Katarzyna Konopka

Microstructure and the fracture toughness of the Al2O3–Fe composites

Abstract In the present paper, Al 2 O 3 –Fe composites with various weight contents of iron were analysed. The morphology and distribution of the Fe particles within the ceramic matrix were described. Moreover, the fracture toughness ( K IC ) of these composites was also measured. The microstructure of the composites contains mostly spherical Fe particles distributed uniformly throughout the Al 2 O 3 matrix. Because the iron is melted during sintering, the Fe particle size in the composites was compared with the starting particle size of iron powder. The K IC depends on the Fe content in matrix and is limited by FeAl 2 O 4 spinel formed in the composite during sintering.

The generation of dislocations from twin boundaries and its effect upon the flow stresses in FCC metals

Abstract This paper is concerned with an analysis of the variation of flow stresses with the frequency of twin boundaries present in the microstructure of copper and austenitic steel. The increase of the frequency of twin boundaries results in a reduction of the flow stresses. This effect is explained in terms of dislocations being generated at the twin boundaries. The source of the dislocation generation from these boundaries was found during in situ strain experiments under the transmission electron microscope. The direct observations of dislocations generated from twin boundaries are presented in the paper.

The effect of the twin boundaries on the yield stress of a material

Abstract The aim of the study was to analyse theoretically how the twin boundaries formed in a material during recrystallization affect its yield stress. The yield stress (σ) was considered to be the sum of the two components: the stress ( σ GB ) due to the effect of grain boundaries upon the propagation of slip and the stress ( σ TB ) due to the presence of twin boundaries: σ = σ GB + σ TB Various correlations between the grain size, the frequency of twin boundaries and the yield stress of the material were considered in the analysis. It has been found that the effect of twin boundaries varies depending on whether they present strong or weak obstacles to the dislocation movements during plastic deformation or whether they themselves generate dislocations. Hence, the stress due to twin boundaries ( σ TB ) can be expended into three components that represent the individual effects of the twin boundaries observed during plastic deformation.

Characterization of composites containing NiAl2O4 spinel phase from Al2O3/NiO and Al2O3/Ni systems

This paper focused on the characterization of the composites containing nickel aluminate spinel from Al2O3/NiO and Al2O3/Ni systems. The composites were prepared by die pressing of powders and subsequent sintering of green bodies in air atmosphere. Composites were characterized by XRD, SEM, EDS and DTA/TG/MS analyses. The physical properties of the composites were measured by Archimedes method. Quantitative description of the composites microstructure was made on the basis of SEM images using computer image analysis. The XRD studies and SEM observations of composites confirmed the presence of two phases Al2O3 and NiAl2O4 in the whole volume of samples from both systems. Spinel phase was evenly distributed throughout the volume of the material. Morphology of NiAl2O4 obtained from both systems was characterized by the presence of voids. The DTA/TG/MS measurements showed the characteristics of organic binder decomposition and type of gases released to the atmosphere during thermal treatment. Moreover, the DTA/TG analysis showed the temperature of spinel-phase formation for both systems. It was found that the spinel-phase NiAl2O4 formation retards the process of densification. Therefore, it can be concluded that densification of samples with spinel phase depends mainly on the volume of spinel phase in composite material and does not depend on the substrates used to prepare spinel phase. The values of the selected properties of Al2O3–Ni- and Al2O3–NiO-based materials confirmed that the physical properties depend on the type of substrates used in the fabrication of composites. The type of powder influences the open porosity of samples. For composites produced using NiO powder, open porosity is lower than for samples formed with nickel powder.

Al2O3-Fe Functionally Graded Materials Fabricated under Magnetic Field

This paper describes technology, which can be used to obtain ceramic-metal composites with a gradient of metal particles concentration. Graded composites, have been obtained by slip casting. The gradient of iron concentration was induced by magnetic field. Microstructures of the specimens have been investigated using a light and scanning electron microscopy. Quantitative analysis of microstructures has been carried out with the help of image analyzer. The obtained results prove the possibility to produce Al2O3-Fe functionally graded materials under the magnetic field.

