introduction to shape memory alloys...

Introduction to Shape Memory Alloys (SMAs)

•  (Micro)structural aspects of martensitic transformations (MT)

•  Thermodynamic issues of MT

•  Self-accomodation modes

•  Detwinning

•  One-way shape memory effect

•  Superelasticity

•  Two-way shape memory effect

•  Superelasticity – Pseudoelasticity

•  Nitinol alloys

•  Applications

SMAs

(Micro)structural aspects

BAIN STRAIN

Thermally induced martensitic transformation

SMAs


austenite

martensite

SMAs


MARTENSITE:

Monoclinic (B19)

low simmetry

AUSTENITE:

Cubic (B2)

high simmetry

SMAs


SMAs


Thermodynamic issues

Af As

Ms

Ms Mf

Main features of the MT: •  First order •  Athermic •  Non diffusive •  Reversible

Af As

Ms Mf


Ms Ms Ms

Ms

Ms

As


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0 10 20 30 40 50 60 70

Temperatura

% d

i fa

se a

uste

nit

ica

riscaldamento

raffreddamento


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Temperatura (°C)

% d

i fa

se

tra

sfo

rma

ta

% di fase martensitica

% di fase austenitica


Self accommodation modes

INVARIANT SHEAR STRAIN: Mechanisms for accommodating shape and/or volume changes

SLIP



INVARIANT SHEAR STRAIN: Mechanisms for accommodating shape and/or volume changes

TWINNING

TWINNING: Thermally reversible strain!

Some features of twins: •  Low energy •  High mobility •  No break of chemical bonds


SEVERAL VARIANTS... i.e., ORIENTATIONS Self accommodation modes

About thermal hysteresis: a comparison between transformation nature… THERMOELASTIC (Au-Cd) Fully MARTENSITIC, i.e., no THERMOELASTIC (Fe-Ni)

Au-Cd: A->M - nucleation/growth M->A - inversion

Fe-Ni: A->M & M->A nucleation/growth


Result of self accommodation: the shape is preserved. Self accommodation modes

DETWINNING

Detwinning

MACRO & MICROSCOPIC ASPECTS of DETWINNING

Detwinning

Detwinning

T<Mf

Detwinning

T<Mf Detwinning

Tension Compression

Tension

Compression

Detwinning

Detwinning

Compression 4%

Tension 4%

Detwinning

One-way shape memory effect

1)

2)

3)

1) austenite->martensite (thermally induced) 2) deformation of martensite 3) martensite->austenite (thermally induced)…SHAPE RECOVERY!

Requirements for shape recovery:

• Twinning as accommodation mode of Martensite;

• Very small or nil volume changes involved in the

• A<->M transformation (<<1%).


Leghe a memoria di forma

FIRST CICLE

SECOND CICLE

Shape change occurs just once:

One Way Shape Memory Effect (OWSME)

A M

A

A M

M


Superelasticity Mechanically induced martensitic tranformation

OWSME

Superelasticity

T>Af

Superelasticity

Superelasticity

SUPERELASTICY: elevated strains (8%)... Fully recoverable (reversible)

Superelasticity

€

dσdT

=ΔHM >A

ΔVdefTt

Superelasticity

Superelasticity


Sistema Isteresi Ms RangeNiTi 20-30 °C -60/60 °C

NiTiCu 5-10°C 30°C/50 °CAgCd 15°C -80°C

CuZnAl 10°C -20/50°CCuAlNi 35°C 80/130 °CNiTiNb 40-120°C -20/-50°C

Supe

rela

stic

rang

e

Superelasticity

Superelasticity

Superelasticity – One-way shape memory effect

Superelasticity - Pseudoelasticity

T<Mf T>Af

T>Md T≈Md

T<Mf

T>Md



0%

1.2%

4% 6%

T<Mf


Two-way shape memory effect: phenomenology

One-Way SME

Two-Way SME

A M

A

A M

M A …...

Superelastic (mechanical) cycles

One-way shape memory cycles

High strain of martensite

Two-way shape memory effect: training

A M A …...

Two Way Shape Memory Effect (TWSME)


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Numero cicli termici

Allu

ngam

ento

%

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Allu

ngam

ento

%

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Allungamento %

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rzo

(M

Pa

)

Two-way shape memory effect: decay of the effect

introduction to shape memory alloys...

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