Introduction to Shape Memory Alloys...
Transcript of 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
(Micro)structural aspects
austenite
martensite
SMAs
(Micro)structural aspects
MARTENSITE:
Monoclinic (B19)
low simmetry
AUSTENITE:
Cubic (B2)
high simmetry
SMAs
(Micro)structural aspects
SMAs
(Micro)structural aspects
Thermodynamic issues
Af As
Ms
Ms Mf
Main features of the MT: • First order • Athermic • Non diffusive • Reversible
Af As
Ms Mf
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues
Ms Ms Ms
Ms
Ms
As
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues
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Thermodynamic issues
Thermodynamic issues
Self accommodation modes
INVARIANT SHEAR STRAIN: Mechanisms for accommodating shape and/or volume changes
SLIP
Self accommodation modes
Self accommodation modes
INVARIANT SHEAR STRAIN: Mechanisms for accommodating shape and/or volume changes
TWINNING
Self accommodation modes
TWINNING: Thermally reversible strain!
Some features of twins: • Low energy • High mobility • No break of chemical bonds
Self accommodation modes
SEVERAL VARIANTS... i.e., ORIENTATIONS Self accommodation modes
Self accommodation modes
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
Self accommodation modes
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!
One-way shape memory effect
Requirements for shape recovery:
• Twinning as accommodation mode of Martensite;
• Very small or nil volume changes involved in the
• A<->M transformation (<<1%).
One-way shape memory effect
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
One-way shape memory effect
One-way shape memory effect
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
Superelasticity
Leghe a memoria di forma
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
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Superelasticity
Superelasticity
Superelasticity – One-way shape memory effect
Superelasticity – One-way shape memory effect
Superelasticity - Pseudoelasticity
T<Mf T>Af
T>Md T≈Md
T<Mf
T>Md
Superelasticity - Pseudoelasticity
Superelasticity - Pseudoelasticity
Superelasticity - Pseudoelasticity
Leghe a memoria di forma
0%
1.2%
4% 6%
T<Mf
Superelasticity - Pseudoelasticity
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)
Two-way shape memory effect: training
Two-way shape memory effect: training
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Two-way shape memory effect: decay of the effect