Staff

Abstract:
This paper introduces a first-time investigation into the impact of power ultrasound (PUS)-assisted preparation on the physicochemical and mechanical properties of cement-granulated blast furnace slag (GBFS) composite pastes. Pastes containing deposited GFBS with varying particle size fractions, partially replacing Portland cement, were prepared using PUS (ultrasonic horn tip, 20 kHz, 700 W) in pulse mode in a vertical jacketed glass sonoreactor with closed-circuit cooling. Cement paste incorporating 20 wt.% GBFS as mass substitution with varying particle size fractions was characterized by several physicochemical techniques at different curing ages. Exploring the cement and GBFS interaction induced by PUS, the compressive and flexural strength, the elastic modulus and indentation hardness, the heat of hardening, the mineral composition of hydration products, and the specific surface area BET were evaluated for a curing time of up to 28 days. The grain size distribution of GFBS and the reaction mixture's pH were measured. Both mechanical properties, heat of hydration and nanoporosity exhibited strong sensitivity to PUS treatment. Sonofragmentation of GBFS particles (especially the 125–250 μm fraction) increased with increasing sonication time, resulting in a relative increase of fraction <63 μm and a decrease of fraction >125 μm by 275 % and 60 %, respectively. Using the obtained SEM-EDS data, a simplified mechanism is proposed to explain the effects induced by PUS treatment.

Keywords:
Power ultrasound treatment, Portland cement, Granulated blast furnace slag, Early strength development, C-S-H/C-A-S-H, Seeding effect

Abstract:
Functionally graded metal matrix composites have attracted the attention of various industries as materials with tailorable properties due to spatially varying composition of constituents. This research work was inspired by an application, such as automotive brake disks, which requires advanced materials with improved wear resistance on the outer surface as combined with effective heat flux dissipation of the graded system. To this end, graded AlSi12/Al2O3 composites (FGMs) with a stepwise gradient in the volume fraction of alumina reinforcement were produced by hot pressing and spark plasma sintering techniques. The thermal conductivities of the individual composite layers and the FGMs were evaluated experimentally and simulated numerically using 3D finite element (FE) models based on micro-computed X-ray tomography (micro-XCT) images of actual AlSi12/Al2O3 microstructures. The numerical models incorporated the effects of porosity of the fabricated AlSi12/Al2O3 composites, thermal resistance, and imperfect interfaces between the AlSi12 matrix and the alumina particles. The obtained experimental data and the results of the numerical models are in good agreement, the relative error being in the range of 4 to 6 pct for different compositions and FGMstructure. The predictive capability of the proposed micro-XCT-based FE model suggests that this model can be applied to similar types of composites and different composition gradients.

Abstract:
This study investigates the effect of Nb concentration on the mechanical properties, superelasticity, as well as deformation behavior of metastable β-phase Ti–Nb alloys produced via powder metallurgy. The alloys were fabricated through mechanical alloying, followed by consolidation using hot pressing. The resulting microstructure comprises fine β-phase grains with TiC carbide precipitates at the grain boundaries. The study reveals non-linear variations in the values of yield strength for the manufactured materials, which were attributed to the occurrence of various deformation mechanisms activated during the loading. It was found that the mechanisms change with the increasing concentration of Nb in the manner: stress-induced martensitic transformation, twinning, slip. However, all these mechanisms were activated at a reduced concentration of Nb compared to the materials obtained by casting technology previously reported in the literature. This is most probably associated with the elevated oxygen content, which affects the stability of the parent β-phase. The study revealed that superelasticity in Ti–Nb-based alloys prepared using powder metallurgy may be achieved by reducing the content of β-stabilizing elements compared to alloys obtained by conventional technologies. In this study, the Ti–14Nb (at. pct) alloy exhibited the best superelasticity, whereas conventionally fabricated Ti–Nb alloys displayed superelasticity at an Nb concentration of approximately 26 at. pct. The developed material exhibited a non-conventional, one-stage yielding behavior, resulting in a superelastic response at significantly higher stresses compared to conventionally fabricated Ti–Nb alloys.

Abstract:
Ionogels are an attractive class of materials for smart and flexible electronics and are prepared from the combination of a polymer and ionic liquid which is entrapped in this matrix. Ionogels provide a continuous conductive phase with high thermal, mechanical, and chemical stability. However, because of the higher percentage of ionic liquids it is difficult to obtain an ionogel with high ionic conductivity and mechanical stability, which are very important from an application point of view. In this work, ionogel films with high flexibility, excellent ionic conductivity, and exceptional stability were prepared using polyvinyl alcohol as the host polymer matrix and 1-ethyl-3-methylimidazolium hydrogen sulfate as the ionic liquid using water as the solvent for energy storage application. The prepared ionogel films exhibited good mechanical stability along with sustaining strain of more than 100% at room temperature and low temperature, the ability to withstand twisting up to 360° and different bending conditions, and excellent ionic conductivity of 5.12 × 10−3 S/cm. The supercapacitor cell fabricated using the optimized ionogel film showed a capacitance of 39.9 F/g with an energy and power densities of 5.5 Wh/kg and 0.3 kW/kg, respectively confirming the suitability of ionogels for supercapacitor application.

Keywords:
Ionic liquid, Gel polymer electrolyte, Ionic conductivity, 1-Ethyl-3-methylimidazolium hydrogen sulfate, Supercapacitors

Abstract:
Metal-ceramic composites by their nature have thermal residual stresses at the micro-level, which can compromise the integrity of structural elements made from these materials. The evaluation of thermal residual stresses is therefore of continuing research interest both experimentally and by modeling. In this study, two functionally graded aluminum alloy matrix composites, AlSi12/Al2O3 and AlSi12/SiC, each consisting of three composite layers with a stepwise gradient of ceramic content (10, 20, 30 vol%), were produced by powder metallurgy. Thermal residual stresses in the AlSi12 matrix and the ceramic reinforcement of the ungraded and graded composites were measured by neutron diffraction. Based on the X-ray micro-computed tomography (micro-XCT) images of the actual microstructure, a series of finite element models were developed to simulate the thermal residual stresses in the AlSi12 matrix and the reinforcing ceramics Al2O3 and SiC. The accuracy of the numerical predictions is high for all cases considered, with a difference of less than 5 % from the neutron diffraction measurements. It is shown numerically and validated by neutron diffraction data that the average residual stresses in the graded AlSi12/Al2O3 and AlSi12/SiC composites are lower than in the corresponding ungraded composites, which may be advantageous for engineering applications.

