SHAPE MEMORY ALLOYS Fundamentals and applications RICCARDO CASATI MILANO, FEBRUARY 2022 Shape memory alloys are functional materials that show two peculiar properties: Video Shape Memory Effect • Shape Memory Effect (SME) • Pseudoelasticity/Superelasticity Both effect are based on a martensitic transformation Video Superelasticity 2 • The SMA functional properties are due to a particular martensitic transformation (MT), i.e. shear-dominant diffusionless solid-state phase transformation occurring by nucleation and growth of the martensitic phase from a parent austenitic phase. • MT is associated with an inelastic deformation of the crystal lattice with no diffusive process involved. Parent and Martensite phases coexist during the phase transformation (first order transition) and as a result there exists an invariant plane, which separates the parent and product phases. • This transformation is crystallographically reversible. • Since the crystal lattice of the martensitic phase has lower symmetry than that of the parent austenitic phase, several variants of martensite can be formed from the same parent phase crystal. • Martensite can be Thermally Induced (TIM) or Stress Induced (SIM) 3 Diffusive transformations • Formation of a phase with a new chemical composition • They occur thanks to the transport of atoms over relative distances • Diffusion of atoms takes time to take place (time dependent); • They can be suppressed by quenching. • They are defined as isothermal because they can proceed over time at a constant temperature. Displacive transformations • They do not modify the composition of the parent phase, but only the crystalline structure • Atoms rearrange themselves in short-range cooperative motion • Since no atomic migration is necessary, displacive transformations progress almost independently of time, with the speed of the interface between two phases able to move at the speed of sound. • They are referred to as athermic transformations, since they do not progress at a constant temperature and the amount of the new phase present depends only on temperature, not time. • Martensitic transformations are displacive transformations. The term martensite refers to the stable phase at low temperature and austenite refers to the stable parent phase at high temperature. 4 Cooling from austenite: • Martensite start • Martensite finish Heating from martensite: • Austenite start • Austenite finish 5 It takes place through two distinct contributions: • Bain deformation (lattice deformation) which consists of all the atomic movements that serve to produce the new crystalline structure. Very small atomic shifts are required to advance the interface • Lattice-invariant shear: accommodation of the shape variation that allows the new martensitic phase to stay within the austenitic matrix that surrounds it. This accommodation can occur through the phenomenon of sliding dislocations (slip) as in steels or through the formation of twinning as occurs in many SMAs. During martensitic transformation the crystalline lattice undergoes a shearing parallel to a particular crystallographic plane. The different potential orientations in which such shearing can be produced are named variants Slip Twinning 6 Twinning causes the formation of a new variant and plays a key role in the martensitic transformation, in the deformation of martensite, in the shape memory effect and in superelasticity. • On the twinning plane, atoms are part of the two adjacent lattice regions, no dislocations are required, and the energy accumulated by the boundary is low. Twinning in a BCC structure • Twin boundaries are very mobile, small movements are enough to make a row of atoms move and advance the twin boundary. 7 • The martensite formed by cooling the austenite has a twinned structure, forming several variants. Twin boundaries can move easily (low energy, high mobility) and certain martensite variants can grow or shrink with small system energy changes. The growing of most favorable variants instead of others when a load is applied is named DETWINNING, it is possible to change the shape of a Nitinol artifact by spending little energy. • If the deformation is sufficiently high, a martensitic grain consisting of multiple variants can transform into a grain consisting in a first approximation of a single variant. • Detwinning is exploited for SME 8 Austenite (Shape I) Austenite Detwinned-M Twinned-M Detwinned-M (Shape II) 1. Austenite (shape I) 2. Cooling T< Mf (TIM) 3. Loading – Detwinned Martensite 4. Unloading – recovery of elastic deformation (shape II) Twinned-M (Shape I) 1. Heating – recovery of shape I 9 . • The unique ability of martensite to accommodate the change in shape through the growth of some variants allows it to accommodate the deformations and reduce the internal energy of the system. LOAD UNLOAD • At temperatures where the chemical energy of austenite is less than that of martensite, the total energy of the martensitic system could be less if a stress is applied. Hence the formation of stress-induced martensite (SIM) is energetically favored. 