Recent macroscopic experimental and theoretical evidence on the stress-strain-temperature behavior of NiTi beyond the strain recoverability limits (large strain, high stress, high temperature), where reversible martensitic transformation tends to proceed together with irreversible plastic deformation processes, is reviewed. Model predictions on the transformation plasticity coupling are laid out based on the mathematical theory of martensitic microstructures and the crystal plasticity theory. A particular attention is paid to the strain compatibility at moving phase interfaces that may have a direct impact on the plasticity accompanying the martensitic transformation. It is suggested that strong transformation-plasticity coupling shall be expected during the reverse martensitic transformation. Macroscopic models from the literature capable of simulation of thermomechanical responses of NiTi polycrystals due to coexisting martensitic transformation and plastic deformation are reviewed. Dedicated thermomechanical loading experiments on superelastic and actuator NiTi wires aimed at improving our understanding of the coupling between martensitic transformation and plasticity are presented. Based on the results of in-situ studies during thermomechanical loading experiments (electric resistance, synchrotron Xray diffraction, surface strain by DIC, relaxations) and characterization microstructures in deformed wires by TEM, it is shown that: (i) microstructures and consequently functional properties of annealed NiTi wires can be purposely manipulated by thermomechanical processing, (ii) shape setting of NiTi can be performed at relatively low temperatures (< 300 degrees C), (iii) strain drift of NiTi actuators can be brought under control utilizing the knowledge derived from the presented experiments.

Three neutron instruments at the Neutron Physics Laboratory (NPL) in Řež near Prague — small-angle scattering (SANS) MAUD, strain scanner SPN-100 and strain diffractometer TKSN-400 — have been modernized recently with new 2D position-sensitive detectors (PSDs) from JINR, Dubna. Here we report on the progress made in relation to the possibilities of the diffractometers due to the improved performance of the detectors. The first part of the paper is dedicated to a detailed description of the hardware and software of the PSDs, as well as its integration with the in-house experimental control software. Then practical examples of neutron scattering experiments for each of the upgraded facilities are presented.

The stress-induced martensitic transformation in tensioned nickel-titanium shape-memory alloys proceeds by propagation of macroscopic fronts of localized deformation. We used three-dimensional synchrotron x-ray diffraction to image at micrometer-scale resolution the grain-resolved elastic strains and stresses in austenite around one such front in a prestrained nickel-titanium wire. We found that the local stresses in austenite grains are modified ahead of the nose cone-shaped buried interface where the martensitic transformation begins. Elevated shear stresses at the cone interface explain why the martensitic transformation proceeds in a localized manner. We established the crossover from stresses in individual grains to a continuum macroscopic internal stress field in the wire and rationalized the experimentally observed internal stress field and the topology of the macroscopic front by means of finite element simulations of the localized deformation.

The Beamline for European Materials Engineering Research (BEER) will be built at the European Spallation Source (ESS). The diffractometer utilizes the high brilliance of the long- pulse neutron source and offers high instrument flexibility. It includes a novel chopper technique that extracts several short pulses out of the long pulse, leading to substantial intensity gain of up to an order of magnitude compared to pulse shaping methods for materials with high crystal symmetry. This intensity gain is achieved without compromising resolution. Materials of lower crystal symmetry or multi-phase materials will be investigated by additional pulse shaping methods. The different chopper set- ups and advanced beam extracting techniques offer an extremely broad intensity/ resolution range. Furthermore, BEER offers an option of simultaneous SANS or imaging measurements without compromising diffraction investigations. This flexibility opens up new possibilities for in-situ experiments studying materials processing and performance under operation conditions. To fulfil this task, advanced sample environments, dedicated to thermo- mechanical processing, are foreseen.

