Shape Memory Alloys Theresa Valentine ENMA490 Fall 2002

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

Shape Memory Effect Free-energy versus temperature curves for the parent (G p ) and martensite (G m ) structures in a shape memory alloy. From Otsuka (1998), p.23, fig Martensite-austenite phase transformation in shape memory alloys. From

Superelasticity  Stress-induced martensite formation above transition temperature Martensite immediately reverts to austenite once stress is removed Large recoverable deformation From

Nickel-Titanium  Near-equiatomic NiTi most widely used SMA today PropertyValue Transformation temperature -200 to 110  C Latent heat of transformation5.78 cal/g Melting point 1300  C Specific heat0.20 cal/g Young’s modulus83 GPa austenite; 28 to 41 GPa martensite Yield strength195 to 690 MPa austenite; 70 to 140 MPa martensite Ultimate tensile strength895 MPa annealed; 1900 MPa work-hardened % Elongation at failure25 to 50% annealed; 5 to 10% work-hardened From

Nickel-Titanium B2 (cesium chloride) crystal structure. From 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

Macroscale SMAs  “Magic flower”

Macroscale SMAs  “Climbing koala”

Macroscale SMAs  Eyeglass frames – superelastic NiTi

MEMS Applications for SMAs  TiNi pneumatic microvalve

MEMS Applications for SMAs  NiTi microbubble (UCLA)

MEMS Applications for SMAs  Flow control

MEMS Applications for SMAs  Surgical micro-wrapper

MEMS Applications for SMAs  One-axis translation stage

MEMS Applications for SMAs  Micro-gripper

Future Research  Thin films for micro-actuators, micro-devices Compositional changes during sputtering Accurate phase diagram necessary  Two-way shape memory effect  Alloying NiTi with small percentages of other metals (Cu, Fe)