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
(Micro)structural aspects Thermally induced martensitic transformation BAIN STRAIN SMAs
(Micro)structural aspects SMAs
(Micro)structural aspects austenite martensite SMAs
(Micro)structural aspects AUSTENITE: Cubic (B2) high simmetry MARTENSITE: Monoclinic (B19) low simmetry SMAs
(Micro)structural aspects SMAs
Thermodynamic issues M s M f M s A s A f
Thermodynamic issues M f M s A s A f Main features of the MT: First order Athermic Non diffusive Reversible
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues s M s s s A s M s
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues
Thermodynamic issues % di fase austenitica 100 80 60 40 20 0 0 10 20 30 40 50 60 70 riscaldamento raffreddamento Temperatura
Thermodynamic issues 100 90 80 % di fase trasformata 70 60 50 40 30 20 10 0 30 35 40 45 50 55 60 Temperatura ( C) % di fase martensitica % di fase austenitica
Thermodynamic issues
Self accommodation modes
Self accommodation modes INVARIANT SHEAR STRAIN: Mechanisms for accommodating shape and/or volume changes SLIP
Self accommodation modes INVARIANT SHEAR STRAIN: Mechanisms for accommodating shape and/or volume changes TWINNING
Self accommodation modes
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.
Detwinning DETWINNING
Detwinning MACRO & MICROSCOPIC ASPECTS of DETWINNING
Detwinning
Detwinning T<M f
Detwinning T<M f
Detwinning Tension Compression Tension Compression
Detwinning
Detwinning Compression 4% Tension 4%
One-way shape memory effect 1) 3) 2) 1) austenite->martensite (thermally induced) 2) deformation of martensite 3) martensite->austenite (thermally induced) SHAPE RECOVERY!
One-way shape memory effect
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 FIRST CICLE A M M A SECOND CICLE A M Shape change occurs just once: One Way Shape Memory Effect (OWSME) Leghe a memoria di forma
One-way shape memory effect
Superelasticity Mechanically induced martensitic tranformation OWSME
Superelasticity
Superelasticity T>A f
Superelasticity
Superelasticity SUPERELASTICY: elevated strains (8%)... Fully recoverable (reversible)
Superelasticity dσ dt = ΔH M >A ΔV def T t
Superelasticity
Superelasticity
Superelasticity Superelastic range Sistema Isteresi Ms Range NiTi 20-30 C -60/60 C NiTiCu 5-10 C 30 C/50 C AgCd 15 C -80 C CuZnAl 10 C -20/50 C CuAlNi 35 C 80/130 C NiTiNb 40-120 C -20/-50 C Leghe a memoria di forma
Superelasticity
Superelasticity One-way shape memory effect
Superelasticity One-way shape memory effect
Superelasticity - Pseudoelasticity T<M f T>A f T M d T>M d
Superelasticity - Pseudoelasticity T<M f T>M d
Superelasticity - Pseudoelasticity
Superelasticity - Pseudoelasticity
Superelasticity - Pseudoelasticity 1.2% T<M f 4% 6% 0% Leghe a memoria di forma
Two-way shape memory effect: phenomenology One-Way SME A M M A Two-Way SME A M A...
Two-way shape memory effect: training High strain of martensite One-way shape memory cycles Superelastic (mechanical) cycles
Two-way shape memory effect: training Two Way Shape Memory Effect (TWSME) A M A...
Two-way shape memory effect: training
Two-way shape memory effect: decay of the effect 1.6 1.4 1.2 Allungamento % 1 0.8 0.6 0.4 0.2 0 0 100 200 300 400 500 600 700 800 900 1000 1100 Num ero cicli termici 5 4.5 4 Sforzo (MPa) 240 210 180 150 120 90 60 30 0 0 1 2 3 4 5 6 7 Allungamento % Allungamento % 3.5 3 2.5 2 1.5 1 0.5 0 0 100 200 300 400 500 600 700 800 900 1000 1100 Num ero cicli termici