Shape memory behavior in Fe 3 Al-modeling and experiments

The Fe 3 Al alloy with D0 3 structure exhibits large recoverable strains due to reversible slips. Tension and compression experiments were conducted on single crystals of Fe 3 Al, and the onset of slip in forward and reverse directions were obtained utilizing high-resolution digital image correlation technique. The back stress provides the driving force for reversal of deformation upon unloading, resulting in a superelastic phenomenon as in shape memory alloys. Using density functional theory simulations, we obtain the energy barriers (GSFE -generalized stacking fault energy) for {1 1 0}〈1 1 1〉 and {1 1 2} 〈1 1 1〉 slips in D0 3 Fe 3 Al and the elastic moduli tensor, and undertake anisotropic continuum calculations to obtain the back stress and the frictional stress responsible for reversible slip. We compare the theoretically obtained slip stress magnitudes (friction and back stress) with the experimental measurements disclosing excellent agreement

Reversible Martensitic Transformation under Low Magnetic Fields in Magnetic Shape Memory Alloys

Magnetic field-induced, reversible martensitic transformations in NiCoMnIn meta-magnetic shape memory alloys were studied under constant and varying mechanical loads to understand the role of coupled magneto-mechanical loading on the transformation characteristics and the magnetic field levels required for reversible phase transformations. The samples with two distinct microstructures were tested along the [001] austenite crystallographic direction using a custom designed magneto-thermo-mechanical characterization device while carefully controlling their thermodynamic states through isothermal constant stress and stress-varying magnetic field ramping. Measurements revealed that these meta-magnetic shape memory alloys were capable of generating entropy changes of 14 J kg(−1) K(−1) or 22 J kg (−1) K(−1), and corresponding magnetocaloric cooling with reversible shape changes as high as 5.6% under only 1.3 T, or 3 T applied magnetic fields, respectively. Thus, we demonstrate that this alloy is suitable as an active component in near room temperature devices, such as magnetocaloric regenerators, and that the field levels generated by permanent magnets can be sufficient to completely transform the alloy between its martensitic and austenitic states if the loading sequence developed, herein, is employed

Compressive Response of Polycrystalline NiCoMnGa High-Temperature Meta-magnetic Shape Memory Alloys

The effects of the addition of quaternary element, Co, to polycrystalline NiMnGa alloys on their magnetic and shape memory properties have been investigated. NiCoMnGa polycrystalline alloys have been found to demonstrate good shape memory and superelasticity behavior under compression at temperatures greater than 100 °C with about 3% transformation strain and low-temperature hysteresis. It is also possible to train the material to demonstrate a large two-way shape memory effect

Mechanical Oscillations in TiNi Under Synchronized Martensite Transformations Experimental Procedure

