Materials Science Forum Vol. 782

Abstract: A simple oxidative polymerization of pyrrole (Py) directly onto the surface of iron (Fe) microparticles was applied to increase the content of carbon in resulting material. Detection and quantification of the PPy (polypyrrole) coatings obtained were performed by means of pyrolysis gas chromatography (Py-GC). A powder consisting of such particles was compacted. The effect of PPy coating on the compressibility of coated iron powder was analysed. Namely, a set of specimens was uniaxially pressed in a steel die. Compaction pressures ranged from 50 MPa up to 600 MPa. It was found that PPy coating has a positive effect on the compaction behaviour of iron powders in the low to moderate pressure region. At higher pressures, the brittleness of PPy coating adversely affected the compressibility. Both the light and the scanning electron microscopy (LM, SEM) were used to characterize the morphology of coated powders and the microstructure of pressed samples.

Abstract: The commercial carbonyl iron powder coated with iron phosphate (20 wt.%) was dried (60°C for 2 h in air), calcinated at 400°C for 3 h in air, compacted at 600 MPa into cylindrical samples and subsequently sintered at 820, 900 and 1110°C for 30 min in N₂-10%H₂ atmosphere. By means of EDX and XRD analyses the phase composition of the coating and sintered microstructure was studied. Microstructure resulting from sintering at 820 and 900°C was formed by initial iron particles surrounded with the crystalline FePO₄ and α-Fe₂O₃ phases. Due to liquid phase sintering at 1110°C a mixed microstructure containing spheroidized α-Fe phase surrounded by solidified liquid phase consisting of iron oxides and phosphorous compounds has been formed. In order to prepare a network composite microstructure the compacts based on spherical iron particles size of 100-160 µm coated with 2 wt.% of iron phosphate were dried, calcined at 400°C, compacted and liquid phase sintered at 980°C.

Abstract: Iron was considered a good material candidate for temporary implants in cardiovascular and orthopedic surgery. Mechanical properties of iron are attractive, however, a higher degradation rate is required. The contribution deals with the effect of silver content on microstructure and corrosion behavior of materials prepared from Ag coated iron powders. Using electroless deposition, Fe-powders with 0.29 and 2.1 wt.% of silver were prepared. Cylindrical specimens compacted at a pressure of 200 MPa were isothermally sintered at 1120°C for 60 min. The microstructure of the sintered specimens consisted of iron matrix with Ag-precipitates. The corrosion behaviour of sintered compacts was studied using the potentiodynamic polarization technique in Hank’s solution and complemented with SEM analysis. It was found out that corrosion resistance of material decreased with an increase in silver content.

Abstract: A La_0.67Ca_0.33-xSr_xMnO₃ (x=0.33; 0.03; 0) perovskite magnetocaloric powders were prepared by solid state synthesis in air. Phase transformations were characterized by thermal analyses and the phase composition was confirmed by X-ray powder diffraction. Pressed pellets were sintered at different temperatures. Microstructure analyses were done by polarized light microscopy and scanning electron microscopy. The influence of sintering conditions on porosity, grain size and content of secondary phases is shown.

Abstract: Abstract: Reactions during compaction of Mo/Mo silicide wires with Ni interlayers are qualitatively assessed in this work. It appeared that due to extreme high temperature strength of MoSi₂ hot pressing even at 1800°C/60 min/30 MPa in vacuum had not been sufficient to compact the Mo/Mo silicide wires in the absence of any additional interfacial layer. Therefore Ni had been chemically coated on the surface of Mo/Mo silicide wires that were subsequently compacted by hot pressing. Structural analysis revealed the reaction between Ni and MoSi₂ resulting in the formation of ternary (MoNiSi) compounds. These established an interfacial bonding with minimal porosity.

Abstract: Tungsten as refractory material and high thermal conductive carbon fibres are promising candidates for production of copper matrix composites for high temperature applications. Three types of rod-like samples were prepared by gas pressure infiltration of different carbon/tungsten fibre preforms with copper and/or copper alloy (Cu-0.5Cr) respectively. The fibres are aligned parallel to rod axis and were combined with the tungsten wire cloth. The microstructure of prepared hybrid composites was examined. The samples were thermally cycled 3 times up to 550 °C at a relatively high heating/cooling rate (10 K/min) to touch real condition in applications where high heat is formed during short time. The thermal expansion behaviour in radial direction was also analysed. Results show that a combination of both types of reinforcements in rod-shapes samples insures good protection against composite disintegration during high temperature thermal loading.

Abstract: The thermal expansion behaviour of Cu-1Cr/C composite subjected to 5 thermal cycles in the temperature range 30 - 1000 °C was investigated. The coefficients of thermal expansions (CTEs) as low as 0.7 x 10^-6 K^-1 in longitudinal and as large as 24.0 x 10^-6 K^-1 in transversal direction were obtained. Electron microscopy observations confirmed the high structural stability of the thermally cycled composite as no signs of disintegration were observed within the applied thermal cycling conditions.

Abstract: Nickel aluminides exhibit very attractive high temperature properties. However, due to high melting temperatures they are difficult to prepare. Gas pressure reactive infiltration is a relatively cheap technology that provides composites where nickel aluminides are formed due to mutual reaction between Ni powder and molten aluminium forced to penetrate into powder preform. The feasibility of this concept is demonstrated in this work. Ni powder and/or Ni+25 vol. % Al₂O₃ powder mixture, respectively, were mechanically pressed and then infiltrated with aluminium using 5 MPa argon gas pressure at the temperature of 750 °C for 120 s. Al/Al₂O₃ composite using loose alumina powder was prepared in similar manner for comparison. The microstructure of composites was observed by scanning electron microscopy and newly formed intermetallic phases were analysed by energy-dispersive X-ray spectroscopy. Relative elongations during additional thermal cycling up to 800 °C had been recorded. Composites were additionally characterized by hardness measurements.

Abstract: Compacts for the synthesis of composite zones in castings were obtained by cold pressing powders of the TiC reactants under a pressure of 250, 300, 500 and 600 MPa. The all compacts made under different pressures were placed in a mould cavity and poured with liquid unalloyed low-carbon steel. From the resulting casting, four composite zone A, B, C, D, produced in this casting by in situ method. In all composite zones, TiC and ferrite (αFe) were obtained. Additionally, in zones A, B and C the presence of graphite (C_gr) was also stated. The surface friction (S_f) of C_gr decreases in composite zones A ÷ D, while both the S_f of TiC and hardness VH30 increase in these zones with increasing compaction pressure of the reactants. Too low compaction pressure applied to the TiC reactants impedes the effective propagation of the reaction of synthesis.

Abstract: Fe/SiO₂/shellac composites based on iron powder consisting of irregularly and/or spherical shaped particles coated with SiO₂ layer or with admixed SiO₂ powder were prepared either by mixing the Fe-SiO₂ powder with shellac or by vacuum/pressure impregnation (VPI) of sintered Fe-SiO₂ compacts with shellac dissolved in ethanol. The effect of iron particle shape, density of compacts and method of shellac application on electrical resistivity and magnetic coercivity was evaluated and correlated with microstructure and features of insulating layer. For VPI composites based on spherical particles a uniformly thick insulating layer was created. This resulted in the electrical resistivity and coercivity of 780-1120 μΩ.m and 177-290 A/m. For composites based on irregularly shaped particles the shellac admixing is more preferable.