Internal Crack Initiation and Growth Starting from Artificially Generated Defects in Additively Manufactured Ti6Al4V Specimen in the VHCF Regime

The aim of the present work was to investigate the ‘fine granular area’ (FGA) formation based on artificially generated internal defects in additively manufactured Ti6Al4V specimens in the early stage of fatigue crack growth in the ‘very high cycle fatigue’ (VHCF) regime. Fatigue tests were performed with constant amplitude at pure tension-compression loading (R = −1) using an ultrasonic fatigue testing setup. Failed specimens were investigated using optical microscopy, high-resolution ‘scanning electron microscopy’ (SEM), and ‘focused ion beam’ (FIB) techniques. Further, the paper introduces alternative proposals to identify the FGA layer beneath the fracture surfaces in terms of the ‘cross section polishing’ (CSP) technique and metallic grindings with special attention paid to the crack origin, the surrounding microstructure, and the expansion of the nanograin layer beneath the fracture surface. Different existing fracture mechanical approaches were applied to evaluate if an FGA formation is possible. Moreover, the results were discussed in comparison to the experimental findings

Phase transformations of stoichiometric mixtures of hematite and iron under FAST conditions

Since the mechanism of the synthesis of magnetite from a stoichiometric mixture of hematite and iron isstill under debate, systematic studies of the phase transformations in such powder mixture processedunder field assisted sintering conditions, are presented. Phase contributions, grain sizes and stoichiometriesof the sintered composites were determined using scanning electron microscopy, high energyX-ray diffraction and M€oßbauer spectroscopy. It was shown that with an increasing sintering temperaturean accelerated growth of magnetite can be observed, while the amount of hematite decreases.Additionally, intermediate wustite phase was observed with a maximum intensity where iron vanishedfrom the samples. Therefore, it was concluded that the transition from hematite - iron mixture tomagnetite actually takes place in two steps. In the first step, iron reduces hematite to magnetite andoxidizes itself to wustite. In the second step, wustite enables the nucleation of magnetite and with thehelp of hematite it transforms into nearly pure stoichiometric magnetite at higher sintering temperatures.In composites sintered from pure hematite under the same conditions only a minor transition tohighly nonstoichiometric magnetite was observed emphasizing the above mentioned route oftransformation

Structural properties of Mn-substituted hercynite

In this work spinel series with the general formula Fe1-xMnxAl2O4 (where x = 0, 0.3, 0.5 and 0.7) were synthesized and characterized with respect to their structure and microstructure. X-ray diffractometry (XRD) was used to identify the phase composition that revealed a single phase spinel material. Rietveld refinements of the XRD patterns were carried out in order to determine the lattice and oxygen positional parameters of the spinel compounds. Mössbauer effect measurements were performed at room temperature to determine the local chemical environment of the Fe ions, their valences, and degrees of spinels inversion. It was shown that an increase in the Mn content led to a decrease in the ratio of Fe2+ to Fe3+. The results obtained from Mössbauer spectroscopy (MS) were used to establish the chemical formulas of the synthesized spinels. Finally, the microstructure that was observed using scanning electron microscopy (SEM) showed a compact microstructure with an octahedral crystal habit

Structural changes and pseudo-piezoelectric behaviour of field assisted sintered calcium titanate

The polycrystalline perovskite calcium titanate has an orthorhombic crystal structure at room temperature, which belongs to a centro-symmetric point group. Due to this fact, it does not show piezoelectric behaviour. However, such behaviour is observed in nanostructured calcium titanate prepared by sol-gel synthesis and field assisted sintering. Whereas, the conventionally sintered sample does not show this behaviour. Presumably, the instability of regular TiO$_6$ octahedra results in the off-centering of titanium positions of the field assisted sintered calcium titanate. This phenomenon leads to the generation of electric dipoles due to the lattice distortions produced by the formation of highly localized defects, i.e. oxygen vacancies, during densification by the field assisted sintering. As a result, pseudo-piezoelectric behaviour is observed, which confirms that the field assisted sintering triggers the piezoelectric effect but not the conventional sintering. The charge ( Q ) produced in the field assisted sintered sample and the piezoelectric constant (d$_{33}$*) values have been determined to be Q = (2.1 ± 0.3) pC and d$_{33+}$* ~(7.13 ± 0.4) pm/V or d$_{33-}$* ~(-5.95 ± 0.3) pm/V, respectively. This particular response of nanostructured calcium titanate is of great interest in biomedicine because it can improve the osseointegration of an implant