Keywords:
Finite element modeling,Micro-XCT,Thermal residual stress,Hot pressing,Aluminum matrix composites

Abstract:
Functional antibacterial textiles fabricated from a hybrid of organic waste-derived and bio-inspired materials offer sustainable solutions for preventing microbial infections. In this work, we developed a novel antibacterial textile created through the valorization of spent coffee grounds (SCG). Electrospinning and electrospraying techniques were employed to integrate the biowaste within a polymeric nanofiber matrix, ensuring uniform particle distribution and providing structural support for enhanced applicability. Modification with polydopamine (PDA) significantly enhanced the textile's photothermal performance. Specific attention was paid to understanding the relation between temperature change and key variables, including the surrounding liquid volume, textile layer stacking, and applied laser power. Developed platforms demonstrated excellent photothermal stability. While the SCG-based textile demonstrated exceptional biocompatibility, the PDA-modified textile effectively eradicated Staphylococcus aureus (S. aureus) under near-infrared (NIR) irradiation. The developed textiles in our work demonstrate a dynamic balance between biocompatibility and on-demand antibacterial functionality, offering adaptable solutions in accordance with the desired application.

Keywords:
Organic waste valorization, Spent coffee grounds, Micro-nanostructured textiles, Bio-inspired photothermal agents, Polydopamine, Antibacterial textiles

Abstract:
The low fracture toughness of NiAl at room temperature is one of the critical issues limiting its application in aircraft engines. It has been previously shown that a small addition of rhenium and alumina significantly improves the fracture toughness of hot-pressed NiAl. In this work, NiAl with an admixture of rhenium and alumina was produced by laser powder bed fusion additive technology (LPBF). The purpose was to compare the fracture toughness, bending strength, and microhardness of the NiAl-Re-Al2O3 material produced by LPBF and hot pressing (HP). Our results show that the LPBF material has lower fracture toughness and bending strength compared to its hot-pressed equivalent. Microcracks generated by thermal stresses during the LPBF process were the primary cause of this behavior. To improve the LPBF material, a post-processing by HP was applied. However, the fracture toughness of the (LPBF + HP) material remained at 50% of the KIC of the HP material. This study supports hot pressing as a suitable processing method for NiAl with rhenium and alumina additions. However, a hybrid approach combining LPBF and HP proved to be highly effective on the raw NiAl powder, resulting in superior fracture toughness of the final material compared to that consolidated by singular HP.

Keywords:
NiAl intermetallic,additive manufacturing ,hot pressing ,mechanical properties

Abstract:
This study presents a novel, straightforward approach for synthesizing graphitic carbon nitrides (g-C3N4, g-CN) from melamine, requiring merely 30 min of thermal holding at temperatures ranging from 400 to 550 °C in an atmosphere comprising either nitrogen or air. Elemental analysis, X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), diffuse reflectance (UV–Vis DRS), photoluminescence (PL) spectroscopy and scanning electron microscopy (SEM) were employed to assess the quality of the as-prepared powders. Furthermore, theoretical calculations utilizing the Density Functional Theory (DFT) method were conducted to reinforce the experimental findings of the research. A further investigation of the thermal stability of the selected sample was conducted using a unique combination of thermogravimetry–coupled with differential scanning calorimetry, quadrupole mass spectrometry (TG-DSC-MS) and advanced temperature-programmed desorption (TPD) analyzes. The current study focuses on the effect of synthesis conditions (temperature and nitrogen/air environment) on the structure, morphology, and photocatalytic performance of g-C3N4 compounds synthesized using this approach. The g-C3N4-based materials were examined as potential photocatalysts using the acid orange 7 (AO7) photodegradation methodology. To enable comparison of the photodegradation experiments, two separate lamps with wavelengths of 360 nm (UV light) and 420 nm (VIS light) were utilized. The primary objective was to present a novel method for the synthesis of g-C3N4-based materials. This was achieved by demonstrating that organic composites generated at lower temperatures have the best photocatalytic capabilities. Furthermore, the approach to achieving high-quality photocatalysts was shown to be cost-effective, environmentally friendly, and scalable.

Abstract:
Atomic force microscopes could be used in wide range of nanotribology experiments but probes available on the market are only made of silicon or silicon nitride with a stiffness in the range of 0.01–100 N/m, which significantly limits the possible research. We strive to solve this problem by designing all-metal probes. The proposed fabrication method is characterised by the use of a copper substrate and electrodeposition in a mould prepared by indentation and photolithography. Prototype probes fabricated with this method were made of nickel with a stiffness of 20 N/m and 2800 N/m and were used for topography and friction measurements. Both the method and all-metal probes showed flexibility and great potential, especially in the field of nano/microtribology.

Keywords:
Atomic force microscopy, Microfabrication, Cantilever, Metal probe, Friction, Nanotribology

Abstract:
The development of powder metallurgy methods in recent years has caused traditional casting methods to be replaced in many industrial applications. Using such methods, it is possible to obtain parts having the required geometry after a process that saves both manufacturing costs and time. However, there are many material issues that decrease the functionality of these methods, including mechanical properties anisotropy and greater susceptibility to cracking due to chemical segregation. The main aim of the current article is to analyze these issues in depth for two powder metallurgy manufacturing processes: laser powder bed fusion (LPBF) and hot-pressing (HP) methods—selected for the experiment because they are in widespread use. Microstructure and mechanical tests were performed in the main manufacturing directions, X and Z. The results show that in both powder metallurgy methods, anisotropy was an issue, although it seems that the problem was more significant for the samples produced via LPBF SLM technique, which displayed only half the elongation in the building direction (18%) compared with the perpendicular direction (almost 38%). However, it should be noted that the fracture toughness of LPBF shows high values in the main directions, higher even than those of the HP and wrought samples. Additionally, the highest level of homogeneity even in comparison with wrought sample, was observed for the HP sintered samples with equiaxed grains with visible twin boundaries. The tensile properties, mainly strength and elongation, were the highest for HP material. Overall, from a practical standpoint, the results showed that HP sintering is the best method in terms of homogeneity based on microstructural and mechanical properties.

