Shape Memory Materials

Question 1

what is the shape memory effect?

Answer 1

The ability of the materials to recover their original shape from a
significant and seemingly plastic deformation when a particular
stimulus is applied

Question 2

what are shape changing materials?

Answer 2

Shape-changing materials (SCM) are materials that change shape in
response to a specific stimulus
- If the change is to/from a programmed shape, then the material is
termed SMM.

Question 3

electrostrictive material

Answer 3

Dielectric materials display a shape change
with respect to the direction of the field,
due to the train caused by the movement of
ions in the crystal lattice

Question 4

magnetostrictive materials

Answer 4

The change in the materials shape or
dimensions due to the influence of an
external magnetic field (usually in
ferromagnetic)

Question 5

what are the 2 categories of shape memory alloys?

Answer 5

According to the type of stimulus, SMAs fall in two categories, one is
thermo-responsive, and the other is magneto-responsive

Question 6

what are famous SMAs in use?

Answer 6

nickel–titanium (Ni-Ti, Nitinol), copper–zinc–
aluminum–nickel, and copper–aluminum–nickel

Question 7

In these 2 types what is the cause of the shape change?

Answer 7

the reversible martensitic transformation is the
underlying driving mechanism.

Question 8

SMAs have the ability to what?

Answer 8

Ability to change from one crystallographic structure to another in
response to a stimulus.

Question 9

What are these 2 cystallographic structures?

Answer 9

the low-temperature
martensite phase and the high-temperature austenite phase

Question 10

why is the term “twinned martensite” sometimes given?

Answer 10

because the
diffusionless/shear transformation creates a twinned structure

Question 11

Why is the shape change not visible macroscopically?

Answer 11

because the twinned
martensite occupies the same space as the austenite

Question 12

What happens to the strain that exists in the deformed martensite?

Answer 12

it is completely recovered, and the martensite recaptures its unique shape during the
process of heating to the austenite phase.

Question 13

At low temperatures SMAs have what?

Answer 13

SMAs have the SME (needs heat to recover the
shape),

Question 14

At high temperatures SMAs?

Answer 14

recovery can be achieved instantly and
simultaneously upon releasing the applied load, just like rubber band
(super-elasticity).

Question 15

The shape memory effect is the characteristic of?

Answer 15

Shape memory materials

Question 16

Superelasticity is the characteristic of?

Answer 16

Shape changing materials

Question 17

SMAs in aerospace?

Answer 17

• Boeing variable geometry chevron
• Adaptive wing structure
  Thermal stimuli initiate shape changes

Question 18

SMAS in medical?

Answer 18

Development of shape memory cardiovascular stent

Question 19

In shape memory ceramics (SMC) what is the elastic strain change associated with?

Answer 19

the electric field–induced phase transition

Question 20

Examples of shape memory ceramics?

Answer 20

stabilized zirconia and ferroelectric lead
zirconate titanate.

Question 21

what are the mechanisms for shape recovery in shape memory ceramics?

Answer 21

viscoelastic,
martensitic, ferroelectric, and ferromagnetic

Question 22

Viscoelastic mechanism?

Answer 22

The shape is fixed upon cooling and the original shape is recovered
upon heating again.
* The elastic strain energy due to the deformation strain stored in the
glassy phase provides the driving force for shape recovery.

Question 23

What does the viscoelastic mechanism mostly happen in?

Answer 23

Mostly in glass (40%–60% mica in a glass matrix)
* Mica glass recovery of up to 0.5% of strain at high temperatures

Question 24

Ailicon nitride, silicon carbide, zirconia, and alumina also show what?

Answer 24

viscoelastic properties but the strain energy is lower

Question 25

How does the martensitic transformation happen in zirconia ceramic?

Answer 25

the phase transition is between a
tetragonal and a monoclinic structure, which is induced thermally or
by the application of stress

Question 26

Shape memory ceramics exhibit what?

Answer 26

pseudoelastic deformation

Question 27

martensitic mechanism in ceramics?

Answer 27

On loading at room temperature (cooling the austenitic phase), the
shape change is effected by a stress-induced transformation from
austenite to martensite, and when the load is removed, the new
shape remains

Question 28

How is the shape recovered in martensitic mechanicsm in ceramics?

Answer 28

Subsequent heating above the transition temperature causes the
martensite to revert to its original shape by retracting the loading
path

Question 29

ferroelectric and ferromagnetic mechanism?

Answer 29

On applying an electric field, the ions inside the material become
polarized, and the electric dipole moments in each crystal unit cell are
arranged in parallel in the FE phase and against each other in the AFE
phase.

Question 30

the material is non polarized in what phase?

Answer 30

During the PE phase (paraelectric), which upon cooling
forms the AFE(antiferroelectric) phase with a specific dipole moment

Question 31

The application of an external field transforms the AFE phase to the?

Answer 31

FE phase, and the SMC is thereby deformed, as with the martensite in
SMAs.

Question 32

This deformed state is?

Answer 32

unstable and is based purely on the applied
field orientation, as it can revert to AFE upon field reversal

Question 33

Upon heating beyond the critical temperature, the FE phase does what?

Answer 33

gains
back its original memorized shape.

Question 34

Viscoelastic material

Answer 34

property of materials that exhibit both viscous and elastic characteristics when undergoing deformation.

Question 35

what is pseudoelasticity?

Answer 35

sometimes called superelasticity, is an elastic (reversible) response to an applied stress, caused by a phase transformation between the austenitic and martensitic phases of a crysta

Question 36

what is twinning?

Answer 36

results in the formation of a domain crystal inside a parent crystal, where the two share some of the crystal lattice points in a symmetrical manner and are separated by twin boundaires

Question 37

what is the diffusionless shear mechanism?

Answer 37

Diffusionless transformations occur when atoms in a crystal move cooperatively and nearly simultaneously, distorting the crystal into a new shape.