Staff

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:
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:
This work presents a comprehensive analysis of heat transfer and thermal conductivity of porous materials manufactured by spark plasma sintering. Intermetallic nickel aluminide (NiAl) has been selected as the representative material. Due to the complexity of the studied material, the following investigation consists of experimental, theoretical and numerical sections. The samples were manufactured in different combinations of process parameters, namely sintering temperature, time and external pressure, and next tested using the laser flash method to determine the effective thermal conductivity. Microstructural characterisation was extensively examined by use of scanning electron microscopy and micro-computed tomography (micro-CT) with a special focus on the structure of cohesive bonds (necks) formed during the sintering process. The experimental results of thermal conductivity were compared with theoretical and numerical ones. Here, a finite element framework based on micro-CT imaging was employed to analyse the macroscopic (effective thermal conductivity, geometrical and thermal tortuosity) and microscopic parameters (magnitude and deviation angle of heat fluxes, local tortuosity). The comparison of different approaches toward effective thermal conductivity evaluation revealed the necessity of consideration of additional thermal resistance related to sintered necks. As micro-CT analysis cannot determine the particle contact boundaries, a special algorithm was implemented to identify the corresponding spots in the volume of finite element samples; these are treated as the resistance phase, marked by lower thermal conductivity. Multiple simulations with varying content of the resistance phase and different values of thermal conductivity of the resistance phase have been performed, to achieve consistency with experimental data. Finally, the Landauer relation has been modified to take into account the thermal resistance of necks and their thermal conductivity, depending on sample densification. Modified theoretical and finite element models have provided updated results covering a wide range of effective thermal conductivities; thus, it was possible to reconstruct experimental results with satisfactory accuracy.

Keywords:
thermal conductivity, porous materials, spark plasma sintering, micro-computed tomography, nickel aluminide, finite element modelling, tortuosity

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:
Copper-matrix composites reinforced with silicon carbide (SiC) particles are heat sink materials with potential application in the electronic industry. A major challenge in the manufacturing of these materials, involving sintering process, is to prevent the decomposition of SiC and the subsequent dissolution of silicon in the copper matrix. This is overcome by coating SiC particles with metallic layers. In this study, a combined experimental and computational micromechanics approach was used to investigate thermal conductivity of Cu-matrix composites reinforced with silicon carbide particles coated with chromium, titanium, or tungsten layers. Plasma Vapor Deposition (PVD) was used to produce the metallic layers on SiC particles, while Spark Plasma Sintering (SPS) to consolidate the powder mixtures of copper and coated silicon carbide. Thermal conductivities of the fabricated three-phase composites Cu/SiC/Cr, Cu/SiC/Ti and Cu/SiC/W were evaluated using the Laser Flash technique. Finite Element Method (FEM) and Variational Asymptotic Method (VAM) based homogenization techniques were used for computational modeling of thermal conductivity. In the numerical models complex material microstructures were accounted for using micro-CT images of the sintered compacts. Comparison of the experimental results with simulations highlighted the importance of including the effect of imperfect interfaces to accurately model thermal conductivity of the investigated composites.

Keywords:
metal-matrix composites, powder metallurgy, plasma vapor deposition, imperfect interface, thermal conductivity, numerical modeling

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 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 review paper deals with flaws in aluminum–alumina composites and FGMs induced by their manufacturing processes. Aluminum–alumina composites have been studied for many years as potentially interesting materials for applications, for example, in the automotive sector due to their enhanced mechanical strength, wear resistance, good heat conductivity and low specific weight. The focus here is on the interpenetrating phase composites (IPCs) manufactured by infiltration of porous alumina preforms with molten aluminum alloys. The primary objective is to provide an updated overview of research findings on a variety of flaws occurring at different stages of the manufacturing processes. Some precautions on how to avoid processing induced flaws in aluminum–alumina bulk composites and FGMs are mentioned.