Mechanical vibrations in alloys with thermoelastic martensitic transformations have some specific features. The main one is the existence of damping peaks at temperatures of austenite<-*martensite transitions (Van Humbeeck, 1989; Naturally, experiments including fast phase transformations with the duration of a small fraction of the period of vibrations (when tpi, < T) do not allow correctly judging the internal friction as of material damping capacity. However, such experiments are interesting from the point-of-view of active control of vibrations by fast changes of the phase composition. When tph < T, the object under investigation must be considered as a solid with periodically varying strain, in which martensitic transformation occurs on some stages of the deformation. In TiNi-based alloys the transformation is accompanied with such phenomena of martensitic nonelasticity as shape memory, transformation plasticity, generation, or relaxation of stresses. In other words, it leads to a change of the stressedstrained state of the body. If such changes occur one or several times during one period of vibrations, they will necessarily influence the whole mechanical process and cause a variation of the amplitude and frequency of vibrations, level of damping. The result of such influence will certainly depend on what stage of a vibration period the transformation takes place, is it direct or reverse, etc. On the whole, the existing knowledge of martensitic nonelasticity allows us to state that an effective control of vibrations can be achieved by specially organized fast changes of the material structural state. This is confirmed by the results of the preliminary studies by The main goal of this work is the analysis of the influence of fast martensitic transformations on the unforced oscillations of a TiNi alloy wire torsional pendulum. Experimental Procedure The vibrating system under investigation was a torsional pendulum. The specimen used as a working body has been made of Ti-50at.%Ni wire with the length 400 mm and the diameter 0.5 mm. After annealing the alloy had the transformation temperatures Af, = 330 K, Mf = 320 K, A, = 355 K, Af = 370 K. At the room temperature the specimen had the structure of martensite. The upper end of the specimen was fixed in an unmovable conical grip and the lower end could rotate freely together with an attached beam with weights. The length of the beam and the mass of weights have been chosen because the frequency of pendulum vibrations was about 0.05 Hz. The beam was equipped with a transparent rim with scores. Pendulum rotation by one angular degree corresponded to an interval between the scores. The angle of rotation was measured by the number of scores which passed through an optical registration system consisting of a lamp, a collimator, and a photo-indicator. Heating of the specimen had been done by the passing of alternating current through the circuit: upper grip-specimensteel rod fastened to the lower grip and aligned along the pendulum axis-electrolyte (water solution of copper sulfate) -copper blade contact immersed into the electrolyte. The use of the electrolytic bath as part of the circuit allowed securing a reliable electric contact with the specimen and reduce friction to a minimum. Cooling of the specimen after the break of the current occurred by natural heat exchange with the air. The mean temperature was obtained by measuring the resistivity of 0.01 mm diameter copper wire coiled around the specimen on all its length. The deformation y of the specimen was calculated by the formula y = irip)/L, where r and L are radius and length of the specimen, tp is the rotation angle in radians. The initial angular deflection of the pendulum from equilibrium corresponded to 7o = 0.3% deformation. Martensitic transformation was provoked by heating of the specimen with 0.2 s impulses of 3.5 A current. During an impulse the specimen was transformed from martensitic state into an austenitic one. Synchronization of the impulses with the mechanical oscillations is illustrated by Heating impulses were applied at a frequency twice that of the vibrations and as one may see from the figure the specimen experienced the transition from martensite to austenite and back in the course of each semiperiod of the vibrations. The moment of time corresponding to the maximum deflection of the pendulum from equilibrium in each semiperiod was registered by the equipment (by the minimum of the angular speed of the beam) and in a specified delay time At a heating impulse was given

In situ, 3D characterization of the deformation mechanics of a superelastic NiTi shape memory alloy single crystal under multiscale constraint

Microstructural elements in NiTi shape memory alloys (SMAs) e precipitates, phase boundaries, inclusions

Glassy phonon heralds a strain glass state in a shape memory alloy

Shape memory strain glasses are frustrated ferroelastic materials with glasslike slow relaxation an

Ultrahigh tensile transformation strains in new Ni50.5Ti36.2Hf13.3 shape memory alloy

We report on unprecedented transformation strains exceeding 20% in tension for Ni50.5Ti36.2Hf13.3 shape memory alloy (SMA). The strain measurements were made at multiscales utilizing advanced digital image correlation. The display of excellent strain reversibility in shape memory (isothermal deformation between Mf and Af), isobaric thermal cycling (between Mf and Af), and superelasticity experiments (deformation above Af) confirms a wide range of functionality. The ultrahigh strains in [111] orientation exceed the lattice deformation theory predictions possibly pointing to contributions from mechanical twinning effects. The high strength levels and large strains result in very high work outputs compared to other SMAs

Hysteresis in NiTi alloys

Large changes in hysteresis are reported as a function of applied stress under thermal cycling experiments in NiTi single crystals. In the low Ni alloy the thermal hysteresis expanded with increasing stress while in the high Ni alloy the thermal hysteresis contracted with increasing stress. The results in both cases meet the limit obtained from Differential Scanning Calorimetry at zero stress. The changes are attributed to the relaxation of elastic stored energy which is primarily due to dislocations emanating at martensite/austenite interfaces. Modifications in thermodynamics formulation are proposed to account for the change in hysteresis via change in the elastic stored energy. Memory effects due to dislocation arrangements imposed under high stress thermal cycles on subsequent thermal hysteresis under low stresses were found to be significant, while variations in thermal hysteresis from cycle to cycle under constant stress are noted to be rather small

High temperature shape memory behavior of Ni50.3Ti25Hf24.7 single crystals

In this work we present the high temperature functional behavior of the new Ni50.3Ti25Hf24.7 shape memory alloy (SMA). Very high transformation strains were measured during isobaric experiments at temperatures up to Af = 420 °C. For the [111]B2 orientation in tension, we measured with digital image correlation (DIC) averaged transformation strain of 5.15%, while in small domains 7.74%. For the [011]B2 orientation in compression, we measured averaged transformation strain of 4.6%, locally 5.3%. The remarkable results in terms of actuation strains at T > 400 °C define the Ni50.3Ti25Hf24.7 alloy as one of the most promising shape SMA for high temperature applications