Keywords:
Haynes 282 nickel alloy, LPBF, HP, SEM , Static tensile test

Abstract:
The sustainable transformation of bio-waste into usable, material has gained great scientific interest. In this paper, we have presented preparation of an activated carbon material from a natural mushroom (Suillus boletus) and explor its properties for supercapacitor and dye adsorption applications. The produced cell exhibited a single electrode capacitance of ∼247 F g−1 with the energy and power density of ∼35 Wh kg−1 and 1.3 kW kg−1, respectively. The cell worked well for ∼20,000 cycles with ∼30% initial declination in capacitance. Three cells connected in series glowed a 2.0 V LED for ∼1.5 min. Moreover, ultrafast adsorption of methylene blue dye onto the prepared carbon as an adsorbent was recorded with ∼100% removal efficiency in an equilibrium time of three minutes. The performed tests indicate that the mushroom-derived activated carbon has the potential to become a high-performance electrode material for supercapacitors and an adsorbent for real-time wastewater treatment applications.

Keywords:
Activated carbon, Amorphous material, Biomass, Polymer gel electrolyte, Supercapacitor, Dye adsorption

Abstract:
Utilization of CoO@Co3O4-x-Ag (x denotes 1, 3, and 5 wt% of Ag) nanocomposites as supercapacitor electrodes is the main aim of this study. A new low-temperature wet chemical approach is proposed to modify the commercial cobalt oxide material with silver nanoparticle (NP) balls of size 1–5 nm. The structure and morphology of the as-prepared nanocomposites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption–desorption measurements. Hydrogels known to be soft but stable structures were used here as perfect carriers for conductive nanoparticles such as carbons. Furthermore, hydrogels with a large amount of water in their network can give more flexibility to the system. Fabrication of an electrochemical cell can be achieved by combining these materials with a layer-by-layer structure. The performance characteristics of the cells were examined by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge discharge (GCD). Cobalt oxide modified with 5 wt% Ag gave the best supercapacitor results, and the cell offers a specific capacitance of ∼38 mF cm−2 in two-electrode configurations.

Abstract:
This work presents a growth of Fe3O4 nanoparticle films on silicon substrate. The iron oxide is deposited applying a pulsed laser deposition technique. The process is performed in open air in the absence and presence of external magnetic field. In fact, the morphologies of the obtained Fe3O4–Si samples are similar. The Fe3O4 nanoparticles are spherical with average diameters of 30 nm and are densely agglomerated on the Si substrate. The Fe3O4–Si material prepared in the absence of magnetic field has revealed more intense signals during X-ray diffraction and Raman measurements. The magnetic investigations indicate that the Fe3O4 nanoparticles are significantly coupled with the Si substrate and do not exhibit superparamagnetic behavior. Moreover, the Verwey transition is 98 K for both investigated Fe3O4–Si samples.

Keywords:
Fe3O4 nanoparticles,magnetic materials,pulsed laser deposition

Abstract:
A novel approach is proposed to determine a physically meaningful length-scale parameter for the phase-field modeling of macroscale fracture in metal–ceramic composites on an example of chromium–alumina composite fabricated by powder metallurgy. The approach is based on the fractography analysis by the scanning electron microscopy (SEM) with the aim to measure the process zone size and use that value as the length-scale parameter in the phase-field modeling. Mode I and mixed-mode I/II fracture tests are conducted on Cr–Al2O3 composites at different reinforcement volume fractions and particle sizes using single-edge notched beams under four-point bending. The fracture surfaces are analyzed in detail by SEM to determine the size of the process zone where the microscale nonlinear fracture events occur. The model adequately approximates the experimentally measured fracture toughness and the fracture loads. It is shown that the model prediction of the crack initiation direction under the mixed-mode loading is in agreement with the experiments and the generalized maximum tangential stress criterion. These outcomes justify using the process zone size as the scale parameter in the phase-field modeling of macroscale fracture in chromium–alumina and similar metal–ceramic composites.

Abstract:
Metallic coatings are often applied on steel tubes in power generation boilers to improve their performance and extend the lifetime. Besides the high-temperature corrosion and erosion protection, the coatings should manifest good adhesion and cohesion strength, and relatively low residual stresses. In this study, three processing techniques: high velocity oxygen fuel spraying (HVOF), atmospheric plasma spraying (APS), and direct laser cladding were employed to obtain novel NiCr-Re and NiCr-Re-Al2O3 coatings intended for application in combustion boilers. The main objective was to assess the suitability of these three techniques to deposit NiCr-Re and NiCr-Re-Al2O3 composite coatings on a 16Mo3 steel substrate. For this purpose, a comparative analysis of the coatings behavior in selected tests was conducted. Of the three processing techniques, thermal spraying by HVOF turned out to be the optimum choice for the studied coatings. From among several variants of the HVOF-sprayed coatings, namely NiCr+1%Re, NiCr+2%Re, NiCr+1%Re+5%Al2O3, and NiCr+1%Re+10%Al2O3 (all vol.%), the NiCr+1%Re material exhibited the lowest extent of cracking in the disk bend test, the highest tensile strength (405 MPa) in the in-plane tension test, the highest Vickers hardness (379 HV2), the lowest specific wear rate (2.23·10-4 mm3/N m), and the lowest level of average residual tensile stress (120 MPa).

Keywords:
alumina, HVOF, mechanical behavior, nickel-chromium coatings, rhenium, residual stress, wear resistance

Abstract:
Squeeze casting and powder metallurgy techniques were employed to fabricate AlSi12/Al2O3 composites, which are lightweight structural materials with potential applications in the automotive industry. The impact of the processing route on the material properties was studied. Comparative analyses were conducted for the Vickers hardness, flexural strength, fracture toughness, thermal conductivity, thermal residual stresses, and frictional wear. Our results show that the squeeze cast composite exhibits superior properties to those obtained using powder metallurgy.