Microstructures and thermomechanical responses of Ni-rich superelastic wires can be tuned in relatively large extent by varying the degree of cold work and heat treatment. What frequently remains unknown is the superelastic fatigue performance of the wires – whether and/or how it depends on the heat treatment. In this study, five superelastic NiTi wires (d=0.051mm) having similar transformation stresses but different microstructures and transformation strains (3.1, 3.9, 4.7, 5.6, 6.7 %) were produced from one spool of a hot worked NiTi wire by applying different cold work/heat treatments to investigate the influence of the wire microstructure (transformation strain) on its structural and functional fatigue performance. The wires were cycled in tension beyond the end of superelastic plateau at constant temperature in strain rate controlled mode until failure. It is found that various cold work/heat treatments do affect the superelastic fatigue performance of NiTi wires in a defined manner. As concerns the transformation strain dependence, the wires exhibiting large transformation strain show decreasing fatigue performance with increasing transformation strain, the wires exhibiting low transformation strain show opposite trend. On average, the furnace treated NiTi wires (recovered and precipitation hardened microstructure) showed better fatigue performance than the electropulse treated wires (nanosized but partially recrystallized microstructure).

Cyclic tensile superelastic deformation of NiTi was investigated using high resolution in-situ synchrotron x-ray diffraction method. Diffraction patterns were recorded under applied stress prior and after the stress induced martensitic transformation, analyzed and interpreted in view of the evolution of microstructure in the transforming polycrystal. A simple bicrystal scheme of the polycrystal transformation is used to rationalize the experimental evidence on microstructure evolution during cycling. It is claimed that the instability of cyclic superelastic response of NiTi is due to the gradual rise of internal stresses originating from the accumulation of plastic deformation accompanying the martensitic transformation in constrained polycrystalline environment.

In the context of the ongoing research addressing problems of random clinical failures of braided esophageal NiTi stents, we introduce novel in-situ electrochemical methods for investigation of corrosion fatigue of superelastic NiTi wires or springs subjected to cyclic mechanical loadings in simulated body fluids. Corrosion fatigue properties of NiTi are very different from conventional materials due to the intrinsic recoverable deformability of the surface originating from martensitic transformation. It is shown that the localized corrosion of phase transforming NiTi is strongly facilitated by cracking of the surface oxide, crack opening/closing and passivation, mutually competing during cyclic mechanical loading in fluids.

Motivated by an assumption that the instability of the cyclic tensile superelastic behavior of NiTi polycrystal is linked to its fatigue performance (number of cycles till failure), the instability was investigated by high resolution in situ synchrotron X-ray diffraction method. NiTi wires were cyclically deformed in tension at room temperature while X-ray diffraction patterns were recorded in three preselected states along the superelastic stress–strain curve, analyzed and interpreted in terms of the gradual evolution of microstructural state during cycling. It is found that the cyclic instability is due to the gradual redistribution of internal stresses originating from the accumulation of incremental plastic strains accompanying the stress induced martensitic transformation in constrained polycrystalline environment. The degree of cyclic instability increases with the increasing involvement of slip in the hybrid slip/transformation process, which depends on initial microstructure (grain size, defects, precipitates), martensitic transformation (crystallographic incompatibility between transforming phases), temperature and parameters of the cyclic loading (strain rate, amplitude, stress state, type of loading etc.).

After preliminary results obtained and published recently ill, in our contribution focusing and reflectivity properties of the dispersive double bent-crystal arrangement are presented in much more detail. It has been found that two different bent perfect crystals in (+n,-m) setting can be good candidates for high efficiency neutron microfocusing as well as high-resolution monochromatisation. Due to the (+n,-m) setting of two different bent perfect crystals, a high resolution is expected in both Delta(2 theta) (2 theta is the scattering angle) as well as Delta lambda/lambda (lambda is the neutron wavelength). Experimental tests were carried out with the setting employing the bent Si(111) slab and Si(220)-sandwich, which contained either one, or two or four 1.3 mm thin simply stacked slabs. Thanks to a high reflection probability of both bent elements and an easy manipulation with the curvature of the Si(220)-sandwich, an excellently focused intensive monochromatic beam of the width from one to several millimetres was obtained. The properties of the double bent-crystal setting were studied in Rez at the neutron optics diffractometer for the neutron wavelength of 0.162 nm and for various thicknesses and curvatures of the Si(220)-sandwich. It has been also found that besides an excellent focusing and reflectivity properties of the dispersive double bent-crystal setting the obtained monochromatic neutron current is sufficiently high for standard high-resolution diffraction experiments even at the medium power research reactor.

