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Special Technologies
An Introduction to Shape Memory Alloys (SMAs)
Mehrshad [email protected] University of RomeDepartment of Mechanical and Aerospace Engineering
Department of Mechanical and Aerospace Engineering
Special TechnologyTypes of Materials
- Metals- Ceramics- Polymers- Composites- Semiconductors
REVIEW!
Department of Mechanical and Aerospace Engineering
Special TechnologyPiezoelectric materialsShape memory alloysMagnetic shape memory alloysMagnetorheologicalPH sensitive polymersHalochromic materialsThermochromic materialsChromogenic systemsElectrochromicSmart Grease
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Department of Mechanical and Aerospace Engineering
Special TechnologyWhat are Shape Memory Alloys? Shape Memory Alloys (SMAs) are metallic alloys that undergo a solid-to-solid phase transformation which can exhibit large recoverable strains. Example: Nitinol
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Department of Mechanical and Aerospace Engineering
Special TechnologyTimeline of Memory Metals1932 - A. lander discovers the pseudoelastic properties of Au-Cd alloy.1949 - Memory effect of Au-Cd reported by Kurdjumov & Kandros.1967 At Naval Ordance Laboratory, Beuhler discovers shape memory effect in nickel titanium alloy, Nitinol, which proved to be a major breakthrough in the field of shape memory alloys.1970-1980 First reports of nickel-titanium implants being used in medical applications.Mid-1990s Memory metals start to become widespread in medicine and soon move to other applications.
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Special TechnologyNumber of publications per year
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Special TechnologyGeographic distribution
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Special TechnologyShape Memory Alloys
AlloyTransformation CompositionTransformation Temp. Rang (C)Hysteresis (C)Ag-Cd44/49 at % Cd-190 to -50~15Au-Cd46.5/50 at % Cd30 to 100~15Cu-Al-Ni14/14.5 wt %Al, 3/4.5 wt %Ni-140 to 100~35Cu-Sn~15 at % Sn-120 to 30Cu-Zn38.5/41.5 wt % Zn-180 to -10~10Cu-Zn-X (X=Si,Sn,Al)few wt % X-180 to 200~10In-Ti18/23 at % Ti60 to 100~4Ni-Al36/38 at % Al-180 to 100~10Ni-Ti~49/51 at % Ni-50 to 110~30Fe-Pt~25 at % Pt~-130~4Mn-Cu5/35 wn % Cu-250 to 180~25Fe-Mn-Si32 wt % Mn-200 to 150~100
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Special TechnologyFerromagnetic shape-memory alloysShows shape-memory effect in response to a magnetic field Deformation due to magnetic field is known as magnetoelastic deformation.Ni-Ti is non-magnetic Examples of ferromagnetic SMAs: Ni2MnGa, Fe-Pd, Fe3Pt
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Special TechnologyMaterial behavior in Micro-size
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MfMsAsAfAusteniteMartensiteTEMPERATURE
MfMsAsAfAusteniteMartensite
TEMPERATURE(twinned)(twinned)Characteristic temperatures:Mf=Martensitic FinishMs=Martensitic StartAs=Austenitic StartAf=Austenitic FinishThermally Induced Phase Transformation in SMAs
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How are SMAs working?
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Special TechnologyTriggers for Martensitic Transformation (MT)StressTemperature
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Special TechnologyHow does it work? Step 1: Austenite PhaseHigh TemperatureThe atoms arrange themselves in their permanent shape
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Step 2: Martensite PhaseLow temperatureCubic structure becomes folded or twined
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Step 3:Bend the WireIt remains in its Martenesite Phase
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Special TechnologyStep 4: Austenite PhaseHeat the wire above the transition temperature of 50 degrees It moves back to its original position!
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Special TechnologyAustenite & Martensite Phases
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SMAs
Pseudoelasticity or Superelasticity (SE)
Shape Memory Effect (SME)
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Special TechnologyShape Memory Alloys
Pseudoelasticity or Superelasticity (SE)When the metal is changed to the martensite phase simply by strain. The metal becomes pliable and can withstand strains of up to 8%.
Shape Memory Effect (SME)These materials have an ability to remember its austenite phase. As the metal is cooled to the martensite phase, it can be easily deformed. When the temperature is raised to the austenite phase, it reforms to the original shape of the material.A mix of roughly 50% nickel and 50% titanium is the most common SMA. Also CuZnAl and CuAlNi are widely used.
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Special TechnologySuperelastic behavior (SE)SMAs 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 transformation.