Abstract:
This work presents the evolution of morphological, structural and magnetic properties of amorphous Fe-Ni wire-like nanochains caused by thermal oxidation. The initial Fe1−xNix samples (x = 0.75; 0.50; 0.25) were prepared through the magnetic-field-induced synthesis, and then they were heated in dry air at 400 °C and 500 °C. These treatments led to two competing simultaneous processes occurring in the investigated samples, i.e., (i) a conversion of amorphous material into crystalline material, and (ii) their oxidation. Both of them strictly affected the morphological and structural properties of the Fe-Ni nanochains which, in turn, were associated with the amount of iron in material. It was found that the Fe0.75Ni0.25 and Fe0.50Ni0.50 nanochains were covered during thermal treatment by the nanoparticle oxides. This coverage did not constitute a good barrier against oxidation, and these samples became more oxidized than the Fe0.25Ni0.75 sample which was covered by oxide nanosheets and contained additional Ni3B phase. The specific morphological evolutions of the Fe-Ni nanochains also influenced their saturation magnetizations, whereas their coercivities did not vary significantly. The obtained results constitute an important source of information for future application of the thermally treated Fe-Ni nanochains which could be applied in the energy storage devices or catalysis.

Abstract:
Thermal treatment is a post-synthesis treatment that aims to improve the crystallinity and interrelated physical properties of as-prepared materials. This process may also cause some unwanted changes in materials like their oxidation or contamination. In this work, we present the post-synthesis annealing treatments of the amorphous Fe1−xCox (x = 0.25; 0.50; 0.75) Wire-like nanochains performed at 400 °C in two different atmospheres, i.e., a mixture of 80% nitrogen and 20% hydrogen and argon. These processes caused significantly different changes of structural and magnetic properties of the initially-formed Fe-Co nanostructures. All of them crystallized and their cores were composed of body-centered cubic Fe-Co phase, whereas their oxide shells comprised of a mixture of CoFe2O4 and Fe3O4 phases. However, the annealing carried out in hydrogen-containing atmosphere caused a decomposition of the initial oxide shell layer, whereas a similar process in argon led to its slight thickening. Moreover, it was found that the cores of thermally-treated Fe0.25Co0.75 nanochains contained the hexagonal closest packed (hcp) Co phase and were covered by the nanosheet-like shell layer in the case of annealing performed in argon. Considering the evolution of magnetic properties induced by structural changes, it was observed that the coercivities of annealed Fe-Co nanochains increased in comparison with their non-annealed counterparts. The saturation magnetization (MS) of the Fe0.25Co0.75 nanomaterial annealed in both atmospheres was higher than that for the non-annealed sample. In turn, the MS of the Fe0.75Co0.25 and Fe0.50Co0.50 nanochains annealed in argon were lower than those recorded for non-annealed samples due to their partial oxidation during thermal processing.

Keywords:
annealing, amorphous materials, Fe-Co nanochains, magnetic-field-induced synthesis, wire-like nanostructure

Abstract:
This work presents a combined experimental and computational study of the deformation and fracture of microcantilever specimens made of chromium(rhenium)-alumina metal–matrix composite (MMC), with a particular focus on the failure properties of the metal–ceramic interfaces. The obtained experimental results show that the bending strength of microcantilevers containing alumina particles in critical cross-sections near specimen's fixed end is considerably higher than that of unreinforced chromium(rhenium) samples. Brittle cracking along chromium–alumina interfaces is the dominant fracture mode of the composite microcantilevers. The interface characteristics are determined in an indirect way by numerical simulations of the experiment with account of the actual specimen microstructure from the scanning electron microscope (SEM) images. A parametric study demonstrates that the overall material response may be reproduced by different sets of model parameters, whereas the actual failure mode permits to discriminate among the possible alternatives. Using this approach, the in situ values of the chromium–alumina interface cohesive strength and the fracture energy are estimated.

Abstract:
In the search for a remedy to increase the fracture toughness of NiAl, the effect of rhenium and aluminum oxide addition is explored. Using a powder metallurgy processing route an optimum composition of NiAl-Re-Al2O3 material is found which manifests KIC over two times higher than as-received NiAl sintered under the same conditions.

Keywords:
fracture behavior, stress/strain measurements, intermetallics, composites, powder metallurgy, grains and interfaces

Abstract:
This paper explores an anomalous size effect in thermal residual stresses occurring in the alumina matrix of Al2O3/Cr sintered composite upon varying the particle size of the chromium reinforcement. When a coarse chromium powder (45 µm mean particle size) is used the average residual stress in the alumina phase after cooling is compressive in accordance with the classical Eshelby solution. However, in the case of a fine chromium (5 µm mean particle size) it switches to tension. This effect, detected by photoluminescence piezospectroscopy, is also confirmed by X-ray and neutron diffraction experiments. As the classical micromechanics models are incapable to capture it, a finite element model is developed with the actual composite microstructure being reconstructed from the microtomography images. It is shown by numerical simulations that the anomalous size effect is associated with the complex microstructure of the composite fabricated with the fine chromium powder. It is also pointed out that the temperature dependence of the coefficients of thermal expansion of the matrix and the reinforcement affects the residual stress levels.

