Development of NiTi shape memory alloy bimorph towards energy harvesting and micro- mechatronics application

Abstract

Shape memory alloy (SMA) are smart material, when deformed below the transformation temperature yields thermoelastic martensite (detwinned martensite), and upon heating, it regains its parent austenite phase. This unique property is called the shape memory effect (SME) happens owing to twinning (i.e., shear lattice distortion) rather than long-range diffusion of atoms. The Shape memory effect is shown in Fig. 1a. Along with this property of SME, the shape memory alloy also possess pseudoelasticity (superelasticity), high power to weight ratio, high damping capacity, and biocompatibility. The pseudoelastic nature of NiTi stent is shown in Fig. 1b. The SME provides significant motion and high forces resulting in increased work output. Super elasticity offers extreme flexibility finding application in the biomedical field and the Micro Electro Mechanical Systems (MEMS) field. 1.1 Shape memory effect Shape memory alloy are the material that remembers the shape due to its unique property of shape memory effect. The shape memory effect is usually known as temperature-induced transformation. The shape memory effect refers to the ability to recover or memorize its parent shape through consequent heating after getting deformed in its lower temperature martensite state. The nucleation and growth of the martensite phase from the parent austenite phase result in a diffusion less solid-state phase transformation mechanism. The crystal structure changes due to the cooperative and homogenous movement of many atoms during diffusion less solid-state transformation. However, the material remains in a solid state because the movement is small usually less than the interatomic distances. In the absence of applied load, the parent phase austenite transforms during cooling to a martensite variant called twinned martensite with a monoclinic structure. The variant twinned martensite changes to detwinned martensite upon applied load. The SMA material regains the shape during heating to the symmetric austenite phase. The phase transition from austenite to martensite is called forward transformation and during heating, in the martensite phase, the material returns to the parent austenite phase is called reverse transformation. The schematic of the shape memory effect is shown in Fig. 1.2. Consider point a as the starting point of transformation. When the SMA material cools down below the martensite transformation temperatures, it reaches point b, which is the formation of twinned martensite. On applying a load σ (point c) the reorientation starts results in the formation of certain favorably oriented martensitic variants. The stress level for reorientation of the variants is far lower than the permanent plastic yield stress of martensite. The detwinning martensite formation (point d) is completed by applying a certain amount of load. During detwinning, the SMA material accommodated the stress applied without any slip or breakage in the bonding of atoms. This accommodation through shear lattice deformation makes them unique compared to other alloy materials. The release of load from the SMA material slightly recovers a part of strain, reaching point e in Fig. 1.2. Now the material is heated without applying any load allowing the SMA to reach its parent austenite phase (point f). On heating, the material the reverse transformation will be started. This whole cycle of the shape memory effect can be repeated. Primarily, austenite has cubic crystal structures, whereas martensite has tetragonal, orthorhombic, or monoclinic structures. The austenite phase is called as parent phase, and the martensite phase is called as daughter phase. The phase transition from austenite to twinned martensite happens without any visible changes. However, the change of phase from detwinned martensite to austenite results in shape change. This SME will be very useful in various fields such as aerospace, automobiles, spacecraft, and in the medical field.