S. Kudela

The δ-Al2O3 (saffil) fibre degradation during infiltration with MgLi alloy

The chemical and phase transformations of δ-Al2O3 (Saffil) fibres during their infiltration with Mg-8 wt% Li were studied by scanning and transmission electron microscopy, Auger electron spectroscopy, secondary ion mass spectrometry and X-ray diffraction methods. The infiltration experiments were carried out in autoclave under argon pressure at temperatures of 883–908 K and contact times of 4–30 s as well as at 918 K/420 s. During the course of infiltration, lithium penetrates the Saffil fibres and this process is accompanied by the gradual transformation of the tetragonal δ-Al2O3 lattice towards the cubic spinel LiAl5O8 compound, where part of the Li+ ions is probably substituted by Mg2+ No remarkable interfacial zone at the fibre/matrix interface was observed; however, the Saffil fibres became brittle which had been manifested by the occurrence of fragmentation on the metallographically treated fibre cross-sections. The tensile strength (maximum 220 MPa) of the corresponding metal matrix composite clearly decreased with increased infiltration time.

Phase transformations of δAl2O3 (Saffil) fibres during their interaction with molten MgLi alloys

The phase transformation of δAl2O3 occurring in Saffil fibres during their infiltration with molten Mg-8 wt% Li alloy was studied by secondary ion mass spectroscopy, X-ray diffraction and infrared spectroscopy methods. It has been shown, that lithium penetrates very quickly into the whole fibre volume, attaining up to Li/Al ≈ 0.25–0.30 ion ratio. The metastable spinel-like compound, γ(Li), was formed by incorporation of Li+ ions into the δAl2O3 lattice in which the basic spinel structure unit has been assigned by the formula Al8 [Al(40−x)/3□(8−2x)/3]Lix]O32. During long-term annealing, a further transformation γ(Li) → LiAl5O8 proceeded, and LiAlO2 aluminate was also identified in Saffil fibres with high Li/Al concentration ratio values. In parallel with lithium, magnesium also penetrated the Saffil fibres within an infiltration period; however, the incorporation of magnesium into the spinel lattice has not been observed.

Fracture behavior of Mg–Li matrix composites

Abstract Deformation and fracture of Saffil fiber (SF) reinforced Mg, Mg-8 wt.% Li and Mg-12 wt.% Li composites was monitored by means of in-situ scanning electron microscopy during 3-point bend tests. Mg and MgLi matrix composites (10 vol.% SF) were prepared by the melt infiltration technique. Poor interfacial bond causes premature failure of the SF/Mg composite that is usually triggered by the debonding of perpendicular fibers at the convex composite surface. Strong interfaces in SF/Mg12Li composite result in multiple cross-breakage of aligned fibers which is indicative of their participation in the composite strengthening. Resultant composite strengthening is frustrated by massive stress relaxation of the Mg12Li matrix. Strong interfaces and fairly high strength of the Mg8Li matrix cause remarkable strengthening of SF/Mg8Li composites. The cleavage of phases in two-phase Mg8Li matrix plays an insignificant role in the composite failure.

TEM study of the fibre cross-section attack in δ-Al2O3/Mg8Li metal matrix composites

During manufacturing of δ-Al2O3/Mg8Li composites by melt infiltration reactive diffusion of Mg and Li into the fibre bulk takes place. No remarkable reaction zone was found at the fibre periphery, although, a Mg enrichment zone (approx. 100 nm thick) was detected there by EDX and EELS. In addition, MgO, Li2O and Mg2Si minority phases were identified by SAED within the fibre cross-section.The crystalline structure of the δ-Al2O3 fibres in the Mg8Li matrix remained of the alumina spinel type at all the time, however, a set of crystallographically coherent products assigned as γ(Li) was detected by XRD and SAED within the fibre cross-section. As believed, the γ(Li) results from topotactic Li+ incorporation into the δ-Al2O3 lattice so that no significant changes in terms of size and morphology of the alumina crystallites are observed.

Compatibility between PAN-based carbon fibres and Mg8Li alloy during the pressure infiltration process

The interaction of PAN-based T800H type carbon fibres, which had been coated with (i) pyrocarbon and (ii) pyrocarbon+SiC protective layers, was studied by means of SEM, energy-dispersive X-ray analysis and Auger electron spectroscopy. The interaction occurred during the pressure infiltration process at temperatures from 898 to 908 K and at contact times of 4–30 s. As detected by AES measurements, both Mg and Li penetrated throughout the carbon fibres, and Li2C2 was formed without remarkable reaction zone occurrence, which led to carbon fibre degradation. The pyrocarbon layer (about 50 nm thick) demonstrated a good protective efficiency and the corresponding metal-matrix composite exhibited satisfying strength characteristics; nevertheless, the influence of processing variables (temperature, time) was obvious. On the other hand, the double-coated pyrocarbon+SiC carbon fibres were strongly affected by Li2C2 formation.

