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.).
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.
Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1
...mm 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 (20–50
nm) 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 200
nm.
Carbon nanotube reinforced copper alloy (CNT/Cu) composites with high strength and good wear resistance have been developed using acid treatment, sintering processes and consolidation techniques. The ...effects of CNT reinforcement and grain size refinement on the mechanical properties and surface deformations of CNT/Cu composite coatings are investigated by means of nanoindentation and block-on-ring wear tests, respectively. High-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses reveal that the CNTs are firmly implanted in the Cu alloy due to the formation of Cu-oxides at the CNT-Cu interface. The effect of CNT addition on the CNT/Cu coating strength is significantly greater than that of grain size reinforcement. Finally, the present results indicate that the addition of CNTs to the Cu matrix reduces the surface deformations of the CNT/Cu composite coatings due to the formation of a carbonaceous solid-lubricant film at the contact interface during sliding.
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•Grain size refinement and CNT reinforcement for CNT/Cu composite are studied.•Morphological and structural characterizations of Cu-coated CNT are investigated.•Mechanical properties and surface deformations of CNT/Cu composites are examined.
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.
Polycrystalline NiTi shape memory alloys deformed in tension tend to exhibit localization and propagation of deformation in macroscopic shear bands. The propagation of the deformation bands is ...characterized by a plateau-type stress–strain curve. Such behavior has been reported only for NiTi alloys and only in tension. The reason for this behavior is still unclear although different hypotheses have been proposed in the literature. This article briefly summarizes relevant experimental evidences and offers an explanation based on micromechanics modelling of pseudoelasticity of polycrystalline NiTi.
The stress-induced martensitic transformation in tensioned nickel-titanium shapememory 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.
While outstanding functional properties of thin NiTi wires are nowadays well recognized and beneficially utilized in medical NiTi devices, development of 2D/3D wire structures made out of these NiTi ...wires remains challenging and mostly unexplored. The research is driven by the idea of creating novel 2D/3D smart structures which inherit the functional properties of NiTi wires and actively utilize geometrical deformations within the structure to create new/improved functional properties. Generally, textile technology provides attractive processing methods for manufacturing 2D/3D smart structures made out of NiTi wires. Such structures may be beneficially combined with soft elastomers to create smart deformable composites. Following this route, we carried out experimental work focused on development of 3D flexible NiTi-braided elastomer composites involving their design, laboratory manufacture and thermomechanical testing. We describe the manufacturing technology and structural properties of these composites; and perform thermomechanical tests on the composites, focusing particularly on quasistatic tensile properties, energy absorption, damping and actuation under tensile loading. Functional thermomechanical properties of the composites are discussed with regard to the mechanical properties of the components and architecture of the composites. It is found that the composites indeed inherit all important features of the thermomechanical behavior of NiTi wires but, due to their internal architecture, outperform single NiTi wires in some features such as the magnitude of recoverable strain, superelastic damping capacity and thermally induced actuation strain.
Evolution of the electrical resistivity during thermal and mechanical tests of NiTi wires showing R-phase transformation was investigated by experiments and micromechanical model simulations ...considering B2-R-B19′ transformations. Since reasonable agreement between the simulated and experimental mechanical and electrical resistivity responses was achieved, the apparently curious behavior of electrical resistivity could be rationalized through the model simulations in terms of the activity of multiple transformation and deformation processes taking place in the activated NiTi wires.
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 X-ray 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 °C), (iii) strain drift of NiTi actuators can be brought under control utilizing the knowledge derived from the presented experiments.
