1Shape memoryTopic 11Reading assignment• Lecture notes on “Shape Memory”on the course webpage• Askeland and Phule, The Science and Engineering of Materials, 4thEd., Sec. 11-11 (first page only) and Sec. 11-12.Shape-memory alloy (SMA)• A material that can remember its shape• A class of smart materials• SMA also exhibits superelastic (pseudoelastic) behaviorSuperelastic behaviorSMAs deformed above a critical temperature show a large reversible elastic deformation (recoverable strains up to 10%. much exceeding the elasticity) as a result of stress-induced martensitic transformation2Applications of superelastic behavior• Orthodontal braces• Frames for eyeglasses• Underwires for brassieres• Antennas for cellular phonesApplications of shape-memory effect• Self-expandable cardiovascular stent• Blood clot filters• Engines• Actuators for smart systems• Flaps that change direction of airflow depending upon temperature (for air conditioners)• CouplingsCoupling for Tubing©2003 Brook s/Cole, a division of Thom son Learning, Inc. T homson Learning™is a trademark used herein under l icense.Use of memory alloys for coupling tubing: A memory alloy coupling is expanded (a) so it fits over the tubing (b). When the coupling is reheated, it shrinks back to its original diameter (c), squeezing the tubing for a tight fitExamples of SMAs• Cu-Zn-Al• Cu-Al-Ni• Ni-Ti (50 at.% Ti, nitinol, which stands for Nickel Titanium Naval Ordinance Laboratory)3Origin of shape-memory effectMartensitic phase transformation that occurs as a result of stress or temperature changeTriggers for martensitictransformation• Stress• TemperatureSteps of using an SMA• Betatizing (heating to equilibrate at the austenite phase field of the phase diagram)• Quench to form martensite• Deform the martensite• Heat to return to the austenite phase and to restore the original shape4Martensitic transformation• A diffusionless solid-state phase transformation; no change in composition.• Also known as athermal or displacivetransformations.• Transformation results in a metastablephase known as martensite.• The growth rate is so high that nucleation becomes the rate-controlling step.Eutectic transformation – involves diffusion due to change in compositionMartensite has a twinned microstructureTwinning enables plastic deformation, hence superelasticity.Variants of martensiteDue to various twinning configurations5Coherent interfaceIncoherent interfaceInterface between austenite and martensite phasesMartensitic transformation temperatures• Ms: temperature at which austenite begins to transform to martensiteupon cooling• Mf: temperature at which transformation of austenite to martensite is complete upon coolingMartensitic transformation temperatures• As: temperature at which martensite begins to transform to austenite upon heating• Af: temperature at which transformation of martensite to austenite is complete upon heatingHysteresisMf <Ms <As <Af6Stress generationIf an SMA is constrained from recovering (e.g., within a composite material), a recovery stress if generated.Mechanisms of deformation of martensite• Growth of favorably oriented twins• Deformation twinning (twinning upon shear during deformation)T < AsT > Af<T < AfAs7Superelastic behaviorT > AfHysteresis loop means energy dissipation, hence vibration dampingStressShape memory in polymers using viscoelastic behaviorFerroelasticityT < AsTypes of shape-memory behavior• One-way shape memory: transformation to the desired shape occurs only upon heating, i.e., memory is with the austenite phase.• Two-way shape memory: the deformed shape is remembered during cooling, in addition to the original shape being remembered during heating, i.e., memory is with both austenite and martensite phases (requires training to attain memory during cooling; formation of favorably oriented twins during cooling between Msand Mf)8Ferromagnetic shape-memory alloys• Shows shape-memory effect in response to a magnetic field • Deformation due to magnetic field is known as magnetoelasticdeformation.• Ni-Ti is non-magnetic • Examples of ferromagnetic SMAs: Ni2MnGa, Fe-Pd,