Indentation size effect and 3D dislocation structure evolution in (001) oriented SrTiO3: HR‐ EBSD and etch‐pit analysis

Most crystalline materials exhibit an indentation size effect (ISE), i.e., an intrinsic increase in hardness with decreasing penetration depth. During indentation testing, the material underneath the indenter is heavily deformed, introducing strain gradients in the materials, causing high local dislocation densities. In the present work, the three-dimensional (3D) dislocation structure evolution and ISE in (001) oriented Strontium Titanate (STO) have been studied by direct observation of dislocations using chemical etching and high-resolution electron backscattered diffraction (HR-EBSD) analysis. The sequential polishing, etching and imaging technique was used to reveal the 3D dislocation etch-pit structure at various sub-surface depths using confocal laser and scanning electron microscopy (Fig. 1). The 3D dislocation etch-pit analysis of spherical indentations confirm that, at the early stage of plastic deformation, the dislocation pile-ups were aligned in \u3c100\u3e directions, lying on {110}45 planes, inclined at 45° to the (001) surface. At higher mean contact pressure and larger indentation depth, however, dislocation pile-ups along \u3c110\u3e directions appeared, lying on {110}90 planes, perpendicular to the (001) surface. These observations were qualitatively confirmed by corresponding direct Molecular Dynamics Simulations. Please click Additional Files below to see the full abstract

Ductile‐brittle‐transition of flash annealed Fe‐based metallic glass ribbons

Fe-based metallic glasses show a ductile-brittle-transition (DBT) after annealing above a critical temperature, which makes the further processing of annealed ribbons complicated. However, the annealing step is necessary to improve the soft magnetic properties of these materials for industrial applications. For the future development of ductile (partial-) nanocrystalline Fe-based ribbons with excellent soft magnetic properties, it is important to understand the mechanisms behind the DBT. Therefore, tensile and bending tests were performed to determine the DBT of 15-20 µm thin, flash annealed Fe85.2B9.5P4Cu0.8Si0.5 ribbons in terms of critical stress intensity factor and bending ductility. Microstructure analysis has been done via X-ray diffraction (XRD), differential scanning calorimetry (DSC) and atom probe tomography (ATP). Please click Additional Files below to see the full abstract

Solid solution hardening effects on structural evolution and mechanical properties of nanostructured high entropy alloys

The equiatomic Cantor alloy and Ni-enriched derivates of it ((CrMnFeCo)xNi1-x with x = 0.8, 0.4, 0.08 and 0) were deformed by high pressure torsion to the saturation regime and subsequently annealed up to 900°C. The HEA alloy compositions exhibits the highest solid solution strengthening, leading to the smallest saturation grain size as well as highest thermal stability, but a phase decomposition at intermediate temperatures. The differences in microstructural stability are also reflected by the mechanical properties as studied via Nanoindentation strain rate jump and constant contact pressure experiments. All alloys show pronounced pileup around the indentation, which is even increasing after annealing, leading seemingly to a peak in hardness and modulus of the HEA alloys. After correction, the hardness of the HEA type alloys remains constant up to 450 °C (Ni60) or even increases up to 500 °C (Ni20) followed by a softening at higher annealing temperatures. Transients are observed during strain rate change, with a slightly enhanced rate sensitivity of the dilute alloys. Furthermore, stress reduction experiments indicate higher rate sensitivities at low applied contact stresses and small deformation rates. However, the Ni-based alloys remain fairly stable at RT deformation

Exploring the compositional parameter space of high-entropy alloys using a diffusion couple approach

In this paper the phase stability and solubility limits of high-entropy alloys are studied, using a diffusion couple approach. Diffusion couples have been fabricated using the established Cantor alloy (CrMnFeCoNi) and a commercial fcc CoNiCrMo alloy (MP35N®) with constituent or foreign elements as diffusion partners. Chemical gradients within the interdiffusion zone as well as the phase stability are quantified using EDX and EBSD. For comparability of the results, new phase stability diagrams are presented. The experimental results show no general correlation between maximum solubility of individual elements and atomic size mismatch, whereas the valence electron concentration model (VEC) seems to be a good approximation for fcc to bcc phase transitions for most of the investigated diffusion couples

Dislocation and grain boundary interaction in oxides: Slip transmission or cracking?

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{001}-textured Pb(Zr, Ti)O₃ thin films on stainless steel by pulsed laser deposition

In this work, we report nearly single oriented {001}-textured ferroelectric PbZr0.52Ti0.48O3 thin films grown by pulsed laser deposition onto AISI 304 stainless steel substrates. Pt, Al2O3, and LaNiO3 buffer layers promote the PbZr0.52Ti0.48O3 {001} texture and protect the substrate against oxidation during deposition. The dominant {001} texture of the PbZr0.52Ti0.48O3 layer was confirmed using x-ray and electron backscatter diffraction. Before poling, the films exhibit a permittivity of about 350 at 1 kHz and a dielectric loss below 5%. The films display a remanent polarization of about 16.5 μC cm⁻² and a high coercive field of up to Ec ¼ 135.9 kV cm⁻¹. The properties of these PbZr0.52Ti0.48O3 thin films on stainless steel are promising for various MEMS applications such as transducers or energy harvesters

