Investigation of residual stress effects in an alloy reinforced ceramic/metal composite

This study aims at investigating the thermal expansion behavior and internal residual strains in metal reinforced ceramic matrix composites (CMCs). A variety of Al₂O₃/A356 CMCs composites with an interpenetrating network structure and varying metal content, ranging from 10 to 40 vol.%, were produced using the pressure infiltration technique of Squeeze casting. Values of coefficients of thermal expansion (CTEs) were found to vary significantly with temperature, indicating an influence of the flow characteristics of the metal. Comparisons are made with well known methods for predicting CTEs values of metal/ceramic composites. The overall strain was found to increase with temperature and scaled proportionally with the metal content of the composite. Comparisons were also made with non-infiltrated porous ceramic preforms and a pure metallic sample. The uniform heating and cooling curves for the composite samples were found to exhibit hysterisis. Residual stress analysis and failure simulation were performed based on thermomechanics and the finite element method (FEM). This analysis is often utilized for the analysis of stress distribution or deformation of a structure. High angle X-ray and CTEs mismatch equation analysis were utilized to analyze the residual stresses at the ceramic/metal interface of the Al₂O₃/A356 composites. The relationship of residual stresses and the contact area of the ceramic/metal interface are also discussed.     

In situ measurement of lattice strains in mixed ceramic cutting tools under thermal and mechanical loads using synchrotron radiation

To completely understand wear mechanisms of mixed ceramic cutting tools (Al₂O₃–TiC), residual stress states and the superposition of external loads during hard turning should be investigated. This can be done via X-ray diffraction using high-energy synchrotron radiation to determine lattice strains in the material. For this reason, in first model tests, strain states in mixed ceramics were determined during the application of external loads. An experimental setup was developed to measure lattice strains in the different phases of the ceramic material in situ during thermal, mechanical and thermo-mechanical loading for first reference. The accuracy of the setup was sufficient to clearly determine shifts in lattice parameters in the different phases due to external loads. By applying a thermal load on the mixed ceramic material the two main phases showed different elastic lattice strains. Thus, a slightly lower coefficient of thermal expansion in the Al₂O₃-phase than in the Ti(O,C)-phase could be determined. This indicated the development of compressive stresses in the Al₂O₃-phase and tensile stresses in the Ti(O,C)-phase at room temperature. By applying external bending stresses to the mixed ceramic material, for both phases equal lattice strains could be determined. From these strains stresses could be calculated for both phases which were in the same order of magnitude as external stresses. With further in situ investigations of strain and stress states in the different phases of mixed ceramics during friction and turning experiments a more comprehensive characterization of wear mechanisms is possible.     

Residual stress and diffraction line-broadening analysis of Al2O3/Y-TZP ceramic composites by neutron diffraction measurement

Time-of-flight neutron diffraction and Rietveld analysis have been used to investigate the residual stresses, crystallite structure and microstructural features in Al₂O₃/Y-TZP ceramic composites fabricated by two different green processing techniques (a novel tape casting and conventional slip casting) and with different zirconia content (5 and 40 vol.% Y-TZP). The change in lattice parameters, individual peak shifts and peak-broadening were analyzed, in order to calculate the uniform residual stress field (mean phase stresses and peak-specific stresses) and the non-uniform microstrains. Peak-specific residual stresses were calculated for different hkl reflections both for the Al₂O₃ matrix and the Y-TZP particulates. The sign and the magnitude of the peak-specific residual stress are highly dependent on the individual hkl reflection studied and on the volume fraction of zirconia. Peak broadening was observed in the Y-TZP reflections, due to non-uniform microstrains. Both the mean phase stress field and the non-uniform microstrains were mainly influenced by the Y-TZP content in the studied Al₂O₃/Y-TZP composites, irrespective of the measured direction and the fabrication process.     