Superelastic behaviorT > AfHysteresis loop means energy dissipation, hence vibration dampingStress
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Special TechnologyApplications of superelastic behaviorOrthodontal bracesFrames for eyeglassesUnderwires for brassieresAntennas for cellular phones
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Special TechnologyShape Memory Effect (SME)Martensitic phase transformation that occurs as a result of stress or temperature change
Department of Mechanical and Aerospace EngineeringThe Shape Memory Effect
se
T
Cooling
Detwinning
Heating/Recovery
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Special TechnologyApplications of shape memory effect
Self-expandable cardiovascular stentBlood clot filtersEnginesActuators for smart systemsCouplingsFlaps that change direction of airflow depending upon temperature (for air conditioners)
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SHAPE MEMORY ALLOY (NITINOL)Inactive
Active - Bending
Active - Twist
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Special TechnologyBEFORE TRANSFORMATION
DURING TRANSFORMATION
AFTER TRANSFORMATION
SHAPE MEMORY POLYMERS
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AFTER TRANSFORMATION
BEFORE TRANSFORMATION
DURING TRANSFORMATION
SHAPE MEMORY POLYMERS
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Special Technology SHAPE MEMORY POLYMERS
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Special TechnologyTypes of shape-memory behaviorOne-way shape memory: transformation to the desired shape occurs only upon heating, i.e., memory is with the austenite phase.
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Special TechnologyTypes of shape-memory behaviorTwo-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 Ms and Mf)
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Special TechnologyPhysical properties of Nitinol (versus SS)
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Special TechnologyThe applications of SMAs
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Special TechnologyMicro-actuatorsMobile phone antennasOrthodontic archwiresPenile implantPipe couplingsRobot actuatorsRock splittingRoot canal drillsSatellite antenna deploymentScoliosis correctionSolar actuatorsSpectacle framesSteam valvesStentsSwitch vibration damperThermostatsUnderwired brasVibration dampersZIF connectorsAids for disabledAircraft flap/slat adjustersAnti-scald devicesArterial clipsAutomotive thermostatsBraille print punchCatheter guide wiresCold start vehicle actuatorsContraceptive devicesElectrical circuit breakersFibre-optic couplingFilter strutsFire dampersFire sprinklersGas dischargeGraft stentsIntraocular lens mountKettle switchesKeyhole instrumentsKey-hole surgery instrumentsCurrent examples of applications of shape memory alloys
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Special TechnologyExisting and potential SMA applications in the biomedical domain
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Special TechnologySMAs in Bio-medical Devices
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Bone AnchorsRobotic arms
Medical Stents
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Special TechnologySMA orthodontic wires
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Nitinol is used in medicine for stents: A collapsed stent can be inserted into a vein and heated (returning to its original expanded shape) helping to improve blood flow. Also, as a replacement for sutures where nitinol wire can be weaved through two structures then allowed to transform into it's pre-formed shape which should hold the structures in place.
SMAs Stents
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Bone staple
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Special TechnologyPhotographs of (a) brassiere and (b) various designs of superelastic NiTi underwires.
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Special TechnologyExisting and potential SMA applications in the automotive domain
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Special TechnologyExisting and potential SMA applications in the aerospace domain
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Picture of wing with SMA wires The wires in the picture are used to replace the actuator. Electric pulses sent through the wires allow for precise movement of the wings, as would be needed in an aircraft. This reduces the need for maintenance, weighs less, and is less costly.
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Department of Mechanical and Aerospace Engineering
Special TechnologySMAs Microactuators
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Department of Mechanical and Aerospace Engineering
Special TechnologySMAs Micro-actuatorsSMAs can generate the mechanical work in shape of large actuation force and displacement during the phase transformation. This deformation is produced due to cooling and heating cycle as two significant elements in SMA actuators. The main parts of the SMA actuators include; SMA part, mechanical system, electronic control system, the fixture body and the effective element with recovering capability based on the employed stress.
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Special TechnologyAdvantages and dis-advantages of shape memory materials
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Special TechnologySMAs Micro-actuatorsThe most common applications of NiTi thin films are concentrated on microactuators due to their particular capabilities in MEMS, such as micropumps, microvalves, microgrippers, micropositioners, microsprings, microspacers, and microwrappers, etc.
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The schematic of NiTi microvalve in (a) close position, (b) open position
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(Left) Scanning electron micrograph of a NiTi micro-gripper (Right) (1) The sketch of operational mode (2) Thermo-mechanically cycle (3) The working principle due to heating/cooling
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Magnetic microgripper (a) the schematic of the body, (b) Gripping a micro-object by the micro-tip
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A NiTi microgripper with a hook structure and two C-shape probes (a) cooled state (b) heated state
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The micro wrapper based on NiTi thin film
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Special TechnologySchematics of the operation of MicroSwitches
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Special TechnologyProblems With SMAsFatigue from cyclingCauses deformations and grain boundariesBegin to slip along planes/boundariesOverstressA load above 8% strain could cause the SMA to completely lose its original austenite shapeDifficulty in machining processDifficulty with computer programmingMore expensive to manufacture than steel and aluminum
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