Mechanical softening of CuX alloys at elevated temperatures studied via high temperature scanning indentation

The thermal stability and temperature dependent hardness of ultrafine-grained Cu-alloys CuSn5 and CuZn5 after high pressure torsion are investigated using the high temperature scanning indentation (HTSI) method. Fast indentations are carried out during thermal cycling of the samples (heating-holding-cooling) to measure hardness and strain rate sensitivity as a function of temperature and time. The microstructures after each thermal cycle are investigated to characterize the coarsening behaviour of both alloys. Results show that the thermal stability of the tested alloys can be expressed in terms of several temperature regimes: A fully stable regime, a transient regime in which growth of individual grains occurs, and finally a regime in which the microstructure is fully coarsened. The onset of grain growth is accompanied by high strain rate sensitivity on the order of 0.2–0.3. Furthermore, the obtained hardness and strain rate sensitivity values are in good agreement with continuous stiffness measurement (CSM) and strain rate jump (SRJ) experiments. This highlights the applicability of the HTSI method to the characterization of the thermomechanical properties of ultrafine-grained alloys

Dislocation toughening in single‐crystal KNbO₃

The growing research interest in dislocation‐tuned functionality in ceramics is evident, with the most recent proofs‐of‐concept for enhanced ferroelectric properties, electrical conductivity, and superconductivity via dislocations. In this work, we focus on dislocation‐tuned mechanical properties and demonstrate that, by engineering high dislocation densities (up to 10¹⁴ m⁻²) into KNbO₃ at room temperature, the fracture toughness can be improved by a factor of 2.8. The microstructures, including dislocations and domain walls, are examined by optical microscopy, electron channeling contrast imaging, piezo‐response force microscopy, and transmission electron microscopy methods to shed light on the toughening mechanisms. In addition, high‐temperature (above the Curie temperature of KNbO₃) indentation tests were performed to exclude the influence of ferroelastic toughening, such that the origin of the toughening effect is pinpointed to be dislocations

Enhanced Conductivity and Microstructure in Highly Textured TiN1–x/c-Al2O3 Thin Films

Titanium nitride thin films are used as an electrode material in superconducting (SC) applications and in oxide electronics. By controlling the defect density in the TiN thin film, the electrical properties of the film can achieve low resistivities and a high critical temperature (Tc) close to bulk values. Generally, low defect densities are achieved by stoichiometric growth and a low grain boundary density. Due to the low lattice mismatch of 0.7%, the best performing TiN layers are grown epitaxially on MgO substrates. Here, we report for the first time a Tc of 4.9 K for ultrathin (23 nm), highly textured (111), and stoichiometric TiN films grown on 8.75% lattice mismatch c-cut Al₂O₃ (sapphire) substrates. We demonstrate that with the increasing nitrogen deficiency, the (111) lattice constant increases, which is accompanied by a decrease in Tc. For highly N deficient TiN thin films, no superconductivity could be observed. In addition, a dissociation of grain boundaries (GBs) by the emission of stacking faults could be observed, indicating a combination of two sources for electron scattering defects in the system: (a) volume defects created by nitrogen deficiency and (b) defects created by the presence of GBs. For all samples, the average grain boundary distance is kept constant by a miscut of the c-cut sapphire substrate, which allows us to distinguish the effect of nitrogen deficiency and grain boundary density. These properties and surface roughness govern the electrical performance of the films and influence the compatibility as an electrode material in the respective application. This study aims to provide detailed and scale-bridging insights into the structural and microstructural response to nitrogen deficiency in the c-Al₂O₃/TiN system, as it is a promising candidate for applications in state-of-the-art systems such as oxide electronic thin film stacks or SC applications

Nanoindentation pop‐in in oxides at room temperature: Dislocation activation or crack formation?

Most oxide ceramics are known to be brittle macroscopically at room temperature with little or no dislocation-based plasticity prior to crack propagation. Here, we demonstrate the size-dependent brittle to ductile transition in SrTiO₃ at room temperature using nanoindentation pop-in events visible as a sudden increase in displacement at nominally constant load. We identify that the indentation pop-in event in SrTiO₃ at room temperature, below a critical indenter tip radius, is dominated by dislocation-mediated plasticity. When the tip radius increases to a critical size, concurrent dislocation activation and crack formation, with the latter being the dominating process, occur during the pop-in event. Beyond the experimental examination and theoretical justification presented on SrTiO₃ as a model system, further validation on α-Al₂O₃, BaTiO₃, and TiO₂ are briefly presented and discussed. A new indentation size effect, mainly for brittle ceramics, is suggested by the competition between the dislocation-based plasticity and crack formation at small scale. Our finding complements the deformation mechanism in the nano-/microscale deformation regime involving plasticity and cracking in ceramics at room temperature to pave the road for dislocation-based mechanics and functionalities study in these materials