The Effect of Platinum on β-NiAl/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Interfacial Tensile Stress: A Combined Ab Initio DFT and Mechanics-Based Model Study

The beneficial effect of adding Pt in diffusion β-NiAl coating on reducing the tensile stress normal to the coating/α-Al₂O₃ scale interface has been investigated using a combination of ab initio density functional theory (DFT), phonon dispersion theory and mechanics-based interfacial stress modeling. The coefficient of thermal expansion (CTE) for Pt, β-NiAl and β-NiAl-6.25 at% Pt was calculated using the total energy DFT combined with the phonon dispersion theory. The calculated CTE of β-NiAl and β-NiAl-6.25 at% Pt and experimentally measured CTE of α-Al₂O₃ were used to evaluate the tensile stress of the undulated β-NiAl/α-Al₂O₃ and β-NiAl-6.25 at% Pt/α-Al₂O₃ interfaces resulting from the CTE mismatch between the coating and the oxide scale during cooling from elevated temperatures. It was found that the addition of Pt to β-NiAl is capable of lowering the interfacial tensile stress as a result of the reduced CTE of the Pt-modified β-NiAl coating, thus beneficial for improving thermal cyclic durability of the coating. The calculated results showed that the interfacial tensile stress is a function of oxide scale thickness and the interfacial wave amplitude and wavelength of an undulated interface. A thicker oxide scale and a rougher interface with a larger ratio of wave amplitude versus wavelength yield higher interfacial tensile stresses during thermal cycling. The addition of 6.25 at% Pt to β-NiAl coating reduces the coating/oxide scale interfacial tensile stress by about 27% over a wide range of scale thickness and interfacial wave amplitude and wavelength.     

Effect of thermal-cooling cycle treatment on thermal expansion behavior of particulate reinforced aluminum matrix composites

Two micron SiC particles with angular and spherical shape and the sub-micron Al2O3 particles with spherical shape were introduced to reinforce 6061 aluminium by squeeze casting technology.Microstructures and effect of thermal-cooling cycle treatment(TCCT) on the thermal expansion behaviors of three composites were investigated.The results show that the composites are free of porosity and SiC/Al2O3 particles are distributed uniformly.Inflections at about 300 °C are observed in coefficient of thermal expansion(CTE) versus temperature curves of two SiCp/Al composites,and this characteristic is not affected by TCCT.The TCCT has significant effect on thermal expansion behavior of SiCp/Al composites and CTE of them after 3 cycles is lower than that of 1 or 5 cycles.However,no inflection is observed in Al2O3p/Al composite,while TCCT has effect on CTE of Al2O3p/Al composite.These results should be due to different relaxation behavior of internal stress in three composites.     

Load partitioning in Al2O3–Al composites with three-dimensional periodic architecture

Interpenetrating composites are created by infiltration of liquid aluminum into three-dimensional (3-D) periodic Al₂O₃ preforms with simple tetragonal symmetry produced by direct-write assembly. Volume-averaged lattice strains in the Al₂O₃ phase of the composite are measured by synchrotron X-ray diffraction for various uniaxial compression stresses up to ?350 MPa. Load transfer, found by diffraction to occur from the metal phase to the ceramic phase, is in general agreement with simple rule-of-mixture models and in better agreement with more complex, 3-D finite-element models that account for metal plasticity and details of the geometry of both phases. Spatially resolved diffraction measurements show variations in load transfer at two different positions within the composite.     

Fabrication of SiC particulate reinforced AZ91D composite by vacuum-assisted pressure infiltration technology

The AZ91D Mg matrix composites reinforced by SiC particulate with the sizes of 11 μm, 21 μm and 47 μm were successfully fabricated respectively by vacuum-assisted pressure infiltration technology. Microstructures and particulate distributions were analyzed with scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The coefficient of thermal expansion (CTE) measurements was performed from 75 °C to 400 °C at a heating rate of 5 °C/min. The results show that the uniform distribution of SiC particulate in metal matrix and density over 98% in theoretical density of composites were fabricated. Only MgO phase was detected at the interface and no brittle phases of Al₄C₃ and Mg₂Si were discovered. The desirable coefficients of thermal expansion of composites were achieved. The intensity of dislocation generation nearby SiC particulate increases significantly with the increasing of SiC particulate size. Therefore, this technology is a potential method to fabricate Mg matrix composites reinforced by SiC particulates with the desirable microstructures and CTE.     