This chapter contains sections titled: Introduction Important phenomena in cyclic superelasticity Experimental evaluation of rate dependent superelasticity Modelling of heat effects in superelasticty Conclusion

Recovery processes responsible for evolution of microstructures in 0.1mm thin cold-drawn Ni-Ti shape memory alloy wire heat treated by DC electric pulse were investigated by combination of in-situ tensile stress - strain, electrical resistance and X-ray diffraction measurements. The X-ray data were used to obtain direct experimental information on the evolution of the phase fractions, internal strain and defects in the microstructure evolving through activation of a sequence of recovery processes during the short time electropulse treatment. It is shown that superelastic functional properties of the treated Ni-Ti wire can be precisely set by controlling the progress of the recovery processes by prescribing the time evolution of temperature T(t) and tensile stress s(t) (displacement control) in the treated wire.

A recently developed Velcro-like fastener utilizes superelastic deformation of two interlocked NiTi hooks when pulled apart. This work focuses on experimental analysis (evaluation of normal detachment force at different temperatures) and modeling (simulation by a finite element implemented SMA model) of the unhooking process. It is claimed that nonlinear superelastic deformation of NiTi leads to unique properties of the NiTi hook fasteners such as high strength (similar to 15 000 kg m(-2)), a significant increase of strength with increasing temperature, absorption of impact loads, damping of mechanical vibrations, forceless contact or silent release and better functioning in dirty environments compared to conventional Velcro fasteners.

Neutron strain scanning was employed to investigate the internal stress fields in steel plate coupons with embedded prestrained superelastic NiTi shape memory alloy inserts. Strain fields in steel were evaluated at T=21°C and 130°C on virgin coupons as well as on mechanically and thermally fatigued coupons. Internal stress fields were evaluated by direct calculation of principal stress components from the experimentally measured lattice strains as well as by employing an inverse finite element modeling approach. It is shown that if the NiTi inserts are embedded into the elastic steel matrix following a carefully designed technological procedure, the internal stress fields vary with temperature in a reproducible and predictable way. It is estimated that this mechanism of internal stress generation can be safely applied in the temperature range from −20°C to 150°C and is relatively resistant to thermal and mechanical fatigue. The predictability and fatigue endurance of the mechanism are of essential importance for the development of future smart metal matrix composites or smart structures with embedded shape memory alloy components.

Superelastic deformation of thin Ni–Ti wires containing various nanograined microstructures was investigated by tensile cyclic loading with in situ evaluation of electric resistivity. Defects created by the superelastic cycling in these wires were analyzed by transmission electron microscopy. The role of dislocation slip in superelastic deformation is discussed. Ni–Ti wires having finest microstructures (grain diameter <100 nm) are highly resistant against dislocation slip, while those with fully recrystallized microstructure and grain size exceeding 200 nm are prone to dislocation slip. The density of the observed dislocation defects increases significantly with increasing grain size. The upper plateau stress of the superelastic stress–strain curves is largely grain size independent from 10 up to 1000 nm. It is hence claimed that the Hall–Petch relationship fails for the stress-induced martensitic transformation in this grain size range. It is proposed that dislocation slip taking place during superelastic cycling is responsible for the accumulated irreversible strains, cyclic instability and degradation of functional properties. No residual martensite phase was found in the microstructures of superelastically cycled wires by TEM and results of the in situ electric resistance measurements during straining also indirectly suggest that none or very little martensite phase remains in the studied cycled superelastic wires after unloading. The accumulation of dislocation defects, however, does not prevent the superelasticity. It only affects the shape of the stress–strain response, makes it unstable upon cycling and changes the deformation mode from localized to homogeneous. The activity of dislocation slip during superelastic deformation of Ni–Ti increases with increasing test temperature and ultimately destroys the superelasticity as the plateau stress approaches the yield stress for slip. Deformation twins in the austenite phase ({1 1 4} compound twins) were frequently found in cycled wires having largest grain size. It is proposed that they formed in the highly deformed B19′ martensite phase during forward loading and are retained in austenite after unloading. Such twinning would represent an additional deformation mechanism of Ni–Ti yielding residual irrecoverable strains.