Mechanoacoustic research method of degradation processes in electroceramics materials

The paper presents the concept of mechanoacoustic testing of degradation processes of ceramic materials, which is applied to diagnose high voltage insulators in operation. The method depends on the application of slowly increasing compressive loading acting on the sample, with simultaneous recording of acoustic emission (AE) descriptors. The process of loading is continued to the destruction or stopped at different stresses, and the samples are subjected to microscopic examination. Microscopic analysis of the samples enables determining the effects of stress action. The results were compared with the images of similar materials obtained from the insulators after different periods of operation. On this basis, there were distinguished consecutive stages of ageing of electrotechnical porcelain materials and corresponding effects of their structure degradation. Using slowly increasing quasi-static compressive loading in a relatively short-term mechanoacoustic test makes it possible to get results similar to those of long lasting degradation effects in operated electroinsulating objects. Using this method there is possible to describe the factors which affect short and long-term mechanical strength of the tested materials. There is also possible assessment of the validity and application of the theories explaining the strength of porcelains. There was presented research of the influence of the mullite phase on the short- and long-term mechanical strength of electrotechnical porcelain of different types.

Saffil alumina fibers reinforced dual-phase Mg-Li and Mg-Li-Zn alloys

The gas pressure infiltration technique was used to prepare Saffil alumina fibers reinforced Mg-Li and Mg-Li-Zn matrix composites with a dual-phase matrix structure. There was investigated the effect of variable Li content (6.2–10.3 wt.% Li) and Zn alloying (∼ 1.5 wt.% Zn) on the proof stress Rp0.2 of prepared composites. Rp0.2 values increased monotonously with rising fraction of Saffil fibers (5, 10 and 15 vol.%) reaching the maximum of about 250 MPa for Mg-Li matrix composites. Rp0.2 values of Mg-Li-Zn matrix composites were lower. Strengthening effect of Saffil fibers was promoted by the displacement redox reaction with Mg-Li and Mg-Li-Zn melts in which only Li significantly participated. Zn alloying retarded the displacement redox reaction. Too extensive reaction, however, resulted in the fiber damage and the drop in composite strength. K e y w o r d s: Mg-Li alloys, Saffil fibers, metal matrix composites, short-fiber strengthening, reactive wetting, displacement reactions

Ni-NiO Porous Preform with Controlled Porosity Using Al 2 O 3 Powder

The Ni-NiO skeleton seems to be a good candidate for various applications in industry such as corrosion-proof filters or components in refrigerating systems and as preforms for reactive infiltration with molten metals.The present work was focused on preparation of Ni-NiO composite with higher, controlled porosity. Sintering of pure Ni powder always leads to a substantial closed porosity in almost whole sample volume [1,2]. To eliminate this, we added Al2O3 particles with diameter of-32 +20 μm into the Ni powder (-75 +45 μm diameters) and sintered this loose powder mixture (Ni + 25 vol. % Al2O3) in air by progressive heating up to 800 °C followed by 2 hours isothermal exposure. As a control, pure Ni powder was sintered under the same conditions. Thermal oxidation of loose powder samples performed in alumina crucible indicates that the strongest oxidation occurred in the top part of sample, while the bottom part was the least oxidized. Therefore, it was necessary to run the thermal oxidation once more, but out of the crucible, to ensure the sufficient diffusion of oxygen to the whole volume of sample.

Mechanisms of plastic instability and fracture of compressed and tensile tested Mg-Li alloys investigated using the acoustic emission method

The results of the investigation of both mechanical and acoustic emission (AE) behaviors of Mg4Li5Al alloy subjected to compression and tensile tests at room temperature are compared with the test results obtained using the same alloy and loading scheme but at elevated temperatures. The main aim of the paper is to investigate, to determine and to explain the possible influence of factors related with enhanced internal stresses such as: segregation of precipitates along grain boundaries or solute atoms along dislocations (Cottrell atmospheres) or dislocation pile-ups at grain boundaries which create very high stress concentration leading to fracture. The results show that the plastic instabilities are related to the Portevin–Le Châtelier phenomenon (PL effect) and they are correlated with the generation of AE peaks. The fractography of breaking samples was analyzed on the basis of light (optical), TEM and SEM images.

Microstructure and Properties of Composites Prepared by Reactive Pressure Infiltration of Aluminium into Metal and Ceramic Powder Preforms

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.