Al_(0.25)Cu_(0.75)FeNiCo颗粒增强铝合金的组织与性能

高熵合金是一种新型的结构与功能材料,源于金属-金属间天然的界面结合特性,高熵合金与铝合金基体间的界面润湿性极好。采用Al_(0.25)Cu_(0.75)FeNiCo高熵合金(HEA)颗粒作为增强相来增强铝合金,研究高熵合金含量变化对复合材料显微组织和力学性能的影响。结果表明:高熵合金增强相在基体中分布均匀,随着高熵合金体积分数的增大,局部会出现少量颗粒团聚现象。复合材料的弹性模量和硬度随着高熵合金含量的增加而增大,但复合材料的抗拉强度和延伸率呈现出先增大后减小的趋势。当高熵合金的体积分数为5%时,复合材料的极限抗拉强度和伸长率达到最大值(σb:437.6 MPa,ε:11.42%),比铝合金基体分别提高了20.1%和36.6%。TEM分析表明,高熵合金颗粒和铝合金良好的界面结合状态,使得复合材料具有较高的综合力学性能。     

The Influence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Powder Morphology on the Properties of Cu-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Composites Designed for Functionally Graded Materials (FGM)

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.     

Effect of thermal cycling on the expansion behavior of Al/SiCp composite

The coefficient of thermal expansion (CTE) and accumulated plastic strain of the pure aluminum matrix composite containing 50% SiC particles (Al/SiC_p) during thermal cycling (within temperature range 298–573 K) were investigated. The composite was produced by infiltrating liquid aluminum into a preform made by SiC particles with an average diameter of 14 μm. Experiment results showed that the relationship between the CTE of Al/SiC_p and temperature is nonlinear; CTE could reach a maximum value at about 530 K. The theoretical accumulated plastic strain of Al/SiC_p composites during thermal cycling has also been calculated and compared with the experimental results.     

Functionally gradient (YSZ-20% Al<Subscript>2</Subscript>O<Subscript>3</Subscript>)−SUS422 composites

Functionally gradient materials (FGMs) were prepared by mixing 5 layers comprised of different ratios of (YSZ-20%Al₂O₃) and 422 stainless (SUS422) powders, followed by hot pressing for densification. Two design concepts were proposed: One as a FGM with a monotonic change of the CTE (coefficient of thermal expansion) for each layer, and is designated as the monotonic mode, and the other was a FGM with a change of CTE that is not monotonic for each layer, and is termed the non-monotonic mode. The FGM with a monotonic CTE mode cracked at the ceramic surface after it was removed from the hot pressing furnace. In contrast, the FGM with a non-monotonic CTE mode survived after hot pressing. Based on ABAQUS simulation results, a non-monotonic change in CTE resulted in a decrease of residual stress on the ceramic side but an increase inside the metal-rich layers of the FGMs. The induced change in the stress distribution inside the FGMs was compromised by the deformation of the metal-rich ingredient (SUS422) in the FGM. Thermal shock tests of FGMs were performed between 25°C and 600°C. The non-monotonic FGM endured up to 100 thermal cycles with only slight bending, and was free of delamination and cracking. The use of composition-adjusted layers to manipulate thermal expansion coefficients of each layer greatly changed the stress contour of the FGM. It is noted that a modified functional-gradient FGM can be fabricated with a hard ceramic surface on one side to resist high temperature, and a ductile metallic surface on the other side to provide toughness.     

Combination Effects of Nocolok Flux with Ni Powder on Properties and Microstructures of Aluminum-Stainless Steel TIG Welding-Brazing Joint

A flux consisting of Nocolok and nickel powder was first applied for TIG welding-brazing of aluminum-stainless steel. Results of tensile and impact tests illustrated that a significant improvement in mechanical properties of the butt joint was obtained with the flux, tensile strength increased from 116 to 158 MPa, and impact energy increased from 3.2 to 6.7 J. Investigation results on microstructures of interfaces and seams suggested that Ni addition significantly decreased the thickness of intermetallic compound (IMC) layer on the interfaces, but did not change the phase structure of Al₁₃Fe₄. Furthermore, precipitate phase in the welded seams changed from Al₆Fe to Al₉FeNi, and the quantity of precipitate phases decreased from 12 to 9% approximately. Finally, effect of Ni powder’s addition on the joint was analyzed and discussed. The reduction in the thickness of IMC and quantity of precipitate phases are beneficial to joint properties.     