Microstructural changes taking place during the heat treatment of cold-worked NiTi alloy are of key interest in shape memory alloy technology, since they are responsible for setting the austenite shape and functional properties of the heat-treated alloy. In this work, microstructural evolution during non-conventional electropulse heat treatment of thin NiTi filaments was investigated in a unique high-speed in situ synchrotron X-ray diffraction experiment with simultaneous evaluation of the tensile force and electrical resistivity of the treated wire. The in situ results provide direct experimental evidence on the evolution of the internal stress and density of defects during fast heating from 20°C to ∼700°C. This evidence is used to characterize a sequence of dynamic recovery and recrystallization processes responsible for the microstructure and superelastic functional property changes during the electropulse treatments.

Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1mm diameter as-drawn Ni–Ti wires subjected to a non-conventional heat treatment by controlled electric pulse currents. This method enables a better control of the recovery and recrystallization processes taking place during the heat treatment and accordingly a better control on the final microstructure. Using a stepwise approach of millisecond pulse annealing, it is shown how the microstructure evolves from a severely deformed state with no functional properties to an optimal nanograined microstructure (20–50nm) that is partially recovered through polygonization and partially recrystallized and that has the best functional properties. Such a microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer times and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 200nm.

The present work deals with a non-conventional application of multifunctional materials such as shape memory alloy in engineering components. The concept of active inserts has been adopted in order to redistribute compressive stresses emerged in cutting disc during its operation. According to the present design, the small pre-strained elliptical NiTi elements were placed into openings of steel cutting disc in places with expected maximum stress concentration. To study the stress interaction of the NiTi inserts with steel matrix in detail, the in-situ method of neutron diffraction was employed for residual stress mapping. The diffraction experiments were focused substantially on scan of internal stresses around inserts and their evolution with increased temperature. The performed studies confirm the potential ability of NiTi insert to induce the compressive stress within steel matrix with applied temperature.

In this paper, questions linked to the practical use of superelastic damping exploiting stress-induced martensitic transformation for vibration damping are addressed. Four parameters, particularly vibration amplitude, prestrain, temperature of surroundings, and frequency, are identified as having the most pronounced influence on the superelastic damping. Their influence on superelastic damping of a commercially available superelastic NiTi wire was experimentally investigated using a self-developed dedicated vibrational equipment. Experimental results show how the vibration amplitude, frequency, prestrain, and temperature affect the capacity of a superelastic NiTi wire to dissipate energy of vibrations through the superelastic damping. A special attention is paid to the frequency dependence (i.e., rate dependence) of the superelastic damping. It is shown that this is nearly negligible in case the wire is in the thermal chamber controlling actively the environmental temperature. In case of wire exposed to free environmental temperature in actual damping applications, however, the superelastic damping capacity significantly decreases with increasing frequency. This was explained to be a combined effect of the heat effects affecting the mean wire temperature and material properties with the help of simulations using the heat equation coupled phenomenological SMA model.

Earlier developed micromechanics crystallographic model of SMA polycrystals has been modified by incorporating plastic deformation as additional deformation mechanism. It is assumed that dislocation slip proceeds in austenite and B19' martensite phases in parallel with the processes derived from B2-B19' martensitic transformation in NiTi alloy. The model was used to simulate the responses of NiTi polycrystal in three thermomechanical cycles typically carried out in the SMA research. Based on the simulation results, it is proposed that, due to the dislocation slip occurring simultaneously with transformation related processes, the stresses and strains in the transforming polycrystal are significantly redistributed which in turn modifies the macroscopic thermomechanical responses. In tensile tests at low temperature, the plastic deformation occurs in martensite phase after the stress induced transformation or martensite reorientation, while at medium and high temperatures both deformation mechanisms proceed in parallel. Depending on whether the plastic deformation in tensile tests had taken place in the austenite (typically at high temperatures) or in the martensite (typically at low temperatures) phase, distribution of internal stresses is quite different. Surprisingly, significant plastic deformation activity is predicted for the cooling/heating tests under constant tensile stress. Plastic deformation occurs in this case mainly in the martensite state and redistribution of stress plays significant role. Results of the simulation of the cyclic tensile test at constant temperature explain qualitatively most of the features of the unstable superelastic stress-strain curves observed in cyclic loading experiments. It is proposed that the actual reason for accumulation of unrecovered strain during cyclic loading is in fact the continuously evolving distribution of internal stresses, strains and phase fractions - it causes the plastic deformation to progress even if the macroscopic maximum strain remains constant and peak stress decreases upon cyclic loading.