Novel Ceramic-Metal Composites with Metal Phase from Micro to Nanosize

The present paper is focused on ceramic–metal composites obtained via different technologies which leads to different microstructures in terms of size and distribution of metal phase. Composites analysed in paper were produced by the following methods:(a) infiltration of porous ceramics by metal, (b) consolidation under high pressure and (c) sintering of ceramic powder coated by metal. Their microstructures were investigated by scanning and transmission electron microscopy methods. The three methods of composite fabrication employed in the present study result in specific spatial distribution and dispersion of metal phase. Presureless infiltration of porous ceramics by liquid metal is driven by capillary force and make it possible to produce microstructure with percolation of metal phase in ceramic matrix. The volume fraction of metal phase in this case depends on the size of pores. The size of pores influence also the kinetics and extent of infiltration. Ceramic preforms with small size of pore are not fully infiltrated. This method is useful for composite with size of metal phase in the range of micrometers. Hot pressing under high pressure produces microstructures of composites with metal phase grain size in the range from nano to micrometers. Moreover, it allows to achieve the nanometric size of ceramic grains. In the case of ceramic powders covered by metal, compression and hot pressing preserves nanometric size of metal. The grain growth of ceramic grains is suppressed.

Influence of Temperature and Pressure on the Possibility of Obtaining Al2O3/Ni-P Nanocomposite through Hot Pressing Process

The influence of sintering temperature and high pressure on Ni-P nanoparticles in Al2O3/Ni-P nanocomposites was investigated in this paper. The fine-grained alumina powder was covered with Ni-P nanoparticles of 20 – 50 nm size by electroless nickel plating. The material was sintered in temperatures from 900 to 1400oC using pressures above 5 GPa. It was found that sintering in such conditions give a possibility to maintain nanometrical size of Ni-P particles. In the case of 1400oC the metallic phase melts and non-uniform grain growth of ceramic was observed. Hot pressing at 900oC allowed for the metal to remain in solid state and ensured uniform microstructures of the nanocomposites, with uniform distribution of nanometrical Ni-P grains in the ceramic matrix. In this temperature the grain growth of the ceramic was not observed. The results are discussed in terms of technology for production of ceramic-metal composites for various applications.

Polymer matrix composites with particles of TiC obtained by a sol-gel method

Nanoparticles of carbides, nitrides and carbonitrides can be used to reinforce polymer matrix nanocomposites to obtain the required strength, hardness, corrosion and wear resistance. In order to efficiently achieve the desirable properties the polymer matrix and nanoparticles must be optimised. This paper reports on studies undertaken on TiC reinforced polymer matrix nanocomposites. The TiC nanoparticles were produced by sol-gel method and nanocomposites were obtained in situ, via the reaction and synthesizing of polyether-ester copolymer (PEE). TiC nanoparticles were characterised with a scanning electron microscopy (SEM) and the microstructure of the composites was examined by SEM and atomic force microscopy (AFM). Tensile properties were determined. For comparison, samples of polymer were also studied and composites with submicron size of TiC particles. The results, which are discussed in terms of size of the TiC particles, showed that the particles incorporated in the polymer matrix, influence the strength of the composites.

Studies of Amorphous Ni-P Thin Films Produced by the Chemical Reduction Method

The paper is focused on the crystallization of Ni-P amorphous thin films produced by a chemical reduction method. The results of differential scanning ca lorimetric (DSC) experiments, Xray diffraction analysis (XRD), scanning (SEM) and transmissi on electron microscopy (TEM) studies of the microstructures are presented. The surface morphology of Ni-P amorphous films can be described in terms of arrays of spherical particles and hole s, related to H2, distributed mostly at their borders. The DSC studies of the films showed two exothermic e ffects at 370 and 440 C. This implies that crystallization proceeds in two stages. First, the metastable crystalline phase Ni 12P5 is formed and then transforms into the equilibrium phases Ni 3P and Ni. After annealing at a temperature below the peak of crystallization, the amorphous structure is visible. However, near the edge of perforations of TEM thin foils, the presence of the nanocryst als were revealed and the holes disappeared. Further research of both phenomena, nanocrystallization and heali g of holes, are in progress.

SEM and TEM Characterization of NiAl2O4 Spinel Phase in Al2O3 Matrix Ni Composite

The microstructure and crystallographic relationship development of spinel phase of the composite prepared by sintering of Al2O3 and Ni powders below the melting point of Ni was investigated. Spinel phase is distributed not uniformly and the outer region of sample contains Al2O3 and NiAl2O4 without Ni particles. The differences in the microstructure between the central and surface part of the sample was clearly demonstrated. In the central part of the sample this process was only initiated and start of this reaction can be observed at the Ni/Al2O3 interface. This distribution of spinel phase is connected with the process of its formation and stability. Analysis of the crystallographic relationship between the Ni and spinel (S) indicates that the most common is the crystallographic relationship [001] S || [001] Ni or [001] S || [111] Ni. Similarly there is some statistical preference of the crystallographic relationship between spinel and Al2O3. In this case more often observed relationship is [100] S || [111] Al2O3, however similarly as in the case of Ni some deviations of several degrees are also frequent.