Process study, microstructure, and matrix cracking of SiC fiber reinforced MoSi2 based composites

SiC fiber reinforced SiAlON-MoSi₂ composites have been manufactured by a concurrent fiber winding and low pressure plasma spraying (LPPS) technique to produce a multilayer, circumferentially fiber reinforced composite ring. The LPPS parameters for SiAlON-MoSi₂ powder were optimized by a two-level experimental design approach followed by further optimization, which provided a smooth sprayed surface, low matrix porosity, and high deposition efficiency. The microstructure of SiAlON-MoSi₂ matrix consisted of a lamellar structure built up of individual splats and a uniform distribution of discontinuous SiAlON splats throughout the MoSi₂ matrix. The spray/wind composites exhibited 2% porosity and well-controlled fiber distribution. High temperature consolidation led to the formation of a thick reaction zone at the fiber-matrix interface by a chemical reaction between C coating and MoSi₂. Matrix cracking occurred in SiC _f (15 vol.%)/MoSi₂ after cooling from 1500 to 25 °C and was attributed to the large tensile residual stresses in the matrix developed on cooling because of coefficient of thermal expansion (CTE) mismatch between matrix and fiber. The addition of 40 vol.% SiAlON into the MoSi₂ effectively eliminated the matrix cracking by reducing the matrix-fiber CTE mismatch. Predictions of matrix cracking stress on the basis of residual stresses in the composites showed that the maximum permissible fiber volume fraction to avoid matrix cracking was 6% for SiC _f /MoSi₂ and 23% for SiC _f /SiAlON(40 vol.%)-MoSi₂.     

Piezo-spectroscopy at low temperatures: residual stresses in Al2O3–ZrO2(Y2O3) eutectics measured from 77 to 350 K

High-resolution, piezo-spectroscopic studies were performed in alumina/zirconia eutectic composites with different Y₂O₃ contents and microstructures at different temperatures using the ruby R-line luminescence. Measurements at 77 K allowed the precise determination of the average stress and its distribution in the alumina phase. A normal distribution function was obtained in most of the cases. In some composites the highest stresses are relaxed by micro cracking, giving an asymmetrical distribution function. In the composites with stabilized zirconia the residual stresses originate from the differences in the thermal expansion coefficients of the component phases. A linear dependence of the thermostresses with temperature was obtained in the 77–350 K temperature range. The effective elastic modulus for thermostresses was 118±2.7 GPa. Using the coefficients of thermal expansion of the component phases and making an extrapolation of the low temperature values the stress-free temperature of 1270±35 K was determined.     

Aluminum matrix composites reinforced by in situ Al2O3 and Al3Zr particles fabricated via magnetochemistry reaction

Aluminum matrix composites reinforced by in situ Al₂O₃ and Al₃Zr particles are fabricated from A356-Zr(CO₃)₂ system via magnetochemistry reaction, and the morphologies, sizes and distributions of the in situ particles as well as the microstructures, mechanical mechanisms of the composites are investigated by XRD, SEM, TEM and in situ tensile tests. The results indicate that with the pulsed magnetic field assistance, the morphologies of the in situ particles are mainly with ball-shape, the sizes are in nanometer scale and the distributions in the matrix are uniform. The interfaces between the in situ particles and the aluminum matrix are net and no interfacial outgrowth is observed. These are due to the strong vibration induced by the applied magnetic field in the aluminum melt, which in turn, accelerates the melt reactions. The effects of the magnetic field on the above contributions are discussed in detail.     

Evaluation of crack-tip stress fields on microstructural-scale fracture in Al–Al2O3 interpenetrating network composites

The influence of local microstructure on the fracture process at the crack tip in a ceramic–metal composite was assessed by comparing the measured stress at a microstructural level and analogous finite element modelling (FEM). Fluorescence microprobe spectroscopy was used to investigate the influence of near-crack-tip stress fields on the resulting crack propagation at the microstructural scale. The high spatial resolution was effective at mapping the localized crack-tip stress distributions within the complex Al–Al₂O₃ phase morphologies, where the localized stress distribution about the crack tip within the Al₂O₃ phase could be measured. Regions of high-localized tensile stress within the microstructure resulting from a combination of applied load and thermal residual stress were identified and could be used in predicting the subsequent crack extension direction. Stress distributions calculated from spectroscopy results were compared with microstructural level FEM of the same structure and general agreement between the two techniques was observed.     

