Shape memory alloys’ properties
- Shape memory effect
- Pseudoelstaicity/superelasticity
This effect are based on martensitic transformations
Martensitic transformation
- Origin from a parent austenitic phase
- It is associated to inelastic deformation of cristal lattice with no diffusive process involved
- This transformation is reversibile
- Since the crystal lattice of M has a lower symmetry than Aust several variants of martensite exist.
- Martensite can be thermally induced or stress induced
Diffusive transformastions
Diffusive:
- Formation of a phase with new chemical comp
- They occur thanks to the atoms transp
- Diffusion of atoms takes time to take place
- They can be suppressed by quencing
- Defined as isothermal (they proceed at const temp)
Displacive transf
Displacive:
- Not a new chemical comp
- No atoms transportation
- No time dep, they can occur at the speed of sound
- Athermic transf
Martensitic tranf in SMA
Takes place through two distinct contributions:
- Bain deformation: all atomic movements to produce the new lattice
- Lattice-invariant shear: accomodation of the shape variation that allows the new martensitic phase to stay within the austenitic matrix can occur throught slip or twinning
Twinning
- Causes the formation of a new variant and plays a key role in martensite transformation, deformation and the shape memory effect and superelasticity
De-Twinning
- The martensite formed by cooling austenite without applied stress, has a twinned structure.
- The growing of most favorable variants instead of others when a load is applied is named Detwinning
Shape memory effect
- SMA cicle:
1) austenite
2) Cooling T<Mf Twinning (a lot of twin occurs)
3) Loading (detwinng) only the first twin still exist
4) Removing load the deformation is still.
5) Heating austenite shape is recovered
Stress induce martensite
- At temperatures where the aust phase is stable, martensite can be generated through a load.
- the stress induce martensite formation because it become energetically favored
(posso citare l’energia libera di gibss)
Super elasticity and pseudo elasticity
Superelasticity (or pseudo-elasticity) is the ability of certain materials, specifically shape memory alloys, to undergo large strains when subjected to mechanical stress and to recover their original shape upon unloading without permanent deformation.
SMA actuator
- For small part we can use a SMA as actuator, taking advantage from the contraction in length of austenite phase respect to martensite
Examples of SMA
-NiTi
-Cu
-Iron Fe
- Magnetic
Nitinol
- Nitinol refers to a class of alloys with a quasi-equiatomic composition of nickel and titanium
- “Ni Ti Naval Ordnance Laboratory”
- Is the most used SMA
Discussion about Ni Ti phase diagram
- Number of Ni atoms affect the trans temp.
- Excess in Ti leads to the preciputation of Ti2Ni while an excess in Ti causes Ni3Ti2
Niti B2 autenite
- Ni and Ti occupy well defined position rather than being randomly distribuited
- Each atom of Ti and Ni are in the center of the lattice of the other atom.
Niti B19’ Martensite
- The martensitic phase of NiTi (non sono sicuro) is less symmetric than B2 (un atomo di Ti in meno) e forma un tetraedo schiacciato.
The R phase
NiTi phase has a rhombohedral structure and is a distortion of austenite B2.
The R phase transf from austenite is characterized by an extremely low hysteresis and limited SME SE effects.
They can be improved by addiction of some elements, residual plastic deformation, aging…
Effects of nickel on R phase
- Excess of Ni at high temp
- quencing Ni excess
- Ni in excess causes a drastic reduction in temperatures.
- Ni reduces material ductility
Effects of aging
- Aging leads to formation of Ni rich phases, precipitation of these phaes increase the temp.
Alloying elements effects
- increase hysteresis Nb
- Reduce Hysteresis Cu
- Increase transf temp Pt, Pd, Hf, Zr.
- Reduce transf temp Fe Co V
Oxygen and carbon
Are present in very low concentrations
- Oxygen is very reactive with NiTi and forms a surface and inner oxide, inclusion that reduces fatigue strenght.
- Carbon
Carbon doesn’t dissolve in NiTi and affect fatigue strenght
Niti properties
- High melting temp
- Density low
- High corrosion resistance
- uts high
Vacuum induction melting
- VIM ovens Ni Ti bars are placed in a graphite crucible magnetically heated.
Then is insert into a vacuum chamber.
the advantages are a homogeneous solution
The disadvantages are lots of carbon impurities.
Vacuum arc melting
Ni and Ti are pressed together to form a consumable electrode of VAR.
An electric arc is generated between the Ni and Ti electrode and the bottom of the crucible. The current melt the electrode and the metal is deposited on the bottom of electrode
- Advantages high purity
- Disadvantages poor chemical homogeneity
