc
remelting).
• An electric arc is generated between the Ni and Ti
electrode and the bottom of the crucible, the current is
sufficient to melt the electrode that is deposited on the
bottom of the crucible
• Water-cooled Cu crucible
• Advantages: high purity, no carbon
• Disadvantages: poor chemical homogeneity of the ingot,
which has to be remelted several times.
• Often two techniques (VIM / VAR) are combined
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Hot working at temperatures between 600 ° C and 900 ° C (0.55-0.75 Tm)
Cold working with annealing at temperatures between 600 ° C and 800 ° C
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Final heat treatment to confer the shape (to be remembered) to the material
1) Cold Deformation
2) Bending in the desired shape and fixing.
3) Low temperature treatment (aging) under constraint (350-550°C 5-20 min)
- Ti3Ni4 Precipitation
- Localized stress field in NiTi Matrix
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• One way SME
• Constrained Recovery
• Work Generation / Actuators
• Superelasticity
• High Damping Capacity
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1 Cooling below Mf
2 Deformation at T< Mf (Load/Unload)
3 Free Heating above Af
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Smars NASA rock breaker
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Advantages
Disadvantages
• Sensor-Actuator (Active device)
• Thermal Hysteresis
• Noiseless
• Non linear behaviour
• No Lubrification
• Limited operating ranges
• High reliability
• Functional properties to be stabilized
• No flame or toxic release
• High cost
• Electric circuit
• High power/weight ratio
• Excellent oxidation resistance
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Water mixing valve – Furukawa NT
Air conditioning - Flap movement
Oil valve mixing device Shinkanzen – Furukawa NT
TiNi pinpullers and frangibolt for space deployables
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Prof. Riccardo Casati
Department of Mechanical Engineering
Politecnico di Milano
[email protected]
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