Size dependent strengthening in particle reinforced aluminium

The tensile behaviour of composites produced by infiltrating ceramic particle beds with high purity (99.99%) Al is studied as a function of reinforcement size and chemistry (Al₂O₃ and B₄C). The yield stress is higher in composites containing B₄C particles, increasing with decreasing interparticle distance in both composite systems. The flow stress of the composites, when corrected for damage, displays the same dependence on interparticle distance as the yield stress. The overall strain hardening exponent, however, is independent of the microstructural scale. These observations are rationalized based on the theory of geometrically necessary dislocations.     

Thermal properties of pressureless melt infiltrated AlN–Si–Al composites

Thermal properties of AlN-Si-Al composites produced by pressureless melt infiltration of Al/Al alloys into porous α-Si₃N₄ preforms were investigated in a temperature range of 50-300 °C. SEM and TEM investigations revealed that the grain size of AlN particles was less than 1 μm. In spite of sub-micron grain size, composites showed relatively high thermal conductivity (TC), 55-107 W/(m.K). The thermal expansion coefficient (CTE) of the composite produced with commercial Al source, which has the highest TC of 107 W/(m.K), was 6.5×10^?6 K^?1. Despite the high CTE of Al (23.6×10^?6 K^?1), composites revealed significantly low CTE through the formation of Si and AlN phases during the infiltration process.     

Microstructure, billet surface quality and tensile property of (Al2O3 + Al3Zr)p/Al composites in situ synthesized with electromagnetic field

The aluminum matrix composites reinforced by Al₂O₃ and Al₃Zr particulates were fabricated via in situ chemical reaction between Al-15 wt.% Zr(CO₃)₂ systems. In the process of in situ reaction, a low frequency electromagnetic field (EMF) is employed to improve the conditions of reaction between reactants powder and melt. The optimized electromagnetic density of low frequency EMF is 0.025 T. During the direct chill casting process of composites melt, the custom-designed electromagnetic fields are introduced to control the microstructures and improve the billet surface quality. XRD analysis shows that Al₂O₃ and Al₃Zr reinforcement phases have been obtained. The Lorenz force improves the kinetic condition and accelerates the nucleation of endogenetic particulates. Microstructure analysis by SEM indicates that the average size of particulates and grain size of matrix are refined to 0.5-1 μm and 20-40 μm, respectively. The surface quality of round billet is greatly improved by the high frequency EMF. The results of tensile properties test show that the tensile strength of composites in situ fabricated with EMF is 254.6 MPa, which is increased by about 104 MPa and 69.4% compared with those of composites in situ fabricated without EMF.     

Preparation of Al2O3–ZrO2–Y2O3 Composite Coatings by a Modified Sol–Gel Technique for Thermal Barrier Application

Spatial-network Al₂O₃–ZrO₂–Y₂O₃ composite coatings were prepared by a modified sol–gel technique, so-called thermal pressure and filtration of sol–gel paint. The composite coatings were derived from a composite paint of yttria partially stabilized zirconia (YSZ) particles, Al₂O₃ particles and Al₂O₃–Y₂O₃ sol. Their microstructure showed that YSZ particles were covered with spatial-network Al₂O₃–Y₂O₃ blanket. Cyclic oxidation at 1,050 °C in air for 200 h demonstrates that the oxygen diffusion rate in the coatings could be effectively inhibited. Meanwhile, suitable coefficients of thermal expansion (CTE) gave the composite coatings better spallation resistance than that of Al₂O₃–Y₂O₃ or ZrO₂–Y₂O₃ coatings. The positive results of cyclic oxidation indicated that the composite coating can be used as an interlayer between the bond coat and the top ceramic layer in traditional TBCs. Not only the depletion rate of aluminum-rich phase in MCrAlY alloy could be slowed down by spatial-network Al₂O₃–Y₂O₃, but also different thermal expansion between thermally grown oxides layer and top layer could be relieved by suitable CTE. In this paper, the mechanisms of the inhibition of oxygen diffusion and thermal match between ceramic coating and alloy are also discussed.