Keywords:
thermal residual stress, metal-ceramic composites, size effect, microcomputed tomography, finite element analysis

Abstract:
This work describes optical measurements of processing-induced thermal residual stresses in an alumina matrix reinforced with chromium particles. This ceramic-metal composite is manufactured by the powder metallurgy method comprising powder mixing in a planetary ball mill and consolidation by hot pressing. Two different chromium powders (5 μm and 45 μm mean particle size) are used, while the average alumina particle size is kept constant (1 μm). The residual stresses in aluminum oxide are determined by applying two optical methods: photoluminescence piezo-spectroscopy (PLPS) and Raman spectroscopy (RS). Both experimental techniques reveal a chromium size effect in the residual stress measurements. When the fine chromium powder (5 μm) is used, the average residual stress in the ceramic phase is tensile (unusual effect), whereas for the coarser chromium powder (45 μm) it becomes compressive. The PLPS measurements of the hydrostatic residual stress component in the ceramic phase yield the values of 0.290 and −0.130 GPa for samples with 5 μm and 45 μm chromium powders, respectively. In the RS experiments, the corresponding stress component in the alumina equals 0.351 GPa for the composite with 5 μm chromium and −0.158 GPa for that with 45 μm chromium powder. These values indicate that the residual stress in the alumina reinforced with 5 μm chromium is approximately twice higher than that in the alumina reinforced with 45 μm chromium. Finally, the validity of the results obtained with the optical techniques is confirmed by the neutron diffraction measurements.

Abstract:
The search for new materials capable of replacing nickel-based superalloys in aerospace applications has increased rapidly in the recent years. One of the candidates for this purpose is nickel aluminide NiAl provided that its main drawback, namely the inferior fracture toughness at room temperature is overcome. We propose rhenium as an addition to NiAl to improve its mechanical properties without compromising on the oxidation resistance. Two powder metallurgy techniques (HP and SPS) were used to obtain NiAl/Re sinters. Small amounts of rhenium (0.6 at.%; 1.25at.%; 1.5 at.%) almost doubled the flexural strength of NiAl and improved its fracture toughness by 60%. Microscopic investigations revealed rhenium particles at the boundaries of NiAl grains resulting in an enhanced fracture toughness. Mass changes during oxidation at 900 oC, 1100 oC and 1300 oC were relatively low. Plausible mechanisms of the fracture toughness enhancement and the oxidation behavior are discussed.

Keywords:
nickel aluminide, rhenium, fracture toughness, oxidation resistance, powder metallurgy, grain boundary strengthening

Abstract:
Aluminum-alumina composites with interpenetrating network structure are interesting structural materials due to their high resistance to elevated temperature and frictional wear, good heat conductivity, enhanced mechanical strength and fracture toughness. In this paper aluminum-alumina bulk composites and FGMs are manufactured by pressure infiltration of porous alumina preforms with molten aluminium alloy (EN AC-44200). Influence of the interpenetrating microstructure on the macroscopic bending strength, fracture toughness, hardness and heat conduction is examined. Special focus is on processing-induced thermal residual stresses in aluminium-alumina composites due to their potentially detrimental effects on material performance in structural elements under in-service conditions. The residual stresses are measured experimentally in the ceramic phase by neutron diffraction and simulated numerically using a micro-CT based Finite Element model, which takes into account the actual interpenetrating microstructure of the composite. The model predictions for two different volume fractions of alumina agree fairly well with the neutron diffraction measurements

Keywords:
A. stress measurements, X-ray analysis, finite element analysis, B. composites, C. casting methods

Abstract:
This paper presents a simple way of using X-ray micro-computed tomography (micro-CT) in numerical modeling of material properties of metal-ceramic composites. It shows step by step the proposed methodology with details of the finite element mesh creation, so that it can easily be reproduced by interested researchers. Two case studies are considered to show the proposed approach at work: i) determination of processing-induced residual stresses in hot pressed Cr/Al2O3 and NiAl/Al2O3 particulate composites and ii) determination of J-integral for an interpenetrating phase composite made of porous alumina preform infiltrated with molten copper. The method is straightforward and effective but has its limitations that are pointed out.

Abstract:
W niniejszym artykule zaprezentowano wyniki modelowania zagęszczania proszku stanowiącego wstępny etap procesu prasowania na gorąco. Modelowanie numeryczne zrealizowano metodą elementów dyskretnych z wykorzystaniem kulistych cząstek. Analizę skoncentrowano na badaniu mechanizmów zagęszczania proszku przy ciśnieniu do 50 MPa oraz poszukiwaniu modeli odpowiednich przy zastosowanych warunkach realizacji procesu. Symulacje numeryczne wykonano wykorzystując dwa modele oddziaływania cząstek proszku: sprężysty model Hertza-Mindlina-Deresiewicza oraz plastyczny model Storåkersa. Wyniki numeryczne zostały porównane z wynikami laboratoryjnymi prasowania proszku NiAl. Otrzymano dużą zgodność wyników eksperymentalnych i numerycznych.

This paper presents the results of discrete element simulation of powder compaction which is the initial stage in the hot pressing process. Numerical simulation has been performed by discrete element method with using spherical particles. The research has been focused on densification mechanisms under pressure 50 MPa and models appropriate for these conditions. Numerical simulations have been carried out for two contact models: elastic Hertz-Mindlin-Deresiewicz and plastic - Storåkers. Numerical results and results from laboratory test of the uniaxial pressing of NiAl powder have been compared. The obtained results of numerical simulation and laboratory tests showing a good agreement.

Keywords:
metoda elementów dyskretnych, prasowanie proszków, materiały intermetaliczne, discrete element method, powder compaction, intermetallics

Abstract:
This paper presents numerical studies of powder compaction in cold uniaxial pressing. The powder compaction in this work is considered as an initial stage of a hot pressing process so it is realized with relatively low pressure (up to 50 MPa). Hence the attention has been focused on the densification mechanisms at this range of pressure and models suitable for these conditions. The discrete element method employing spherical particles has been used in the numerical studies. Numerical simulations have been performed for two different contact models—the elastic Hertz–Mindlin–Deresiewicz model and the plastic Storåkers model. Numerical results have been compared with the results of laboratory tests of the die compaction of the NiAl powder. Comparisons have shown that the discrete element method is capable to represent properly the densification mechanisms by the particle rearrangement and particle deformation.

Keywords:
Discrete element method, Simulation, Powder compaction, Cold uniaxial pressing

Abstract:
Over the last few decades intermetallic compounds such as NiAl have been considered as potential high temperature structural materials for aerospace industry. A large number of investigations have been reported describing complex fabrication routes, introducing various reinforcing/alloying elements along with theoretical analyses. These research works were mainly focused on the overcoming of main disadvantage of nickel aluminides that still restricts their application range, i.e. brittleness at room temperature. In this paper we present an overview of research on NiAl processing and indicate methods that are promising in solving the low fracture toughness issue at room temperature. Other material properties relevant for high temperature applications are also addressed. The analysis is primarily done from the perspective of NiAl application in aero engines in temperature regimes from room up to the operating temperature (over 1150 °C) of turbine blades.

