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Smart Materials > Shape memory alloy > Flashcards

Shape memory alloy Flashcards

1
Q

Shape memory alloys’ properties

A
  • Shape memory effect
  • Pseudoelstaicity/superelasticity
    This effect are based on martensitic transformations
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2
Q

Martensitic transformation

A
  • 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
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3
Q

Diffusive transformastions

A

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)

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4
Q

Displacive transf

A

Displacive:
- Not a new chemical comp
- No atoms transportation
- No time dep, they can occur at the speed of sound
- Athermic transf

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5
Q

Martensitic tranf in SMA

A

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

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6
Q

Twinning

A
  • Causes the formation of a new variant and plays a key role in martensite transformation, deformation and the shape memory effect and superelasticity
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7
Q

De-Twinning

A
  • 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
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8
Q

Shape memory effect

A
  • 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
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9
Q

Stress induce martensite

A
  • 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)
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10
Q

Super elasticity and pseudo elasticity

A

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.

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11
Q

SMA actuator

A
  • For small part we can use a SMA as actuator, taking advantage from the contraction in length of austenite phase respect to martensite
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12
Q

Examples of SMA

A

-NiTi
-Cu
-Iron Fe
- Magnetic

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13
Q

Nitinol

A
  • 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
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14
Q

Discussion about Ni Ti phase diagram

A
  • Number of Ni atoms affect the trans temp.
  • Excess in Ti leads to the preciputation of Ti2Ni while an excess in Ti causes Ni3Ti2
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15
Q

Niti B2 autenite

A
  • 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.
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16
Q

Niti B19’ Martensite

A
  • The martensitic phase of NiTi (non sono sicuro) is less symmetric than B2 (un atomo di Ti in meno) e forma un tetraedo schiacciato.
17
Q

The R phase

A

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…

18
Q

Effects of nickel on R phase

A
  • Excess of Ni at high temp
  • quencing Ni excess
  • Ni in excess causes a drastic reduction in temperatures.
  • Ni reduces material ductility
19
Q

Effects of aging

A
  • Aging leads to formation of Ni rich phases, precipitation of these phaes increase the temp.
20
Q

Alloying elements effects

A
  • increase hysteresis Nb
  • Reduce Hysteresis Cu
  • Increase transf temp Pt, Pd, Hf, Zr.
  • Reduce transf temp Fe Co V
21
Q

Oxygen and carbon

A

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
22
Q

Niti properties

A
  • High melting temp
  • Density low
  • High corrosion resistance
  • uts high
23
Q

Vacuum induction melting

A
  • 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.

24
Q

Vacuum arc melting

A

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
25
Q

Hot and cold working

A
  • Hot working at temp between 600 and 900°C cold with annealing 600°C and 800°C
26
Q

Shape setting

A

Final heat to confer the shape to be remembered
1) Cold deformation
2) bending in the desired shape and fixing
3) low temp heating (Aging) under constraints

27
Q

One way SME

A
  • Coupling fasteners
28
Q

SMA actuators

A
  • Air conditioning
  • Oil valves
  • water mixing valve
  • Micro grippers
  • Cameras
29
Q

SMA actuators advantages

A
  • Active device (sensor+actuator)
  • Noisless
  • No lubrifi
  • High reliability
  • Electric circuits
  • oxidation resistance
  • High power/ratio
30
Q

SMA actuators disadvantages

A
  • thermal Hysteresi
  • Non linear behaviour
  • Limited operating ranges
  • functional prop to be stabilized
  • High costs
31
Q

Superelasticity

A

Superelastic materials can undergo substantial elastic deformation without undergoing permanent plastic deformation. This means they can be stretched or compressed to a considerable extent and still return to their original shape once the load is removed.

32
Q

Damping capacity

A

The damping capacity of Shape Memory Alloys (SMAs) refers to their ability to absorb and dissipate mechanical energy during cyclic loading. This property is particularly valuable in applications where vibration reduction and shock absorption are important. The high damping capacity of SMAs arises from their unique phase transformation behavior and the associated internal friction mechanisms.

Smart Materials (12 decks)

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