Abstract:
This paper describes the process of obtaining Cu-SiC-Cu systems by way of spark plasma sintering. A monocrystalline form of silicon carbide (6H-SiC type) was applied in the experiment. Additionally, silicon carbide samples were covered with a layer of tungsten and molybdenum using chemical vapour deposition (CVD) technique. Microstructural examinations and thermal properties measurements were performed. A special attention was put to the metal-ceramic interface. During annealing at a high temperature, copper reacts with silicon carbide. To prevent the decomposition of silicon carbide two types of coating (tungsten and molybdenum) were applied. The effect of covering SiC with the aforementioned elements on the composite's thermal conductivity was analyzed. Results were compared with the numerical modelling of heat transfer in Cu-SiC-Cu systems. Certain possible reasons behind differences in measurements and modelling results were discussed.

Keywords:
copper matrix composites, silicon carbide, interface, thermal conductivity, modelling

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:
Dense chromium–alumina composites doped with rhenium have been developed by the hot pressing method (bulk composite) and plasma spraying (composite coating). The obtained materials show superior mechanical properties, insensitivity to chemically aggressive environment, good resistance to frictional wear and oxidation at elevated temperature. These enhanced properties make the Cr/Re/Al2O3 composites interesting structural materials for energy and transport applications operating in demanding service conditions, e.g. fluidal boilers in power plants or valve seats in combustion engines.

Keywords:
Metal–ceramic composites, Powder technology, Mechanical properties, Coatings

Abstract:
In order to meet the requirements of an increased efficiency applying to modern devices and in more general terms science and technology, it is necessary to develop new materials. Combining various types of materials (such as metals and ceramics) and developing composite materials seem to be suitable solutions. One of the most interesting materials includes Cu-Al2O3 composite and gradient materials (FGMs). Due to their potential properties, copper-alumina composites could be used in aerospace industry as rocket thrusters and components in aircraft engines. The main challenge posed by copper matrix composites reinforced by aluminum oxide particles is obtaining the uniform structure with no residual porosity (existing within the area of the ceramic phase). In the present paper, Cu-Al2O3 composites (also in a gradient form) with 1, 3, and 5 vol.% of aluminum oxide were fabricated by the hot pressing and spark plasma sintering methods. Two forms of aluminum oxide (αAl2O3 powder and electrocorundum) were used as a reinforcement. Microstructural investigations revealed that near fully dense materials with low porosity and a clear interface between the metal matrix and ceramics were obtained in the case of the SPS method. In this paper, the properties (mechanical, thermal, and tribological) of composite materials were also collected and compared. Technological tests were preceded by finite element method analyses of thermal stresses generated in the gradient structure, and additionally, the role of porosity in the formation process of composite properties was modeled. Based on the said modeling, technological conditions for obtaining FGMs were proposed.

Keywords:
functionally gradient material, shot pressing, metal matrix composites, microstructure, spark plasma sintering, thermal conductivity

Abstract:
Copper/alumina composites are extensively used in automotive and aerospace industry for products that are subjected to severe thermal and mechanical loadings, such as rocket thrusters and components of aircraft engines. These materials are well-known for their good frictional wear resistance, good resistance to thermal fatigue, high thermal conductivity and high specific heat. In this paper, the sintering process of copper/electrocorundum composites reinforced by electrocorundum particles with diameters of 3 or 180 μm and 1, 3, 5 vol.% content is presented. The effects of different particle sizes of the ceramic reinforcement on the microstructure, physical, mechanical, tribological and thermal properties of the fabricated composites are discussed.

Keywords:
copper/alumina composites, sintering, modeling of thermal properties, microstructure, mechanical properties

Abstract:
In this paper we present the results of experimental measurements and numerical modeling of the effect of particle size on the residual thermal stresses arising in sintered metal–matrix composites after cooling down from the fabrication temperature. On example of novel Cr(Re)/Al2O3 composites processed by (i) spark plasma sintering and (ii) hot pressing the residual thermal stresses are measured by neutron diffraction technique and determined by a FEM model based on micro-CT scans of the material microstructure. Then numerical model of microcracking induced by residual stresses is applied to predict the effective Young modulus of the damaged composite. Comparison of the numerical results with the measured data of the residual stresses and Young’s modulus is presented and fairly good agreement is noted.

