Shape Memory Alloys: Thermoelastic Martensite

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1 Shape Memory Alloys: Thermoelastic Martensite MatE 152 Thermoelastic Martensite Shape Memory Alloys (SMA) The strain of transformation is much less than the martensitic transformation in steel Thus product microstructures do not become damaged; Transformation can be reversed by reheating

2 Shape memory When a shape memory alloy is in its martensitic form, it is easily deformed to a new shape. However, when the alloy is heated through its transformation temperatures, it reverts to austenite and recovers its previous shape with great force. This process is known as Shape Memory. The temperature at which the alloy remembers its high temperature form when heated can be adjusted by slight changes in alloy composition and through heat treatment. In the Nickel Titanium alloys, for instance, it can be changed from above +100 deg.c to below -100 deg.c. Fundamentals of SMAs The shape recovery process occurs over a range of just a few degrees and the start or finish of the transformation can be controlled to within a degree or two if necessary. Wide variety of alloys known to exhibit SMA

3 Different phases of an SMA Fundamentals of SMAs Shape memory effect (SME) is a consequence of a crystallographically reversible martensitic phase transformation occurring as the solid state Ordered parent Martensite is thermoelastic Individual units self-accomodating First order transformation Hysteresis associated with the transformation Volume fraction of martensite is independent of time

4 Nickel Titanium (Nitinol) SMA A thermoelastic martensitic phase transformation in the material is responsible for its properties. These properties include the shape memory effect, superelasticity, and high damping capability. The properties of Nitinol can be modified to a great extent by changes in alloy composition, mechanical working, and heat treatment. In most cases a trial and error process is required to optimize these factors for a particular application. NiTi shown promising biocompatibility as an implant with substantial amount of recovery strain; lots of data SMA response to heating and cooling No change in shape when cooled from Af (temp at which SMA finishes transforming to Austenite upon heating) to below Mf (temp at which SMA finishes transforming to Martensite upon cooling) When specimen is deformed below Mf it remains deformed until heated. Shape recovery begins at As (temp at which SMA starts transforming to Austenite upon heating. and is completed at Af

5 SMA stress-temperature phase diagram Temperature-induced phase transformation of an SMA without mechanical loading.

6 Manifestation of the Shape Memory Effect (SME) If mechanical load is applied to the material in the state of twinned martensite (at low temperature) it is possible to detwin the martensite. Upon releasing of the load, the material remains deformed. A subsequent heating of the material to a temperature above A 0f will result in reverse phase transformation (martensite to austenite) and will lead to complete shape recovery Shape Memory Effect (SME)

7 Temperature-induced phase transformation with applied load It is also possible to induce a martensitic transformation which would lead directly to detwinned martensite. If load is applied in the austenitic phase and the material is cooled, the phase transformation will result in detwinned martensite. Thus, very large strains (on the order of 5-8%) will be observed. Reheating the material will result in complete shape recovery. The transformation temperatures in this case strongly depend on the magnitude of the applied load. Higher values of the applied load will lead to higher values of the transformation temperatures. Usually a linear relationship between the applied load and the transformation temperatures is assumed Thermomechanical loading path demonstrating the SME in an SMA

8 Stress-strain-temperature curve showing SME

9 Pseudoelastic Behavior It is also possible to induce a phase transformation by applying a pure mechanical load. The result of this load application is fully detwinned martensite and very large strains are observed. If the temperature of the material is above A 0f, a complete shape recovery is observed upon unloading, thus, the material behavior resembles elasticity.

10 Thermomechanical loading path demonstrating pseudoelastic behavior Superelasticity Alloys also show a Superelastic behavior if deformed at a temperature which is slightly above their transformation temperatures. This effect is caused by the stress-induced formation of some martensite above its normal temperature. Because it has been formed above its normal temperature, the martensite reverts immediately to undeformed austenite as soon as the stress is removed. This process provides a very springy, "rubberlike" elasticity in these alloys.

11 Superelasticity Typical Loading and Unloading Behavior of Superelastic NiTi Superelasticity

12 Nitinol Particular alloy has very good electrical and mechanical properties, long fatigue life, and high corrosion resistance. As an actuator, it is capable of up to 5% strain recovery and 50,000 psi restoration stress with many cycles. By example, a Nitinol wire inches in diameter can lift as much as 16 pounds. Nitinol also has the resistance properties which enable it to be actuated electrically by joule heating. When an electric current is passed directly through the wire, it can generate enough heat to cause the phase transformation. Nitinol In most cases, the transition temperature of the SMA is chosen such that room temperature is well below the transformation point of the material. Only with the intentional addition of heat can the SMA exhibit actuation. In essence, Nitinol is an actuator, sensor, and heater all in one material. The advantages of Nitinol become more pronounced as the size of the application decreases.

