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- ItemNovel low modulus beta-type TiāNb alloys by gallium and copper minor additions for antibacterial implant applications(Rio de Janeiro : Elsevier, 2022) Alberta, Ludovico Andrea; Vishnu, Jithin; Hariharan, Avinash; Pilz, Stefan; Gebert, Annett; Calin, MarianaThis study aims to develop novel low-modulus, corrosion-resistant Ti-based alloys with enhanced antimicrobial properties for bone-related implant applications. Novel Ī²-type (Tiā45Nb)-based alloys with minor additions of the antibacterial elements Ga and/or Cu (up to 4 wt.%) were produced by a two-step casting process followed by homogenization treatment. Three nominal compositions (Tiā45Nb)96-4 Ga, (Tiā45Nb)96ā4Cu and (Tiā45Nb)96-2 Gaā2Cu (wt.%) were prepared based on alloy design approach using [Mo]eq and electron per atom (e/a) ratio. The influence of Ga and/or Cu addition on the phase constitution, mechanical response and corrosion characteristics in simulated body fluids (PBS, 37.5 Ā°C) has been investigated. X-ray diffraction studies displayed a single Ī² phase structure for all alloys, with an observed lattice contraction evidenced by the reduction of lattice parameters during Rietveld analysis. Homogenous equiaxed microstructures with grain sizes ranging from 55 Ī¼m up to 323 Ī¼m were observed for (Tiā45Nb)96-4 Ga, (Tiā45Nb)96-2 Gaā2Cu and (Tiā45Nb)96ā4Cu alloys. The alloys displayed excellent plasticity with no cracking, or fracturing during compression tests. Their tensile strength, Young's modulus, maximum tensile strain and elastic energy were measured in the ranges of 544ā681 MPa, 73ā78 GPa, 17ā28% and 2.5ā3.7 MJ/m3, in the order (Tiā45Nb)96-4 Ga > (Tiā45Nb)96-2 Gaā2Cu > (Tiā45Nb)96ā4Cu. In addition, it has been observed that micro-alloying TiāNb alloy with Ga and/or Cu posed no deleterious effect on the corrosion resistance in simulated body fluid conditions. The improvement in strength of the developed alloys has been discussed based on grain boundary and solid-solution strengthening, whereas the improved plasticity is attributed to work hardening.
- ItemInfluence of isothermal omega precipitation aging on deformation mechanisms and mechanical properties of a Ī²-type Ti-Nb alloy(Lausanne : Elsevier, 2022) Pilz, S.; Hariharan, A.; GĆ¼nther, F.; Zimmermann, M.; Gebert, A.In this study, the influence of Ļiso precipitates on the active deformation mechanisms and the mechanical properties of the biomedical Ī²-type Ti-40Nb alloy are revealed. Low temperature heat treatments (aging) at 573 K for durations up to 108.0 ks were carried out for a cold-rolled and recrystallized sample state. After an aging time of 3.6 ks the Ļiso phase was determined by means of synchrotron XRD and the fraction and the crystallite size of Ļiso increased progressively with increasing aging time. Due to the high intrinsic Young's modulus of the Ļiso phase, the Young's modulus increased gradually with the aging time from 63 GPa, for the recrystallized reference condition, to values of 70 GPa (3.6 ks), 73 GPa (14.4 ks), 81 GPa (28.8 ks) and 96 GPa (108.0 ks). Depending on the aging time, also a change of the active deformation mechanisms occurred, resulting in significantly altered mechanical properties. For the single Ī²-phase reference microstructure, stress-induced martensite (SIM) formation, {332} <113> twinning and dislocation slip were observed under tensile loading, resulting in a low 0.2% proof stress of around 315 MPa but a high elongation at fracture of 26.2%. With increasing aging time, SIM formation and mechanical twinning are progressively hindered under tensile loading. SIM formation could not be detected for samples aged longer than 3.6 ks. The amount and thickness of deformation twins is clearly reduced with increasing aging time and for samples aged longer than 14.4 ks deformation twinning is completely suppressed. As a result of the changed deformation mechanisms and the increase of the critical stress for slip caused by Ļiso, the 0.2% proof stress of the aged samples increased gradually from 410 MPa (3.6 ks) to around 910 MPa (108.0 ks). With regard to application as new bone implant material, a balanced ratio of a low Young's modulus of E = 73 GPa and higher 0.2% proof stress of 640 MPa was achieved after an aging time of 14.4 ks.
- ItemPolydopamine-Coated Paraffin Microcapsules as a Multifunctional Filler Enhancing Thermal and Mechanical Performance of a Flexible Epoxy Resin(Basel : MDPI, 2020) Fredi, Giulia; Zimmerer, Cordelia; Scheffler, Christina; Pegoretti, AlessandroThis work focuses on flexible epoxy (EP) composites containing various amounts of neat and polydopamine (PDA)-coated paraffin microcapsules as a phase change material (PCM), which have potential applications as adhesives or flexible interfaces with thermal management capability for electronics or other high-value-added fields. After PDA modification, the surface of PDA-coated capsules (MC-PDA) becomes rough with a globular appearance, and the PDA layer enhances the adhesion with the surrounding epoxy matrix, as shown by scanning electron microscopy. PDA deposition parameters have been successfully tuned to obtain a PDA layer with a thickness of 53 Ā± 8 nm, and the total PDA mass in MC-PDA is only 2.2 wt %, considerably lower than previous results. This accounts for the fact that the phase change enthalpy of MC-PDA is only marginally lower than that of neat microcapsules (MC), being 221.1 J/g and 227.7 J/g, respectively. Differential scanning calorimetry shows that the phase change enthalpy of the prepared composites increases with the capsule content (up to 87.8 J/g) and that the enthalpy of the composites containing MC-PDA is comparable to that of the composites with MC. Dynamic mechanical analysis evidences a decreasing step in the storage modulus of all composites at the glass transition of the EP phase, but no additional signals are detected at the PCM melting. PCM addition positively contributes to the storage modulus both at room temperature and above Tg of the EP phase, and this effect is more evident for composites containing MC-PDA. As the capsule content increases, the mechanical properties of the host EP matrix also increase in terms of elastic modulus (up to +195%), tensile strength (up to +42%), Shore D hardness (up to +36%), and creep compliance (down to ā54% at 60 min). These effects are more evident for composites containing MC-PDA due to the enhanced interfacial adhesion.
- ItemAnelastic-like nature of the rejuvenation of metallic glasses by cryogenic thermal cycling(Amsterdam [u.a.] : Elsevier Science, 2022) Costa, Miguel B.; LondoƱo, Juan J.; Blatter, Andreas; Hariharan, Avinash; Gebert, Annett; Carpenter, Michael A.; Greer, A. LindsayCryogenic thermal cycling (CTC) is an effective treatment for improving the room-temperature plasticity and toughness in metallic glasses. Despite considerable attention to characterizing the effects of CTC, they remain poorly understood. A prominent example is that, contrary to expectation, the stored energy in a metallic glass first rises, and then decreases, as CTC progresses. In this work, CTC is applied to bulk metallic glasses based on Pd, Pt, Ti, or Zr. The effects on calorimetric and mechanical properties are evaluated. Critically, CTC-induced effects, at whatever stage, are found to decay over about one week at room temperature after CTC, returning the properties to those of the as-cast glass. A model is proposed for CTC-induced effects, treating them as analogous to the accumulation of anelastic strain. The implications for analysis of existing data, and for future research on CTC effects, are highlighted.