This work reports on the fabrication and characterization of Al2O3-based composites reinforced with NiAl + 30 wt% Al2O3 pre-alloyed powder, synthesized via mechanical alloying. Two composite series were prepared using slip casting: Series I with 2.5 vol.% and Series II with 5.0 vol.% of the reinforcement phase. The samples were sintered at 1450 degrees C in a reducing H2/N2 atmosphere. X-ray diffraction revealed that, in both series, after sintering, no NiAl was observed; only Ni was detected. SEM and EDS analyses showed uniform dispersion of the reinforcement. Mechanical testing revealed that increasing the metallic phase reduced the hardness from 17.01 +/- 0.61 GPa (Series I) to 15.73 +/- 1.51 GPa (Series II), while the fracture toughness slightly improved from 5.14 +/- 0.30 to 5.20 +/- 0.37 MPa & centerdot;m0.5. The compressive strength reached 25 kN and 17 kN for Series I and II, respectively. The results demonstrate that a tailored reinforcement content enables control over the microstructure and mechanical performance in Al2O3-NiAl composites.
Ensuring the reliable operation of structural elements using modern materials places increasingly high demands on the accuracy of assessing their key characteristics. One of the most important of these is the elastic modulus of the material. An analysis of methods for determining this modulus was carried out. It has been shown that the main methods, developed at the beginning of the 19th century, are still used today as basic methods without taking into account the characteristics of modern composite materials. It is also shown that the simplifications previously adopted in determining elastic moduli from tensile experiments are not justified. They lead to fictitious values of elastic moduli even for isotropic materials. Analytical dependencies are proposed for obtaining reliable values for this class of materials, which are well confirmed experimentally. It has been proven that these dependencies are unacceptable for determining the elastic moduli of anisotropic materials without taking into account the characteristics of the latter. The methods for determining the elastic moduli for two types of composite materials are presented: monotropic and orthotropic. These methods provide good agreement with experimental values. It is shown that the methods used to date for determining the elastic moduli of structural materials give significantly overestimated values of the determined characteristics compared to the actual ones. In the paper a proposal for determining a modified Young's modulus is presented, which could be more precise and, thus the spread of experimental data could be reduced.
The innovative solid state step-sintering reaction technique was used to fabricate lanthanum doped bismuth-nickel ferrite perovskite oxide (Bi1.7La0.3Ni2Fe2O8). The physical features, surface shape, grain dispersion topography, spatial frequency spectrum and temperature-frequency dependent dielectric properties were investigated. By means of X-ray diffraction, the W-H approach, SEM morphology, histograms and EDAX color mapping, the attributes of the produced material concerning crystallite size (47.5 nm), strain (0.00152), dislocation density(4.43 & times;1014 m-2), polycrystalline grain (0.01 mu m-2 mu m) orientations, uniformly aligned grain texture, modest isotropy (50.45 %) and elemental compositions were evaluated. Impedance spectroscopy analysis of this material provides insight into the dielectric dispersion. The low dielectric loss in the high frequency regime signifies negligible dielectric energy dissipation. The findings support the idea that Bi1.7La0.3Ni2Fe2O8 might be a promising mixed metal oxide-based ceramic component for potential multifunctional electronic device applications.
The standard specimens used to determine the compressive strength of unidirectional (UD) composites are of a relatively small cross-section. The compressive strength of such materials depends on the volume of the tested material and decreases with its increase. Therefore, the results of tests carried out using such specimens may have limited applicability to the design of actual structures, and larger ones should replace standard-sized specimens. The performance of a hybrid holder designed for non-standard specimens of relatively large cross-sections, intended to determine the compressive strength of UD composites, was investigated numerically. Selected numerical results were compared against preexisting experimental data. The holder consisted of a metal cup filled with resin surrounding a specimen. The results showed that (i) concentrations of longitudinal stress occurred in two locations: at the tip of the specimen touching the bottom of the cup and in the resin meniscus plane, (ii) the proportion between them could be controlled by (a) the presence or absence of adhesion between the resin and specimen, and the resin and cup wall, (b) by friction at the mentioned interfaces, and (c) by the specimen immersion depth. Eliminating adhesion and friction at the specimen/resin/cup interfaces reduced the differences in the values of the stress concentration factor present at the end of the specimen and in the meniscus plane, indicating the possibility of optimizing the holder design.
The growing demands related to sustainable development and the need to reduce the environmental impact of the materials industry are leading to a search for alternatives to classic thermosetting resins. Vitrimers - polymers containing covalent adaptive networks, combine the good mechanical properties and thermal stability of cross-linked materials with the possibility of recycling and reprocessing. This study presents epoxy compositions obtained from epoxidized vegetable oil, an acid and anhydride hardener, as well as a transesterification catalyst. The mechanical properties, thermal stability and recyclability of the vitrimers were evaluated, indicating their potential for use in composite materials in line with the rules of the circular economy.