Keywords:
Nickel aluminide, Intermetallics, Turbine blades, Fracture toughness, Manufacturing techniques

Abstract:
It is commonly known that the available non-destructive and mechanical methods of the Young modulus measurement yield different results. This paper presents comparison of the results of experimental determination and numerical modeling of the Young modulus of Cr–Al2O3–Re composites (MMC) processed by a powder metallurgical method (SPS). In the computational model a finite element analysis is combined with images of the real material microstructure obtained from micro-computed tomography (micro-CT). Experimental measurements were carried out by four testing methods: three-point bending, resonance frequency damping analysis (RFDA), ultrasonic pulse-echo technique, and scanning acoustic microscopy. The paper also addresses the issue which of the four experimental methods at hand gives results closest to the theoretical predictions of the micro-CT based FEM model.

Keywords:
Finite element analysis (FEA), Micro-CT based FE model, Metal matrix composites, Elastic modulus, Mechanical and nondestructive techniques

Keywords:
anning electron microscopy, in-situ mechanical tests, hot-pressing

Abstract:
Lightweight materials with high wear resistance, good thermal conductivity and enhanced mechanical properties are desired for modern brake discs in the automotive industry. One way to achieve this target is to use functionally graded metal-ceramic composite materials (FGMs). Besides improving the main properties, an FGM must ensure proper thermal conductivity of the system to release heat generated during brake operation and keep the residual stresses at acceptable levels.
Two ceramic materials, SiC and Al2O3, were used as the reinforcing phase of the AlSi12 matrix composites fabricated by powder metallurgy with a stepwise composition gradient (layered composites). The hot-pressing technique was employed to consolidate the powder mixtures with the volume fraction of the ceramics phase ranging from 10 to 30%. High relative density of the composite layers (above 99%) was obtained. Fracture toughness and flexural strength in a four-point bending ranged from 8.7 to 12.94 MPa√m and from 412 to 717 MPa, respectively. In-situ tensile tests under SEM allowed to analyze deformation and crack growth mechanisms on the microscale. Wear tests evidenced high wear resistance of the manufactured materials as compared with the reference material (grey cast iron). Results of the neutron diffraction experiments showed a desired effect of the FGM structure on decreasing the processing-induced residual stresses.
In parallel, FEM simulations based on the actual material microstructure reconstructed from micro-CT images were performed for thermal conductivity and thermal residual stresses to optimize the FGM structure and to answer the question which reinforcement (SiC or Al2O3) better serves the intended application.

Abstract:
The corrosion and erosion processes are detrimental to the lifetime and maintenance costs of steel combustion boilers in the energy sector. One of the remedies for this problem can be novel NiCr based coatings applied on structural elements, which are exposed to aggressive agents. NiCr alloys are known for their resistance to chemical and physical degradation in high temperature. Introducing a small admixture of rhenium and alumina ceramic to NiCr results in further increase of mechanical and wear properties of the coating. Three different deposition techniques were employed (HVOF, laser cladding and plasma spraying) to manufacture the coatings. The primary target of this research was to identify by experiments and numerical simulations the most promising deposition technique for the industrial application of the investigated coatings. Thermal residual stress measurements by XRD have shown that the lowest stresses occurred in the HVOF-deposited coating. The micro-CT based numerical simulations have confirmed this finding. The highest hardness was also manifested by the coating deposited by HVOF. The admixture of 10% vol. of Al2O3has improved the coating wear resistance. Remelting of powders during the laser cladding and low wettability of alumina were the main reasons of the unwished migration of ceramic particles towards the coating surface. A non-standard adhesion test was used to examine the coatings integrity and adhesion to the substrate. The obtained results point to the HVOF technique as the most promising of the three techniques considered. Moreover, HVOF can be easily implemented for complex shapes of the components.

Abstract:
In this paper a numerical model will be presented to investigate the influence of processing-induced thermal residual stresses (TRS) on the fracture (fracture toughness) and mechanical properties (E modulus, bending strength) in particulate bulk metal-ceramic composites. The materials under consideration are hot pressed chromium-alumina bulk composites with different content of alumina (30, 60 and 90 vol. %) and with two different starting sizes of chromium particles to show how the microstructure can influence on the level of TRS.
The reported research includes the processing of composites by powder metallurgy techniques (HP), microscopic analysis of material microstructure with special focus on micro-CT scanning, measurements of TRS by neutron diffraction (ND) method and numerical modelling of TRS by FEM based on micro-CT images of real material microstructure [1-3].
Spatial distributions of TRS measured by ND are considered when interpreting the results of KIC measurements in a four point bending test. Numerical micro-CT based models are proposed to predict the TRS, Young’s modulus and bending strength with account of the TRS-induced damage of the ceramic phase. Our micro-CT based FEM models reproduce the TRS measurements with a good accuracy which may be an asset in applications having in mind the high cost of beam time for ND experiments at neutron sources. Finally, the experimental data and modelling results are compared to assess the TRS/microstructure effect on the fracture toughness of the composites investigated.

[1] W. Weglewski, M. Basista, M. Chmielewski, K. Pietrzak Modeling of thermally induced damage in the processing of Cr–Al2O3 composites. Compos. Part B (2012) 255–264.
[2] W. Weglewski, K. Bochenek, M. Basista, T. Schubert, U. Jehring, J. Litniewski, S. Mackiewicz, Comparative assessment of Young's modulus measurements of metal–ceramic composites using mechanical and non-destructive tests andmicro-CT based computational modeling, Comput. Mater. Sci. 77 (2013) 19–30.
[3] W.Węglewski, M. Basista, A. Manescu, M. Chmielewski, K. Pietrzak, T. Schubert, Effect of grain size on thermal residual stresses and damage in sintered chromium–alumina composites: measurement and modelling, Compos. Part B 67 (2014) 119–124.

