and Engin gA481-42(200g582-589 583 (o in Fig.1).Upon heating above the phase transformation,the parent phase -that has a higher symmetry-nucleates and pro 2.Shape memory alloys:definition and design sapp objectives shape (i.e.the shape corresponding to the parent phase)is the Shape memory alloys (SMAs)are materials that have shape memory effect.Cooling again with no applied stress,self two (or sometimes several)crystallographic phases for which I form (step 2)and no macroscopi mations from one【 other occur through from a macroseonie shane chans noint of view the sme is therefore not intrinsically reversible:a change of shape is with the phase change:the shape memory effect and the supere- serve ting vanan ave nucl lasticity.As these effects hav been extensively documented ctive ore view point of view.it is equivalent to have nosef-accommodating The superelasticity occurs when the martensitic phase trans- s hu leating upon cod nduced oaches have been explored which weclassify respectively upon unloading The magnitude of yelastic strain'can be as high as8%or even more for single crystals. net s consist of m odifying the materia eferabl the addition of an"exteralelement coupled to the SMA mate. ed crys tallographic pha Let us consider a SMA material initially in its par ent nhase 3.Intrinsic methods:tailoring the microstructure (B)(step 1 in Fig.1).On cooling.the material transforms to the martensiti c phase (step 2).TH is phase is c red by marter o the two-way shape memory effect (TWSME).Differen sitic variants with different crystallographic orientations(for hermo-mechani ses that result in a TWSME have ed(step ycling und the mand ep sothermal mechanical cycling in the austenite phase.These point of view,there is no change of shape as the volume gobally hermo-mechanical processes that lead to the TWSME are also remains the same.If a stress is applied on the material (step ses as the materal memonzes pro n Fig 1),variant raining proc that h ve a favo rable on "d-twinning"as self-acco yoricntcomiantsdi A micro-gripper that uses the TWSME [5.22]is shown in appear.Once Fig.2.The millimeter-size device consists of a single piece of reufomaI8oumhickNiTCcolsolle ilimeter 0 The principle is shown in Fig.2('operating mode'):upon gripper jaw opens and o up onheating The Eo pgngom ④ 7① applied betwe te and martens ite phase in th T Austenite defects Themic er shows excellent fatigue properties(0.000cycles)[]and stability regarding Fig.1.lustration of the shape memory effect(way"). mechanical perturbations (23]Y. Bellouard / Materials Science and Engineering A 481–482 (2008) 582–589 583 and in particular thin-film processing are not addressed. Details on these particular aspects can be found for instance in [3,13–15]. 2. Shape memory alloys: definition and design objectives Shape memory alloys (SMAs) are materials that have two (or sometimes several) crystallographic phases for which reversible transformations from one to the other occur through diffusion-less transformations (the so-called “reversible marten￾sitic transformations” [16]). Two remarkable effects are related with the phase change: the shape memory effect and the supere￾lasticity. As these effects have been extensively documented elsewhere (see for instance [16–18]), we just briefly summa￾rize their main features from a microsystems design point of view. The superelasticity occurs when the martensitic phase trans￾formation is stress-induced at a constant temperature. The transformation is characterized by a plateau and a hysteresis upon unloading. The magnitude of reversible ‘pseudo-elastic strain’ can be as high as 8% or even more for single crystals. The shape memory effect (SME) refers to the ability of the material, initially deformed in its low-temperature phase (called “martensite”), to recover its original shape upon heating to its high temperature phase (called austenite or “parent phase”). The SME is a macroscopic effect of thermally induced crys￾tallographic phase changes. An important aspect is that it is a one-way occurrence. This phenomenon is illustrated in Fig. 1. Let us consider a SMA material initially in its parent phase () (step 1 in Fig. 1). On cooling, the material transforms to the martensitic phase (step 2). This phase is characterized by a lower symmetry than the parent phase and has different possible crystallographic orientations (called variants). Multiple marten￾sitic variants with different crystallographic orientations (for instance A, B, C and D in Fig. 1) nucleate so that the deformation strain energy is minimized (step 1 to step 2). These particular variants are called “self-accommodating”. From a macroscopic point of view, there is no change of shape as the volume globally remains the same. If a stress is applied on the material (step 3 in Fig. 1), variants that have a favorable orientation “grow” at the expense of less favorably oriented ones. This effect is called “de-twinning” as self-accommodating variants disappear. Once Fig. 1. Illustration of the shape memory effect (“one-way”). the stress is released (step 4), as the newly formed martensite variants are stable, the material retains the applied deformation (ε0 in Fig. 1). Upon heating above the phase transformation, the parent phase – that has a higher symmetry – nucleates and pro￾gressively replaces the martensite causing the disappearance of the apparent deformation (ε0). The ability to recover its original shape (i.e. the shape corresponding to the parent phase) is the shape memory effect. Cooling again with no applied stress, self￾accommodating variants will form (step 2) and no macroscopic change of volume will be observed. From a macroscopic shape change point of view, the SME is therefore not intrinsically reversible: a change of shape is only observed if non-self-accommodating variants have nucle￾ated. The design objective of an SMA actuator is therefore to achieve a reversible macroscopic shape change, i.e. to have the capacity to switch between two shapes. From a microstructure point of view, it is equivalent to have non-self-accommodating martensitic variants nucleating upon cooling. To provide the necessary reversible shape memory effect, two approaches have been explored which we classify respectively as intrinsic and extrinsic methods. Intrinsic methods consist of modifying the material microstructure so that certain martensitic variants orientations will preferably nucleate upon cooling. Extrinsic methods refer to the addition of an “external” element coupled to the SMA mate￾rial that provides the required stress to induce stress-oriented variants. A third method is based on monolithic design that is a kind of combination of intrinsic and extrinsic methods. 3. Intrinsic methods: tailoring the microstructure The martensitic variants nucleation is influenced by ori￾ented precipitates and oriented defects. Oriented defects lead to the two-way shape memory effect (TWSME). Different thermo-mechanical processes that result in a TWSME have been identified [19–21] like severe deformation of the marten￾sitic phase, thermo-mechanical cycling under constraints and isothermal mechanical cycling in the austenite phase. These thermo-mechanical processes that lead to the TWSME are also called “training processes” as the material “memorizes” pro￾gressively a new shape. A micro-gripper that uses the TWSME [5,22] is shown in Fig. 2. The millimeter-size device consists of a single piece of metal laser-cut from a 180m-thick Ni–Ti–Cu cold-rolled sheet. It is used by a micro-endoscope manufacturer to assemble sub￾millimeter lenses. The principle is shown in Fig. 2 (‘operating mode’): upon cooling the gripper jaw opens and closes up on heating. The heat is provided by a simple resistive layer onto which the grip￾per is glued to. To achieve the reversible finger motion, the gripper is deformed and constrained so that it cannot recover its original shape (step 2 in Fig. 2). About hundred thermal cycles are applied between the austenite and martensite phases. During cycling, stress builds up in the finger hinge introducing permanent oriented defects. The micro-gripper shows excellent fatigue properties (>200,000 cycles) [22] and stability regarding mechanical perturbations [23].