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.
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.
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.
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•At T ≤ 450 °C, Ni-rich grains (NiEGs) segregate at the steel/double oxide interface.•At T ≥ 475 °C, an internal oxidation zone (IOZ) forms under the double oxide scale.•The IOZ ...consists of two phases: a Ni-rich fcc phase and an FeCr-spinel phase.•The IOZ growth relies on fast diffusion paths, such as grain and twin boundaries.
The deployment of Gen-IV lead-cooled fast reactors requires a good compatibility between the selected structural/cladding steels and the inherently corrosive heavy liquid metal coolant. An effective liquid metal corrosion mitigation strategy involves the in-situ steel passivation in contact with the oxygen-containing Pb-alloy coolant. Transmission electron microscopy was used in this work to study the multi-layered oxide scales forming on an austenitic stainless steel fuel cladding exposed to oxygen-containing (CO ≈ 10−6 mass%) static liquid lead-bismuth eutectic (LBE) for 1000 h between 400 and 500 °C. The oxide scale constituents were analyzed, including the intertwined phases comprising the innermost biphasic layer.
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.
The present work reports on the microstructural evolution of a new heat of 24% cold worked austenitic DIN 1.4970 (15-15Ti) nuclear cladding steel subjected to ageing heat treatments of varying ...duration between 500 and 800 °C (by steps of 100 °C). The primary aim was studying the finely dispersed Ti-C nanoprecipitate population, which are thought to be beneficial for creep and swelling resistance during service. Their size distribution and number density were estimated through dark field imaging and bright field Moiré imaging techniques in the transmission electron microscope. Nanoprecipitates formed at and above 600 °C, which is a lower temperature than previously reported. The observed nucleation, growth and coarsening behavior of the nanoprecipitates were consistent with simple diffusion arguments. The formation of nanoprecipitates coincided with significant dissociation of dislocations as evidenced by weak beam dark field imaging. Possible mechanisms, including Silcock's stacking fault growth model and Suzuki segregation, are discussed. Recrystallization observed after extended ageing at 800 °C caused the redissolution of nanoprecipitates. Large primary Ti(C,N) and (Ti,Mo)C precipitates that occur in the as-received material, and M23C6 precipitates that nucleate on grain boundaries at low temperatures were also characterized by a selective dissolution procedure involving filtration, X-ray diffraction and quantitative Rietveld refinement. The partitioning of key elements between the different phases was derived by combining these findings and was consistent with thermodynamic considerations and the processing history of the steel.
•A new heat of DIN 1.4970 cladding steel was aged and characterized.•Ti-C nanoprecipitate number and size distribution evolution were studied by TEM.•They appeared at lower temperatures (≥600 °C) than previously reported.•Results were explained referring to a solubility product and a diffusion model.•Nanoprecipitate formation coincided with dislocation dissociation.
•Successful multi material SP assessments in a wide temperature range shown.•Good robust Rm estimates by different SP force to stress conversion models.•The Chakrabarty models can be used for force ...to stress conversion of non-standard test set-ups.•Good estimates on Rm for curved samples such as cladding tubes by correcting for tube curvature.
The Small Punch (SP) test has shown in a number of applications that it can be successfully used for material ranking and material property estimation, especially where standard tests cannot be applied due to sampling location or the amount of material available. The most sought material properties are the ultimate tensile strength (Rm) and proof strength (Rp02) for the classical SP tests and the equivalent creep stress σ for the SPC creep testing. In the case of SP testing the force to stress conversion is classically done by correlating the Rm to descriptive (test set-up dependent) variables such as measured maximum force divided by the product of displacement at the maximum and the disk thickness. Naturally, if the test set-up or the test samples are not according to the standardized dimensions or low material ductility imposes crack growth instead of plastic deformation, these formulations cannot be applied. In this paper the classical formulations are studied and modifications in the formulations and in the extraction of the best descriptive variables for estimating Rm are proposed. The assessments are done on a range of materials using both standardized flat SP samples as well as curved (tube section) samples. It is claimed that the equivalent stress in both SP and SPC tests can robustly be estimated with the same type of equation, at least for ductile and semi-ductile ferritic/martensitic and austenitic steels. The same equations can further be applied on non-standard test samples and test set-ups using FEA determined conversion factors correcting for curvature. The tensile strengths of ductile P91 steel and 46% cold worked 15-15Ti cladding steel, with clearly reduced ductility, are successfully estimated in a broad temperature range. The determination of tensile strength by small punch testing of engineering steels in general and for nuclear claddings in specific has successfully been shown to give robust and accurate estimates.
•Lamellar twin density is reduced when λ2=1 is approached.•Compound twins are replaced by Type I twins when Au content increases.•Local lattice parameters measured using internal reference of Au ...particles.
The microstructure of various compounds of the Ti–Ni–Au alloy system is investigated by transmission electron microscopy in relation with changing lattice parameters improving the compatibility conditions between austenite and martensite expressed by the λ2=1 equation based on the Geometrically NonLinear Theory of Martensite (GNLTM). Although local differences in microstructure are observed, when increasing the gold content compound twins are replaced by Type I twins, while twinned lamellar structures are replaced by untwinned plates and self-accommodating structures when λ2=1 is approached, all confirming the predictions of the GNLTM.
Nanometer sized (Ti,Mo,Cr)C (MX-type) precipitates that grew in a 24% cold worked Ti-stabilized austenitic stainless steel (grade DIN 1.4970, member of the 15-15Ti austenitic stainless steels) after ...heat treatment were fully characterized with transmission electron microscopy (TEM), probe corrected high angle annular dark field scanning transmission electron microscopy (HR-HAADF STEM), and atom probe tomography (APT). The precipitates shared the cube-on-cube orientation with the matrix and were facetted on {111} planes, yielding octahedral and elongated octahedral shapes. The misfit dislocations were believed to have Burgers vectors a/6 which was verified by geometrical phase analysis (GPA) strain mapping of a matrix-precipitate interface. The dislocations were spaced five to seven atomic planes apart, on average slightly wider than expected for the lattice parameters of steel and TiC. Quantitative atom probe tomography analysis of the precipitates showed that precipitates were significantly enriched in Mo, Cr and V, and that they were hypostoichiometric with respect to C. These findings were consistent with a reduced lattice parameter. The precipitates were found primarily on Shockley partial dislocations originating from the original perfect dislocation network. These novel findings could contribute to the understanding of how TiC nanoprecipitates interact with point defects and matrix dislocations. This is essential for the application of these Ti-stabilized steels in high temperature environments or fast spectrum nuclear fission reactors.
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Three different cladding types were tested for nuclear fuel in traditional light water reactors and generation IV gas-cooled fast reactors. Cr coated Zr cladding was tested in steam atmosphere up to ...1200 °C to demonstrate moderate oxidation and hydrogen production in accident conditions. 15-15Ti stainless steel alloy and SiCf/SiC cladding tube samples were treated in helium atmosphere with different impurities for several hours at 1000 °C. Additional mechanical testing and microstructure examinations were carried out with as-received samples and with specimens after high temperature treatments. The experiments results indicated the applicability of the tested materials for reactor conditions in the investigated range of parameters.
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