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- ItemThermal stability of electron beam welded AlCoCrFeNi2.1 alloy(IOP Publishing, 2024-09-30) Rončák, Ján; Jozefovič, Patrik; Müller, Peter; Adam, Ondřej; Judas, Jakub; Dupák, Libor; Zavdoveev, Anatoliy; Jan, Vít; Zobač, MartinAlCoCrFeNi2.1 alloy, which belongs to the group of eutectic high-entropy alloys (EHEAs), possesses a combination of increased strength and ductility. It should retain these properties over a wide temperature range due to the high entropy effect of the system. At the same time, eutectic alloys are generally considered to have good castability, which increases the possibility of casting the alloy in larger volumes. One of the processes, that the alloy does not avoid when applied in industry, are the various joining techniques including electron beam welding. The weld area is often in a non-equilibrium state, which increases the risk of failure during operation. The paper therefore discusses the stability of the microstructure and mechanical properties of AlCoCrFeNi2.1 alloy when exposed to short-term elevated temperatures. The material heated at 900 degrees C for 1 h in a vacuum furnace was observed using light and electron microscopy, analyzed for chemical and phase composition and finally subjected to HV0.1 hardness measurement and tensile strength test. The resulting condition was compared with the welded joint before exposure to elevated temperature. The microstructure of the weld was formed by a fine lamellar eutectic over the entire observed area. EBSD analysis confirmed the presence of a combination of FCC and BCC phases. The material hardness reached an average value of 370 HV0.1. Maximum tensile strength of the weld joint was measured at 944 MPa with the corresponding displacement of the crosshead 6.1 mm. The welded joint demonstrated sufficient stability and the ability to withstand short-term severe elevated temperature conditions.
- ItemEvaluating the suitability of fast sintering techniques for the consolidation of calcium phosphate scaffolds produced by DLP(ELSEVIER SCI LTD, 2023-11-01) Paredes Sánchez, Claudia Isabel; Roleček, Jakub; Pejchalová, Lucie; Spusta, Tomáš; Salamon, David; Miranda, PedroPorous scaffolds were fabricated via Digital Light Processing (DLP) from beta-TCP powder and sintered by conventional sintering in air (CSA), rapid sintering in air (RSA) and pressure-less spark plasma sintering in vacuum (pl-SPS), at four different temperatures: 1200, 1300, 1400 and 1500 degrees C. Each sintering strategy resulted in scaffolds with different phase composition, microstructure and mechanical properties. Long dwell times or high temperatures were required to achieve a complete beta ->alpha transformation, and rapid cooling rates avoided the reverse transformation. The presence of graphite in the sintering chamber played a crucial role in stabilising the alpha-TCP phase, phase prevailing in SPS-treated scaffolds, hindered their densification and avoided the generation of transformation-induced cracks. All scaffolds exhibited compressive strengths within the range of cancellous bone, with the highest average value of 22 +/- 4 MPa achieved by the RSA scaffolds sintered at 1300 degrees C, thanks to their greater densification and fine microstructure.
- ItemCarboxymethyl starch as a reducing and capping agent in the hydrothermal synthesis of selenium nanostructures for use with three-dimensional-printed hydrogel carriers(ROYAL SOC, 2023-10-11) Vishakha, Vishakha; Abdellatif, Abdelmohsen Moustafa; Michalička, Jan; White, Paul B.; Lepcio, Petr; Tinoco Navarro, Lizeth Katherine; Jančář, JosefThe hydrothermal method is a cost-effective and eco-friendly route for preparing various nanomaterials. It can use a capping agent, such as a polysaccharide, to govern and define the nanoparticle morphology. Elemental selenium nanostructures (spheres and rods) were synthesized and stabilized using a tailor-made carboxymethyl starch (CMS, degree of substitution = 0.3) under hydrothermal conditions. CMS is particularly convenient because it acts simultaneously as the capping and reducing agent, as verified by several analytical techniques, while the reaction relies entirely on green solvents. Furthermore, the effect of sodium selenite concentration, reaction time and temperature on the nanoparticle size, morphology, microstructure and chemical composition was investigated to identify the ideal synthesis conditions. A pilot experiment demonstrated the feasibility of implementing the synthesized nanoparticles into vat photopolymerization three-dimensional-printed hydrogel carriers based on 2-hydroxyethyl methacrylate (HEMA). When submersed into the water, the subsequent particle release was confirmed by dynamic light scattering (DLS), promising great potential for use in bio-three-dimensional printing and other biomedical applications.
- ItemOn energetics of allotrope transformations in transition-metal diborides via plane-by-plane shearing(Elsevier, 2023-06-29) Leiner, Thomas; Koutná, Nikola; Janovec, Jozef; Zelený, Martin; Mayrhofer, Paul; Holec, DavidTransition metal diborides crystallize in the a, y, or w type structure, in which pure transition metal layers alternate with pure boron layers stacked along the hexagonal [0001] axis. Here we view the prototypes as different stackings of the transition metal planes and suppose they can transform from one into another by a displacive transformation. Employing first-principles calculations, we simulate sliding of individual planes in the group IV-VII transition metal diborides along a transformation pathway connecting the a, y, and w structure. Chemistry-related trends are predicted in terms of energetic and structural changes along a transformation pathway, together with the mechanical and dynamical stability of the different stackings. Our results suggest that MnB2 and MoB2 possess the overall lowest sliding barriers among the investigated TMB2s. Furthermore, we discuss trends in strength and ductility indicators, including Young's modulus or Cauchy pressure, derived from elastic constants.
- ItemConstant plane shift model: Structure analysis of martensitic phases in Ni50Mn27Ga22Fe1 beyond non-modulated building blocks(Elsevier, 2023-05-30) Vinogradova, Mariia; Sozinov, Alexei; Straka, Ladislav; Veřtát, Petr; Heczko, Oleg; Zelený, Martin; Chulist, Robert; Lähderanta, Erkki; Ullakko, KariMartensites of Ni-Mn-Ga-based alloys consist of hierarchical twinning domains spanning from micro-to nanoscale. This affects the diffraction pattern and thus can decrease the accuracy of the determination of the crystal structure. We propose a method to obtain different martensitic phases in Ni-Mn-Ga-Fe alloy with simplified variant microstructures and domain sizes of more than 2 micrometers. The use of simplified variant microstructures allows the influence of nanometer-scale domains on diffraction line position to be circumvented and enabls the comparison of the lattice parameters of non-modulated (NM), five-layered modulated (10M), and seven-layered (14M) phases in the same temperature range due to the large hysteresis of the intermartensitic transformations. It is found that the short crystallo-graphic axes in NM, 14M, and 10M martensites at the same temperature have different lengths. As a result, equilibrium NM structure building blocks cannot be used to build the crystal structures of 14M and 10M martensites. Instead, we introduce a constant plane shift model with identical shift values of the nearest planes (110) along [(1) over bar 10] or [1 (1) over bar0] as a replacement for the tetragonal building blocks model. The work demonstrates that plane shift values differ dramatically between mar-tensites, which agrees with ab initio calculations. The application of the constant plane shift and hard sphere models in modulated lattices for atomic-level twinning considerations is discussed.