Keywords:
processing of metal-matrix composites, ceramics, thermal residual stresses, neutron diffraction, microCT FEM

Abstract:
In this paper the influence of material microstructure and thermal residual stresses on the macroscopic fracture toughness, Young’s modulus and bending strength of metal-ceramic composites is studied.
The investigated materials were: (1) Cr/Al2O3 composites (MMC and cermets) with various proportions of the starting powders prepared by hot pressing, and (2) Al2O3/Al infiltrated composites with different volume fractions of the aluminium phase. The two groups of composites (particulate vs. infiltrated) were chosen to examine the effect in question because of their significantly different microstructure.
In the case of hot pressed Cr/Al2O3 composites local thermal residual stresses are generated during cooling from the sintering temperature to RT due to number of factors such as (i) differences in the coefficients of thermal expansion of the ceramic and metal phase, (ii) differences in cooling speeds in different parts of the material, and (iii) irregular shapes of pores causing stress concentrations.
The same problem of formation of thermal residual stresses occurs in the infiltrated Al2O3/Al composite with metal and ceramic phases forming spatially continuous networks throughout the structure (also called Interpenetrating Phase Composites, IPCs).
The fracture toughness and bending strength measurements were performed in a four-point bend test on SEVNB specimens. The microstructural characterization and crack growth analysis were done using scanning electron microscopy.
Our results show that the fracture toughness and other mechanical properties investigated in this study strongly depend on such microstructural features like the amount and distribution of metal and ceramic phase and the type of microstructure (particulate vs. infiltrated). On the other hand the stiffness of reinforcement and matrix, the volume fraction and the grain size of the reinforcement, difference in grain sizes between matrix and reinforcement have an effect on thermal residual stresses distribution, which in turn have an effect on the macroscopic fracture parameters and the crack growth path.

Keywords:
thermal residual stresses, mechanical properties, powder metallurgy, interpenetrating phase composites

Abstract:
Intermetallic compounds such as NiAl manifest an attractive combination of mechanical and physical properties– low dens ity (5.9g/cm 3 ), high melting point (1676 o C), high thermal stability along with good oxidation and corrosion resistance. This has resulted in their numerous non-structural applications such as thermal barrier coatings, but no successful structural application of NiAl has been reported yet. This is caused by its low ductility and poor fracture toughness (<5MPa√m ) at room temperature along with an insufficient impact resistance. There has been a lot of work done already in order to improve NiAl properties and implement this material in aeroengines. The results are very promising, but till now there has been no reported successful application of NiAl - based bulk materials in real in - service conditions [1 - 2 ].

Keywords:
NiAl intermetallics, microstructure, flexural strength, fracture toughness, oxidation resistance, aeroengines

Abstract:
In this paper a numerical model will be presented to investigate the influence of processing-induced thermal residual stresses (TRS) on the mechanical properties (E modulus, bending strength) in particulate bulk intermetallic-ceramic composites. The materials under consideration are hot pressed NiAl/20%Al2O3bulk composites sintered in different temperatures (1300 C deg. and 1400 C deg.). The reported research includes the processing of composites by powder metallurgy techniques (HP), microscopic analysis of material microstructure with special focus on micro-CT scanning, measurements of TRS by neutron diffraction (ND) method and numerical modeling of TRS by FEM based on micro-CT images of real material microstructure. Numerical micro-CT based models are proposed to predict the TRS and Young’s modulus with account of the TRS-induced damage of the ceramic phase. Our micro-CT based FEM models reproduce the TRS measurements with a good accuracy which may be an asset in applications having in mind the high cost of beam time for ND experiments at neutron sources. Finally, the experimental data and modeling results are compared to assess the TRS/microstructure effect on the Young’s modulus of the composites investigated.

Keywords:
sintering, metal-matrix composites, thermal residual stresses, neutron diffraction, microCT based FEM model

Abstract:
The nickel aluminide base composites are considered to be potentially interesting high temperature structural materials for aerospace industry due to their low density (5.9 g/cm3), high thermal conductivity (76 W/mK) and good corrosion and oxidation resistance. However, it is well-known that the main reason limiting this material's application in aerospace industry is related to its low fracture toughness and low ductility at room temperature. Research works on this subject have been carried out by various scientists throughout the world for more than four decades now. After initially high expectations, followed by rather disappointing results reported some 15 years ago, the recent progress in processing technologies in the context of fracture toughness levels is, indeed, remarkable. However, application of this structural material in real working conditions is still to be confirmed, [1].The composite materials investigated in this study were manufactured by powder metallurgy technique. The primary target was to obtain low density nickel aluminide bulk materials with enhanced fracture toughness, flexural strength and high oxidation resistance. The powders of NiAl were mixed in a planetary ball mill with various volume fractions of aluminum oxide, chromium and rhenium. Sintering was conducted in a hot press under the pressure of 30 MPa at 1400oC. Mechanical properties, microstructure and cyclic oxidation at 900oC, 1100oC, 1300oC were investigated. A promising improvement of flexural strength and fracture toughness were observed for each chemical composition. The highest enhancements were measured for the composite with 0.6 at.% addition of rhenium, where the flexural strength increasedfrom the reference level of 428 MPa (pure NiAl) to 808MPa. The oxidation tests showed predominantly high oxidation resistance due to formation of a thin oxide layer preventing significant mass losses. The oxidation experiment was limited to 150 cycles of 1 h duration, hence further tests are necessary to make the final assessment of the oxidation behavior.The second major problem investigated in this paper were thermal residual stresses (TRS) induced in the sintered composites during cooling from high sintering temperature to room temperature, due to CTE mismatch of the constituent materials. The effects of TRS on fracture parameters and other mechanical properties (E modulus, bending strength) were examined experimentally and modelled numerically using micro-CT based FE meshes mimicking the material microstructure. Our micro-CT based FEM models reproduce the TRS measurements by neutron diffraction with good accuracy, which may be an asset for engineering applications considering the high cost of beam time at the neutron sources.