Keywords:
Metal–matrix composites (MMC), Residual/internal stress, Finite element analysis (FEA), Sintering

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

Abstract:
Copper-based composites could be widely used in automotive, electronic or electrical industry due to their very promising thermal properties. In the present paper, Cu-AlN metal matrix composites with ceramic volume fractions between 0.1 and 0.4 were fabricated by hot pressing method in vacuum. Dependence of the coefficient of thermal expansion (CTE) and the thermal conductivity (TC) on the chemical composition of composites has been investigated. The measured values of the thermal expansion coefficient have been compared with the analytical models’ predictions. A numerical model based on FEAP 7.5 in 3D space has been used to evaluate the influence of the porosity on the thermal properties (thermal conductivity) of the composite. A fairly good correlation between the FEM results and the experimental measurements has been obtained.

Keywords:
thermal properties, porosity, copper-based composites

Abstract:
Celem pracy było zbadanie wpływu dodatku renu na właściwości termomechaniczne i użytkowe kompozytów Cr-Al2O3 wytwarzanych metodą spiekania pod ciśnieniem w prasie HP oraz metodą Spark Plasma Sintering (SPS). Uzyskano kompozyty o gęstości przekraczającej 98% gęstości teoretycznej. Właściwości mechaniczne (m.in. moduł Younga, wytrzymałość na zginanie, twardość, odporność na pękanie, granica plastycznośći) oraz odporność na utlenianie wytworzonych materiałów są obiecujące. Zbudowano model numeryczny do obliczeń wielkości naprężeń resztkowych obecnych w materiałach faz kompozytu po procesie spiekania oraz modułów sprężystości. Wykorzystano w tym celu obrazy rzeczywistej mikrostruktury kompozytu otrzymane z tomografii komputerowej. Uzyskano dobrą zgodność wyników modelu z wynikami pomiarów naprężeń metodą XRD. Przedstawiono ponadto porównanie wyników obliczeń numerycznych i pomiarów modułu Younga przy zastosowaniu różnych metod

Keywords:
kompozyty MMC, metalurgia proszków, modelowanie MES, mikrotomografia komputerowa, naprężenia resztkowe

Abstract:
Thermal stresses induced during the cooling of Cr–Al2O3 (MMC) processed by sintering are modeled numerically using the FEA. The composite microstructure is modeled as (i) random distribution of ceramic particles (voxels) in the metal matrix, and (ii) using micro-CT scans of the real microstructure transformed into a FE mesh. Numerical simulations of the thermal residual stresses are compared with the test data measured by X-ray diffraction. A simple numerical model is then proposed to predict the overall elastic properties of the composite with account of the porosity and damage induced by the thermal stresses. Comparison of the model predictions with the measured data for Young’s modulus is presented.

Keywords:
Metal-matrix composites (MMCs), Residual/internal stress, Finite element analysis (FEA), Sintering

Abstract:
This work is focused on the modeling of thermal stresses induced during the fabrication of the metal/ceramic composites. On example of Cr-Al2O3 composite processed by powder metallurgy, thermal stresses after fabrication are determined by FEM model for different contents of metal and ceramic phases. Numerical model of microcracking induced by thermal stresses is then proposed and applied to compute the overall elastic properties of the damaged composite. Comparison of the model predictions with the measur ed data for Young's modulus is presented.

Keywords:
Cr-Al2O3 composite, mechanical properties, thermal stress modeling, microcracking, Young’s modulus modeling

Abstract:
A micromechanical model is proposed to simulate the deformation of cementitious composites exposed to external sulfate attack. The model involves coupled physico-chemical processes of nonsteady diffusion with reaction, topo-chemical reaction of ettringite formation, expansion of ettringite inclusions, microcracking of hardened cement paste and percolation of sulfates through heavily deteriorated mortar. The Fick’s second law with reaction term is assumed to govern the transport of the sulfate ions. The Eshelby solution and the equivalent inclusion method are used to determine the eigenstrain of expanding ettringite crystals in microcracked hardened cement paste. The degradation of transport properties is studied in the effective medium and the percolation regime. An initial boundary value problem (2D) of expansion of a mortar specimen immersed in a sodium sulfate solution is solved and compared with available test data.