10 Temperature is constant (>Af) A→SIM 1 Elastic deformation of Austenite 2 Forward P-M transformation SIM→A 3 Unloading of Martensite 4 Reverse M-P transformation 11 12 13 14 15 Nitinol 16 • The term Nitinol refers to a class of alloys with a quasi-equiatomic composition of Nickel and Titanium. • The term derives from the words "NickelTitanium-Naval-Ordnance-Laboratory" referring to the center where these alloys were initially studied. • NiTi is the most widely used Shape Memory Alloy (SMA) for applications in the industrial and medical fields. William J. Buehler 17 • The NiTi intermetallic compound can accept an excess of Ni atoms, in particular above 600° C • The number of substitutional Ni atoms affects the transformation temperatures • Excess in Ti leads to the precipitation of Ti2Ni, while an excess in Ni causes the precipitation of Ni3Ti2 • It is also possible to create other metastable phases by specific aging treatment 18 • Space group of cesium chloride type (CsCl): each of the two types of atoms (Ni and Ti) form a simple cubic lattice; the two lattices are interpenetrated so that each atom of one type is at the center of a unit cell of the simple cubic lattice of the other atoms. Each unit cell therefore contains 1 Ni atom and 1 Ti atom. • This structure is also defined B2 according to the designation Strukturbericht or Pm3m according to the Hermann – Mauguin notation. • Ni and Ti therefore occupy well-defined positions in the lattice rather than being randomly distributed as in common metal alloys → intermetallic compound Ni Ti 1/8 atomo 19 tetragonal cell The martensitic phase of NiTi has a monoclinic structure, defined B19’ by the designation Strukturbericht or P21/m. Like all martensitic phases, the martensite of NiTi also has less symmetry than the parent phase (B2). Martensite B19‘ inherits a structure ordered from B2 20 B2 transforms into B19‘ lattice, which can have 24 different crystallographic orientations (24 variants), due to the lower symmetry compared to the parent phase. Hence, the martensite variants all have the same crystal structure but different crystallographic orientation. Simplified 2D example: A simple square forms 4 variants with identical structure but different orientation 21 In NiTi alloys a further martensitic transformation can take place which involves the formation of a phase with a rhombohedral structure, the so-called R phase (space group P3 or R3-). The rhombohedral structure is a distorted structure of austenite B2. The rhombohedral distortion consists in stretching the cubic cell along a diagonal of the cube (on of the [111] directions of the B2) which involves a reduction of α from 90 ° to 89 °. The direction [001]B2 does not change at the end of the transformation. The rhombohedral structure has α = β = γ ≠ 90 ° and a = b = c. There are 4 variants of martensite that can form, one for each diagonal of the cube. As for the B19 ', twins are formed to self-accommodate the shape change. 89° Transformation B2→R in a Ti50.3Ni48.2Fe1.5 alloy 22 • The A→R→A transformation is characterized by an extremely low hysteresis, and a limited SME or SE effect. • Phase R training is stimulated by the addition of particular alloying elements, residual plastic deformation, aging treatment, formation of coherent precipitates such as Ni4Ti3, thermomechanical cycling 23 • The austenitic B2 lattice of NiTi allows an excess of Ni at high temperatures. Tang et al. (1999) Frenzel et al. (2010) • If the material is quickly cooled (quenched) from high temperatures, the Ni remains trapped although not in equilibrium configuration at ambient temperatures. • Ni in excess causes a drastic reduction in temperatures. An excess in Ti causes the formation of second Ti2Ni phases without impacting the transformation temperatures. • Ni reduces material ductility. 24 PTT diagram (Nishida et al.) • Aging leads to the formation of Ni-rich phases. Precipitation of such phases enrich the matrix in Ti. Thus, transformation temperatures increase • Alloys with more than 51at.