Jaromír Kopeček

Grain-resolved analysis of localized deformation in nickel-titanium wire under tensile load.

Bend it, shape it, remember it Shape-memory alloys have the useful property of returning to their original shape after being greatly deformed. This process depends on the collective behavior of many small mineral grains in the metal. Using three-dimensional x-ray diffraction, Sedmák et al. tracked over 15,000 grains in a nickel-titanium shape-memory alloy as it moved through this transformation, thus linking microscopic changes to the bulk deformation. Science, this issue p. 559 Synchrotron x-ray diffraction of 15,000 grains allows imaging of internal shear bands in NiTi alloy during deformation. 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.

Structure refinement using precession electron diffraction tomography and dynamical diffraction: tests on experimental data.

The recently published method for the structure refinement from three-dimensional precession electron diffraction data using dynamical diffraction theory [Palatinus et al. (2015). Acta Cryst. A71, 235-244] has been applied to a set of experimental data sets from five different samples - Ni2Si, PrVO3, kaolinite, orthopyroxene and mayenite. The data were measured on different instruments and with variable precession angles. For each sample a reliable reference structure was available. A large series of tests revealed that the method provides structure models with an average error in atomic positions typically between 0.01 and 0.02 Å. The obtained structure models are significantly more accurate than models obtained by refinement using kinematical approximation for the calculation of model intensities. The method also allows a reliable determination of site occupancies and determination of absolute structure. Based on the extensive tests, an optimal set of the parameters for the method is proposed.

Chromium-doped DLC for implants prepared by laser-magnetron deposition.

Diamond-like carbon (DLC) thin films are frequently used for coating of implants. The problem of DLC layers lies in bad layer adhesion to metal implants. Chromium is used as a dopant for improvement of adhesion of DLC films. DLC and Cr-DLC layers were deposited on silicon, Ti6Al4V and CoCrMo substrates by a hybrid technology using combination of pulsed laser deposition (PLD) and magnetron sputtering. The topology of layers was studied using SEM, AFM and mechanical profilometer. Carbon and chromium content and concentration of trivalent and toxic hexavalent chromium bonds were determined by XPS and WDS. It follows from the scratch tests that Cr doping improved adhesion of DLC layers. Ethylene glycol, diiodomethane and deionized water were used to measure the contact angles. The surface free energy (SFE) was calculated. The antibacterial properties were studied using Pseudomonas aeruginosa and Staphylococcus aureus bacteria. The influence of SFE, hydrophobicity and surface roughness on antibacterial ability of doped layers is discussed.

Anomalous lattice softening of Ni2MnGa austenite due to magnetoelastic coupling

Elastic constants of the cubic Ni2MnGa austenite phase and corresponding mechanical damping were determined in the temperature range from 220 K to 400 K and magnetic field up to 2 T using ultrasound pulse-echo method and resonant ultrasound spectroscopy. The shear coefficient c′ increases from 3.6 GPa in the demagnetized state to 5.9 GPa at magnetic saturation, whereas the damping decreased nearly six times. The changes of other elastic constants, c11 and c44 with an applied field were less than 1%. In the ferromagnetic state, the c′ was proportional to the square of magnetization. Above the Curie point, the coefficient c′ and damping were field-independent. The anomalous shear softening is attributed to strong magnetoelastic coupling enhanced by low magnetic anisotropy.

Monitoring Tensile Fatigue of Superelastic NiTi Wire in Liquids by Electrochemical Potential

Fatigue of superelastic NiTi wires was investigated by cyclic tension in simulated biofluid. The state of the surface of the fatigued NiTi wire was monitored by following the evolution of the electrochemical open circuit potential (OCP) together with macroscopic stresses and strains. The ceramic TiO2 oxide layer on the NiTi wire surface cannot withstand the large transformation strain and fractures in the first cycle. Based on the analysis of the results of in situ OCP experiments and SEM observation of cracks, it is claimed that the cycled wire surface develops mechanochemical reactions at the NiTi/liquid interface leading to cumulative generation of hydrogen, uptake of the hydrogen by the NiTi matrix, local loss of the matrix strength, crack transfer into the NiTi matrix, accelerated crack growth, and ultimately to the brittle fracture of the wire. Fatigue degradation is thus claimed to originate from the mechanochemical processes occurring at the excessively deforming surface not from the accumulation of defects due to energy dissipative bulk deformation processes. Ironically, combination of the two exciting properties of NiTi—superelasticity due to martensitic transformation and biocompatibility due to the protective TiO2 surface oxide layer—leads to excessive fatigue damage during cyclic mechanical loading in biofluids.