Keywords:
chemo-damage, micromechanics, concrete, microcracking, sulfate attack

Abstract:
Two micromechanical models are developed to simulate the expansion of cementitious composites exposed to external sulphate attack. The di®erence between the two models lies in the form of chemical reaction of the ettringite formation (through-solution vs. topochemical). In both models the Fick's second law with reaction term is assumed to govern the transport of the sulphate ions. The Eshelby solution and the equivalent inclusion method are used to determine the eigenstrain of the expanding ettringite crystals in microcracked hardened cement paste. The degradation of transport properties is studied in the efective medium and the percolation regime. An initial-boundary value problem (2D) of expansion of a mortar specimen immersed in a sodium sulphate solution is solved and compared with available test data. The obtained results indicate that the topochemical mechanism is the one capable of producing the experimentally observed amount of expansion.

Keywords:
chemo-damage, sulphate attack, topochemical reaction, through-solution reaction, ettringite, micromechanics, microcracking, percolation

Abstract:
This is a review paper on the existing approaches to modelling of discrete cracks (fracture) and diffuse microcracking (damage) in ceramic matrix composites under mechanical or thermal loading. The focus is on Ceramic Matrix Composites (CMC) with metal particle inclusions and on interpenetrating metal ceramic networks. The second phase in form of ceramic inclusions is not considered. The models of toughening mechanisms are discussed in considerable detail. Sections 2-5 deal with discrete cracks while Sections 6-9 with diffuse microcracking. The paper is concluded with identification of unresolved problems and topics for future research in the area of fracture and damage of CMC.

Keywords:
ceramic matrix composites, particles, interpenetrating networks, fracture, damage, toughening mechanisms, bridging, cracks, microcracks, cavitation, debonding

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:
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

Abstract:
There is an urgent technological need for elements performing in demanding service regimes (especially in automotive and aerospace applications) to be made of new materials having superior properties such as higher strength and Young's modulus, enhanced temperature resistance and thermal shock resistance, improved corrosion and wear resistance, as well as reduced specific weight and better recycling potential. The Interpenetrating Phase Composites (IPC) could be a good answer to these industrial and commercial needs. But still a lot of problems appear during the processing of this type of metal-ceramic composites. One severe problem is the large thermal stresses generated during the cooling after the infiltration which can lead to initiation and propagation of microcracks and, thus to harmful decrease of the elastic properties of material.

In this paper, a FEM model is developed for the calculation of thermal residual stresses inside the IPC composites. The FE mesh is generated by the commercial software (ScanFE and ScanIP) based on the real material microstructure obtained from computer microtomography. The stress cracking condition is applied and the influence of the thermal stress induced microcracks on the elastic material parameters is shown. The comparison of the Young modulus furnished by the FEM model with the experimental data is presented.

Keywords:
interpenetrating phase composites, thermal stress, FEM, microcracking

Abstract:
The interpenetrating phase composites (IPC) are strongly different in their morphology, properties and processing than typical metal matrix or ceramic matrix composites. The basic morphological difference in comparison with particulate reinforcement composites is that the two components of IPC form continuous, interpenetrating 3D network. The IPC are more homogeneous, have better mechanical and thermal properties (abrasibility and fracture toughness, thermal conductivity and mechanical stability) than the matrix composites. The processing of IPC is typically done by a pressure or pressureless infiltration of ceramic porous matrix with a molten metal. The infiltration is a high temperature process (e.g. for Cu/Al2O3 IPC the infiltration temperature is above 1200OC and for Al/Al2O3 about 700 OC) which is usually associated with the generation of thermal stresses because of largely different coefficients of thermal expansion of the IPC components. The aim of this work was twofold: (i) to build a numerical model of thermal stress generated during the processing of the interpenetrating phase composites, and (ii) to build a numerical model of the initiation and growth of microcracks induced by the thermal stresses during the processing of the IPC. The results yielded by the models were compared with the experimental data. The models can be used to improve the processing of IPC by providing feedback as to how to reduce thermal residual stresses and how to minimize a risk of the microcracking during the production of the IPC.

Keywords:
interpenetrating phase composites, residual stress, microcracking, damage modelling