Tunable Pseudo-Piezoelectric Effect in Doped Calcium Titanate for Bone Tissue Engineering

CaTiO$_3$ is a promising candidate as a pseudo‐piezoelectric scaffold material for boneimplantation. In this study, pure and magnesium/iron doped CaTiO$_3$ are synthesized by sol‐gelmethod and spark plasma sintering. Energy dispersive X‐ray mapping confirm the homogenousdistribution of doping elements in sintered samples. High‐energy X‐ray diffraction investigationsreveal that doping of nanostructured CaTiO$_3$ increased the strain and defects in the structure ofCaTiO$_3$ compared to the pure one. This led to a stronger pseudo‐piezoelectric effect in the dopedsamples. The charge produced in magnesium doped CaTiO$_3$ due to the direct piezoelectric effect is(2.9 ± 0.1) pC which was larger than the one produced in pure CaTiO$_3$ (2.1 ± 0.3) pC, whereas themaximum charge was generated by iron doped CaTiO$_3$ with (3.6 ± 0.2) pC. Therefore, the pseudopiezoelectricbehavior can be tuned by doping. This tuning of pseudo‐piezoelectric responseprovides the possibility to systematically study the bone response using different piezoelectricstrengths and possibly adjust for bone tissue engineering

A detailed study on the transition from the blocked to the superparamagnetic state of reduction-precipitated iron oxide nanoparticles

Magnetic iron oxide nanoparticles were prepared by salt-assisted solid-state chemical precipitation method with alternating fractions of the ferric iron content. The physical properties of the precipitated nanoparticles mainly consisting of magnetite were investigated by means of transmission electron microscopy, high energy X-ray diffraction, vibrating sample magnetometry and Mössbauer spectroscopy. With particle sizes ranging from 16.3 nm to 2.1 nm, a gradual transition from the blocked state to the superparamagnetic state was observed. The transition was described as a dependence of the ferric iron content used during the precipitation. Composition, mean particle size, coercivity, saturation polarisation, as well as hyperfine interaction parameters and their evolution were studied systematically over the whole series of iron oxide nanoparticles

Formation of maghemite nanostructures in polyol: tuning the particle size via the precursor stoichiometry

This study investigates a thermal synthesis in which ironIJII) and ironIJIII) chlorides are used to formmaghemite nanoflowers in the presence of sodium hydroxide and a solvent mixture ofN-methyldiethanolamine and diethylene glycol. The agglomeration process leading to nanoflower formationis examined by testing temperatures of synthesis ranging from 180 °C to 220 °C and holding timesfrom 2 h to 12 h. A temperature of 220 °C and a holding time of 2 h are confirmed to be suitable synthesisparameters for nanoflower formation. The process of primary particle ordering during agglomeration leadingto cooperative magnetic behaviour is discussed. The stoichiometric ratio of Fe$^{2+}$ and Fe$^{3+}$ ions was variedin the precursor solution and it is shown to have a strong influence on particle and crystallite sizes, andthereby on the magnetic properties. A larger Fe$^{3+}$ ion content during the synthesis leads to larger particlesand crystallites, while a higher content in Fe$^{2+}$ ions favours nucleation at the expense of growth. An additionaltreatment with iron nitrate leads to further growth of crystallites as well as particles. Stable, mostlymonodisperse suspensions of multicore particles with diameters ranging from 18.4 nm to 28.7 nm showBrownian relaxation times between 0.2 μs and 9 μs and dynamic susceptibilities at 25 kHz from about 7 ×10$^{−3}$ m3 kg$^{−1}$ [Fe] to 20 × 10$^{−3}$ m3 kg$^{−1}$ [Fe], making the particles interesting candidates for magnetic hyperthermiaand magnetic particle imaging