13 Physical Properties of Nitinol Density: 6.45gms/cc Melting Temperature: C Resistivity (hi-temp state): 82 uohm-cm Resistivity (lo-temp state): 76 uohm-cm Thermal Conductivity: 0.1 W/cm- C Heat Capacity: cal/gm- C Latent Heat: 5.78 cal/gm; 24.2 J/gm Magnetic Susceptibility (hi-temp): 3.8 uemu/gm Magnetic Susceptibility (lo-temp): 2.5 uemu/gm Mechanical Properties of Nitinol Ultimate Tensile Strength: MPa or ksi Typical Elongation to Fracture: 15.5 percent Typical Yield Strength (hi-temp): 560 MPa, 80 ksi Typical Yield Strength (lo-temp): 100 MPa, 15 ksi Approximate Elastic Modulus (hi-tem): 75 GPa, 11 Mpsi Approximate Elastic Modulus (lo-temp): 28 GPa, 4 Mpsi Approximate Poisson's Ratio: 0.3 Actuation Energy Conversion Efficiency: 5% Work Output: ~1 Joule/gram Available Transformation Temperatures: -100 to +100 C

14 Stress-Strain Characteristics of Nitinol at Various Temperatures High Temperature SMAs High temperature thin film shape memory alloy composed of titanium, nickel, and hafnium (TiNiHF). Film has desirable thermomechanical characteristics with an austenite finish temperature of as high as 170 C. It has high ductility, great strength, and shape recovery of up to 4%. When a force is exerted perpendicular to the plane of the frame, the poppet moves and the TiNiHf microribbons are stretched longitudinally. When an electric current passes through the microribbons it generates Joule heat, the TiNiHf transforms to austenite, contracts and moves the poppet back into the plane of the frame. A force of up to 0.5 N is produced, and displacement as much as 100 µm. Flow rates as high as one liter per minute were recorded with current of 150 ma applied to the actuator.

15 High Temperature SMAs TiNiHf valve actuators microfabricated on a silicon substrate. Each actuator consists of a rectangular frame of silicon, 8 mm long and 5 mm wide, a central poppet etched from the same single crystal silicon wafer as the frame, and four microribbons of TiNiHf that connect the frame and the poppet. Space Application The Clementine spacecraft, successfully deployed its solar panels this morning The device, called the Frangibolt provides a simple, safe, and inexpensive way to anchor spacecraft appendages during launch and release them on cue. decreasing the weight and cost of future space missions as well as increasing their safety and reliability. The Frangibolt comprises a commercially available bolt and a small collar made from shape-changing metal. To release Clementine's solar arrays, a heater coil triggers a change in the collar's shape. material elongates and exerts over 5,000 pounds of force to break a restraining bolt and free the solar panels. Frangibolts replace conventional explosive bolt devices possessing inherent risks that range from handling and installation hazards to unintentional activation and fragmentation.

16 Training SMAs: Thermomechanical Cycling Superelastic behavior an approximation to the actual behavior of SMAs under applied stress. In fact, only a partial recovery of the transformation strain induced by the applied stress is observed. A small residual strain remains after each unloading. Further cooling in the absence of applied stress related to the occurrence of a macroscopic transformation strain contrary to what is observed in the SMA material before cycling. The thermomechanical cycling of the SMA material results in training process.

17 Training SMAs Different training sequences can be used by inducing a non-homogeneous plastic strain (torsion, flexion) at a martensitic or austenitic phase; by aging under applied stress, in the austenitic phase, in order to stabilize the parent phase, in the martensitic phase, in order to create a precipitant phase (Ni-Ti alloys); thermomechanical, either superelastic or thermal cycles. Two-Way Shape Memory Effect (TWSME). Main result of the training process Shape change is obtained both during heating and cooling. The solid exhibits two stable shapes: a hightemperature shape in austenite and a lowtemperature shape in martensite. Transition from the high-temperature shape to the low-temperature shape (and reverse) is obtained without any applied stress assistance. TWSME is an acquired characteristic.

18 TWSME In the heart of the TWSME is the generation of internal stresses and creation of permanent defects during training. The process of training leads to the preferential formation and reversal of a particular martensitic variant under the applied load. Generation of permanent defects eventually creates a permanent internal stress state, which allows for the formation of the preferred martensitic variant in the absence of the external load. And from Sculptor.org Living Sculpture The Art and Science of Creating Robotic Life Written for and published in Leonardo "Octofungi's legs use shape memory alloy as their motive force. This interesting material is shaped like a fine wire, yet has the unique ability to contract when an electrical current is passed through it. The movement produced by the wire is extremely non-linear, producing a pleasing, life-like motion. Also, the wire is silent, so there is no sound of motors or solenoids to mar the aesthetics of the sculpture. "