Structural Changes in Co-Based F-SMA

The Co38Ni33Al29 alloy as a potential ferromagnetic shape memory alloy was investigated. The method of preparation of the unidirectional solidified single-crystals from cast material is described. The high-temperature annealing and subsequent quenching was found to be necessary condition for the shape memory behavior. The martensitic transformation temperatures of annealed samples were about 200 K determined from magnetic measurement while as-cast sample did not exhibit any sharp transformation. All martensitic structures observed at room temperature by microscopic methods are thus stress induced. These results agree with pseudoelastic behavior observed in annealed and quenched samples.

Nano and micro structured targets to modulate the spatial profile of laser driven proton beams

Nano and micro structured thin (μ m-scale) foils were designed, fabricated and irradiated with the high intensity laser system operating at LLC (Lund Laser Centre, Sweden) in order to systematically study and improve the main proton beam parameters. Nano-spheres deposited on the front (laser irradiated) surface of a flat Mylar foil enabled a small enhancement of the maximum energy and number of the accelerated protons. Nano-spheres on the rear side allowed to modify the proton beam spatial profile. In particular, with nanospheres deposited on the rear of the target, the proton beam spatial homogeneity was clearly enhanced. Silicon nitride thin foils having micro grating structures (with different step dimensions) on the rear surface were also used as targets to influence the divergence of the proton beam and drastically change its shape through a sort of stretching effect. The target fabrication process used for the different target types is described, and representative experimental results are shown and discussed along with supporting 3D particle-in-cell simulations.

Nanocrystalline diamond protects Zr cladding surface against oxygen and hydrogen uptake: Nuclear fuel durability enhancement

In this work, we demonstrate and describe an effective method of protecting zirconium fuel cladding against oxygen and hydrogen uptake at both accident and working temperatures in water-cooled nuclear reactor environments. Zr alloy samples were coated with nanocrystalline diamond (NCD) layers of different thicknesses, grown in a microwave plasma chemical vapor deposition apparatus. In addition to showing that such an NCD layer prevents the Zr alloy from directly interacting with water, we show that carbon released from the NCD film enters the underlying Zr material and changes its properties, such that uptake of oxygen and hydrogen is significantly decreased. After 100–170 days of exposure to hot water at 360 °C, the oxidation of the NCD-coated Zr plates was typically decreased by 40%. Protective NCD layers may prolong the lifetime of nuclear cladding and consequently enhance nuclear fuel burnup. NCD may also serve as a passive element for nuclear safety. NCD-coated ZIRLO claddings have been selected as a candidate for Accident Tolerant Fuel in commercially operated reactors in 2020.

Surface analysis of the Heusler Ni49.7Mn29.1Ga21.2 Alloy: The composition, phase transition, and twinned microstructure of martensite

Surface analysis was used to study the dynamics of the martensitic transformation on macro- and mesoscopic scales. The chemical state, morphology, and magnetic and surface structure were monitored at particular stages of the phase transition. At room temperature, the martensitic phase of the Ni49.7Mn29.1Ga21.2 (100) single crystal exhibited macroscopic a/c twinning and a corresponding magnetic domain structure characterized by magnetization vector in and out of the surface plane. Induced by radiation heating, the transformation from martensite to austenite takes place separately at the surface and in the bulk. Its dynamics depend on the history of the sample treatment which affects the crystallographic orientation of twins and minor changes of the surface stoichiometry. The interfaces (twin planes) between twin variants in the martensitic phase were noticeable also in the austenitic phase, thanks to the shape memory effect of this material.

Dependence of NiTi Alloy Microstructure on the Conditions of Powder Metallurgy Production

The aim of this work was to describe the dependence of microstructure of NiTi shape memory alloy on the conditions of powder metallurgy processing route. The technology consisted of blending of elemental Ni and Ti powders, uniaxial cold pressing and reactive sintering. The effects of reactive sintering temperature, heating rate, holding duration and particle size were determined. The proposed technology can be used as the alternative production route of NiTi to minimize the contamination of the alloy.