Shape Memory Effect
Martensite-austenite transformation Austenite is parent, high-temperature phase,
cubic structure Martensite is low-temp phase, usually tetragonal [A] [twinned M] on cooling, diffusionless
shear transformation Deformation of martensite moves twin
boundaries; recovered on heating and transformation to austenite
Shape Memory Effect
Shape memory effect mechanism, showing (a) undeformed parent crystal, (b) martensite, (c and d) deformed martensite through twin boundary movement, and (e) reversion to the parent phase after heating. From Otsuka (1998), p.37, fig. 2.11.
Shape Memory Effect
Free-energy versus temperature curves for the parent (Gp) and martensite (Gm) structures in a shape memory alloy. From Otsuka (1998), p.23, fig. 1.17.
Martensite-austenite phase transformation in shape memory alloys. From http://www.tiniaerospace.com/sma.html.
Superelasticity
Stress-induced martensite formation above transition temperature Martensite immediately reverts to austenite once stress is
removed Large recoverable deformation
From http://www.sma-inc.com/SMAandSE.html
Nickel-Titanium
Near-equiatomic NiTi most widely used SMA todayProperty Value
Transformation temperature -200 to 110 C
Latent heat of transformation 5.78 cal/g
Melting point 1300 C
Specific heat 0.20 cal/g
Young’s modulus 83 GPa austenite; 28 to 41 GPa martensite
Yield strength 195 to 690 MPa austenite; 70 to 140 MPa martensite
Ultimate tensile strength 895 MPa annealed; 1900 MPa work-hardened
% Elongation at failure 25 to 50% annealed; 5 to 10% work-hardened
From http://www.sma-inc.com/NiTiProperties.html
Nickel-Titanium
B2 (cesium chloride) crystal structure. From http://cst-www.nrl.navy.mil/ lattice/struk/b2.html
B19’ crystal structure. From Tang et al., p.3460, fig.5.
Parent β (austenite) phase with B2 structure
Martensite phase with monoclinic B19’ structure
Nickel-Titanium
Intermediate R phase can nucleate in B2, then B19’ phase grows from R
1 and 2 show single dislocations in B2 from which an R phase grows
Nucleation of R-phase in an alloy of Ti-48Ni-2Al from dislocations. From Otsuka (1998), p.56, fig. 3.7.
Shape Memory Alloys Today
Shape memory effect means deforming at low temperature, changing back at high temperature
Shape memory alloys (SMAs) first discovered 1951 NiTi SMA discovered 1963 Macroscale applications as:
Tube couplings Air-directing flaps Spring actuators
MEMS Applications for SMAs
Surgical micro-wrapperhttp://www.afrlhorizons.com/Briefs/Sept02/OSR0203.html
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