% Ni are brittle if not aged TEM image of Ti3Ni4 precipitates N.B. Ti11Ni14 è stato poi indicizzato in lavora successivi come Ti3Ni4 25 90% of applications containing SMA alloys use binary NiTi with a Ni content between 49 and 51% at. without further alloying elements. Interstitial atoms such as carbon and oxygen are present in very low quantities (few hundreds of ppm) and are considered as impurities to be eliminated. For particular applications, substitutional elements are used (NiTiX eg. NiTiFe, NiTiNb, NiTiCr, NiTiCu, NiTiCo, NiTiV, NiTiPt and NiTiHf functional properties) to control the functional properties: • Increase the hysteresis (Nb) • Reduce hysteresis (Cu) • Increase transformation temperature (Pt, Pd, Hf, Zr) • Reduce transformation temperature (or increase the stiffness of B2) (Fe, Co, V, Cr, Mn, Al, Ta) • Increase radiopacity (RE) • Increase mechanical strength, corrosion resistance or a creep 26 27 NiTi is very reactive with oxygen : • Formation of a surface oxide TiO2 (Rutile) • Formation of oxides within the metal matrix such as Ti4Ni2OX • Interstitial oxygen in NiTi lattice Oxygen cannot be avoided (present in raw materials, especially in Titanium, 200-500 ppm). Oxygen has a higher solubility in Ti2Ni than in NiTi. The Ti2Ni precipitates can absorb oxygen until they reach the Ti4Ni2O composition. Ti4Ni2OX compounds (0 <X <1) are isostructural. 28 Ti4Ni2OX compounds (0 <X <1) are insoluble inclusions in the matrix and inert to heat treatment. They can affect the fatigue resistance of the material. They are also present in alloys rich in Ni (superelastic). These Ti-rich inclusions modify the stoichiometry of the alloy, thus lowering the transformation temperatures 29 Carbon does not dissolve in NiTi, it forms carbides with Titanium (TiC). C is usually not present in the starting materials (Ti and Ni), it is introduced into the alloy only if the alloy is melted in a graphite crucible. Ti and Ni absorb C in the liquid phase. About half of the nitinol is produced in copper crucibles, so it does not contain C. TiC particles are generally micrometric in size and have geometric shapes, with sharp edges They can affect the fatigue resistance of the material. To form TiC, the matrix is depleted in Ti, so the alloy composition needs to be slightly corrected. 30 31 • The two most popular melting technologies for the production of NiTi ingots are VIM (vacuum-induction melting) and VAR (vacuum-arc remelting). Starting material are Ni and Ti with high purity. • In VIM ovens, Ni and Ti bars are placed in a graphite crucible which is in turn inserted into a vacuum chamber. The crucible is heated by magnetic induction • Advantage: the molten metal is mixed by magnetic stirring → homogeneous chemical composition and functional properties • Disadvantage: carbon impurities (300-700 ppm) 32 Micrographs: Carbon inclusion in wire produced from VIM ingots 33 • Ni and Ti weighed in the right ratio and pressed together to form the consumable electrode of the VAR (Vacuum arc remelting). • An electric arc is generated between the Ni and Ti electrode and the bottom of the crucible, the current is sufficient to melt the electrode that is deposited on the bottom of the crucible • Water-cooled Cu crucible • Advantages: high purity, no carbon • Disadvantages: poor chemical homogeneity of the ingot, which has to be remelted several times. • Often two techniques (VIM / VAR) are combined 34 Hot working at temperatures between 600 ° C and 900 ° C (0.55-0.75 Tm) Cold working with annealing at temperatures between 600 ° C and 800 ° C 35 Final heat treatment to confer the shape (to be remembered) to the material 1) Cold Deformation 2) Bending in the desired shape and fixing. 3) Low temperature treatment (aging) under constraint (350-550°C 5-20 min) - Ti3Ni4 Precipitation - Localized stress field in NiTi Matrix 36 37 • One way SME • Constrained Recovery • Work Generation / Actuators • Superelasticity • High Damping Capacity 38 1 Cooling below Mf 2 Deformation at T< Mf (Load/Unload) 3 Free Heating above Af 39 40 Smars NASA rock breaker 41 42 Advantages Disadvantages • Sensor-Actuator (Active device) • Thermal Hysteresis • Noiseless • Non linear behaviour • No Lubrification • Limited operating ranges • High reliability • Functional properties to be stabilized • No flame or toxic release • High cost • Electric circuit • High power/weight ratio • Excellent oxidation resistance 43 Water mixing valve – Furukawa NT Air conditioning - Flap movement Oil valve mixing device Shinkanzen – Furukawa NT TiNi pinpullers and frangibolt for space deployables 44 45 46 47 48 49 50 51 52 53 Prof. Riccardo Casati Department of Mechanical Engineering Politecnico di Milano [email protected]