Keywords:
Intermetallics, nickel-aluminides, turbine blades

Abstract:
W niniejszym artykule zaprezentowano wyniki modelowania, zagęszczania proszku stanowiącego wstępny etap procesu prasowania na gorąco, metodą elementów dyskretnych opisaną w [1]. Modelowanie numeryczne zrealizowano metodą elementów dyskretnych, z wykorzystaniem kulistych cząstek. Badania skoncentrowano na mechanizmach zagęszczania proszku przy ciśnieniu 50 MPa oraz modelach odpowiednich przy zastosowanych warunkach procesu. Numeryczne symulacje wykonano z wykorzystaniem dwóch modeli: pierwszy - elastyczny Hertz-Mindlin-Deresiewicz, drugi - plastyczny Storakers, opisanych w pracy [2]. Wyniki symulacji numerycznych zostały porównane z wynikami laboratoryjnymi zagęszczania proszku NiAl w matrycy. W rezultacie otrzymano dużą zgodność wyników eksperymentalnych i numerycznych.

Keywords:
metoda elementów dyskretnych, modelowanie, zagęszczanie proszków, prasowanie

Abstract:
Th is paper investigates the interplay between material microstructure and processing - induced thermal residual stresses (TRS) in particulate bulk MMC’ s with the main objective to explore their combined effect on the macroscopic fracture toughness and material properties (E modulus, bending strength) of the composite. The materials under consideration are hot pressed chromium - alumina bulk composites doped with rhenium, the use of which is motivated by their potential applications in transport and energy sectors. The reported research includes the processing of MMC by powder metallurgy techniques (HP and SPS), microscopic analysis of material microstructure with special focus on micro - CT scanning, measurements of TRS by neutron diffraction (ND) method and numerical modelling of TRS by FEM based on micro - CT images of real material microstructure. Several compositions of Cr(Re)/Al 2 O 3 system and different particle sizes were used in the sintering process to assess the effect of microstructure on the TRS. Spatial distributions of TRS measured by ND are taken as supporting information when interpreting the results of K IC measurements in a four point bending test. Numerical micro - CT based models were developed to predict the TRS, Young’s modulus and bending strength with account of TRS - induced damage of the ceramic phase of MMCs. A good predictive capability of these TRS models was achieved which may become important considering the cost of beam time for ND experiments at neutron sources. Finally, the large pool of experimental data and modelling results is discussed and the conclusions are drawn as to the TRS/microstructure effect on the fracture toughness of the MMCs in question.

Keywords:
thermal residual stress, metal-ceramic composites, fracture toughness, microCT FEM

Abstract:
In processing of metal-ceramic composites thermal residual stresses may result from different CTEs of the constituent materials, variable cooling rates inside the bulk material, or irregular pore shapes causing thermal stress concentrations.This paper investigates the interplay between material microstructure and processing-induced thermal residual stresses (TRS) in particulate bulk metal-matrix composites (MMC) and infiltrated phase composites (IPC) with the main objective to explore thecombined effect of TRS and microstructure on the macroscopic mechanical properties (E modulus, bending strength, fracture toughness) of the composite. The main focus is on numerical modelling of TRS, fracture toughness and effective elastic properties, while taking into account the real material microstructure from micro–computed tomography (micro-CT) experiments. The modelling methodology will be developed on examples ofa hot pressed chromium-alumina bulk MMCdoped with rheniumand on an IPC obtained by squeeze casting infiltrationof an alumina porous preform with molten Al alloyor Cu. Our interest in these particular compositesis motivated by their potential applications in transport and energy sectors. The paperwill includehighlights on the processingtechnologies used(HP, SPS, ceramic tape casting/squeeze casting infiltration), microscopic analysis of material microstructure with special focus on micro-CT scanning, measurements of TRS by neutron diffraction (ND) method, and numerical modelling of TRS by FEM using micro-CT images of real material microstructure. A numerical micro-CT based model developed to predict the TRS, Young’s modulus with account of TRS-induced damage of the ceramic phase will be shown (cf. Fig. 1). The grain size effect on TRS and Young’s modulus will be addressed. A good predictive capability of these TRS models was achieved which may become important considering the cost of beam time for ND experiments at neutron sources. Another model to be presented is concerned with micro-CT FEM modeling of fracture in infiltrated metal-ceramic composites. The model accounts for crack bridging toughening mechanism, large plastic deformations of metal ligaments, and matrix-ligament decohesion. Here the results on J integralin the case of compact-tensiontest specimen made of real interpentrating phase composite will be discussed. Finally, the large pool of obtained experimental data and modelling results will be wrapped up and conclusions will be drawn.

Keywords:
metal-ceramic composites, processing, thermal residual stresses, Youngs' modulus, microCT imaging, numerical modelling

Abstract:
Chromium based composites reinforced with alumina particles combineenhanced thermal, oxidation and wear resistance with mechanical strength and hardness. Because of these valuable properties Cr/Al2O3 composites can be used e.g. in the automotive sector for elements of powertrain. Rhenium due to its good mechanical and thermal properties is primarily used as an admixture of nickel superalloys in the aerospace and chemical industries. In the present paper a powder metallurgy route was used to manufacture dense Cr/Re/Al2O3 bulk composites with rhenium admixture of 2vol% and 5vol%. Composites were processed by hot pressing (HP) and by spark plasma sintering (SPS) techniques. The density of the sintered composites exceeded 98% of the theoretical value. Microstructural characterisation revealed that a solid solution of rhenium in chromium was partially formed. Mechanical properties such as Young’s modulus, bending strength, hardness, plastic limit are promising so are oxidation and corrosion resistance. A numerical FE model was developed for the prediction of thermal residual stresses (TRS) and damage generated in the metal and ceramic phase during cooling from high sintering temperature down to room temperature. The model uses micro-CTimages of the real material microstructure as the input data. A good agreement of the simulation results for TRS and the measurements of by neutron diffraction was achieved. The obtained Cr/Re/Al2O3composites were already tested as demonstrators of valve seats in combustion engines and good preliminary results were reported.

Keywords:
Chromium-alumina MMC, rhenium admixture, powder metallurgy, thermal residual stresses, microCT FEM