A. La Barbera

SiC–SiCf CMC manufacturing by hybrid CVI–PIP techniques: process optimisation

SiC–SiCf ceramic matrix composites (CMC) are candidate structural material for fusion power reactor applications because of their favourable thermo-mechanical and low-activation properties. Among their different manufacturing techniques, present, the most employed ones are chemical vapour infiltration (CVI) and polymer infiltration and pyrolysis (PIP). These two techniques are based on the common principle of filling the porosity among the fibres with SiC resulting from precursor decomposition. CVI process deposits high purity crystalline SiC with good properties onto fibres whereas PIP leaves lower characteristic amorphous SiC with traces of oxygen between fibres. PIP, on the other hand, seems to be much more industrially effective than CVI. In the attempt to maximise the properties and reduce costs, some work has been done on the so called ‘hybrid techniques’ in which CVI and PIP are both employed. The work performed by ENEA and FN S.p.A. consists of a series of combined CVI–PIP process cycles and the subsequent product characterisation.

High temperature corrosion of b2 iron aluminides

The high temperature oxidation of two B2-Fe40Al Iron Aluminides has been studied in isothermal and cyclic conditions in the range 700°C to 1100°C and compared with a FeCrAlY alloy. One of the aluminides was prepared by mechanical alloying with the addition of an Y2O3-dispersion that conferred to it improved mechanical properties. No significative differences in the isothermal oxidation of the aluminides have been observed due to the presence of the Y2O3. Both aluminides presented higher oxidation rates than the FeCrAlY alloy at 758° and 858°C but lower rates at 1118°C. The tests carried out under thermal cycling (100 1-hour cycles: 40 minutes at 900°C plus 20 minutes at room temperature) showed that the FeCrAlY alloy presented a lower weight increase than both Iron aluminides. Scanning electron microscopy observations revealed a similar acicular morphology of the thin oxide layer formed on the aluminides. The oxide grown on the FeCrAlY alloy presented nodule formation.

Development of 2D and 3D Hi-Nicalon fibres/SiC matrix composites manufactured by a combined CVI–PIP route

SiCf/SiC composites with true 3D textures were manufactured in order to investigate the effects of the texture on thermal and mechanical properties. Hi-NicalonTM fibre preforms having a total fibre volume content of about 40% with 25 and 50% relative fibre content through the thickness were infiltrated using a mixed chemical vapour infiltration–polymer infiltration and pyrolysis technique. The issues and the main characteristics of the obtained 3D composites are presented and compared with the ones of 2D composites prepared employing the same materials and process.

Deformations induced by high loading ratios in palladium–deuterium compounds

Abstract The strong concentration gradients produced during the loading process in palladium hydride are responsible for the well known difficulty in obtaining high loading ratios (H(D)/Pd≈1). Experiments on electrolytic loading of palladium with deuterium have been carried out in which the loading ratio was monitored through a four wire resistance technique. Preliminary results show that dynamic loading strongly affects the maximum ratio achievable.

Electron beam cladding and alloying of AISI 316 on plain carbon steel: microstructure and electrochemical corrosion behaviour

Abstract Cladding and alloying of AISI 316 stainless steel on plain carbon steel were performed by means of electron beam processing. The electrochemical corrosion behaviour was investigated at different depths from the original surface, in dilute sulphuric acid. The microstructural transformations were studied by scanning electron microscopy and X-ray microanalysis. In the cladding, a duplex structure was obtained with a fine network of delta ferrite precipitated both at grain and at subgrain boundaries; the electrochemical curves were practically identical with those of the parent material. After the alloying, the dillution of the chromium and nickel induced a martensitic transformation with a remarkable increase in the hardness of the resulting layer. The corrosion behaviour was similar to that of an alloy with a lower chromium content.

Study of deuterium charging in palladium by the electrolysis of heavy water: Heat excess production

SummaryAn experiment based on the electrolysis of heavy water with a palladium cathode is reported. The production of excess power during the electrolysis has been measured with the help of a quite accurate and reliable calorimeter. The correlation of the detected power excess with some meaningful parameters of the experiment is investigated. Some of the features of the experiment permit to investigate the dynamics of deuterium in the palladium lattice, which is thought of as the basic phenomenon for excess power production.

Electrochemical activity of lightweight borohydrides in lithium cells

As substitutes for graphite, the negative electrode material commonly used in Li-ion batteries, hydrides have the potential to overcome both safety and performance limits of the current state-of-the-art Li-ion cells. Hydrides can operate through a conversion process proved for some interstitial hydrides like MgH2: MxAy+nLi= xM+yLimA, where m=n/y. Even if far from optimization, outstanding performances were observed, drawing the attention on the whole hydride family. Looking for high capacity systems, lightweight complex metal hydrides, as borohydrides, deserve consideration. Capacities in the order of 2000-4000 mAh/g can be theoretically expected thanks to the very low formula unit weight. Although the potential technological impact of these materials can lead to major breakthroughs in Li-ion batteries, this new research field requires to tackle fundamental issues that are completely unexplored. Our preliminary findings on the incorporation of borohydrides will be here presented.

Intergranular corrosion behaviour of AISI 316 stainless steel electron-beam clad on plain carbon steel

A previous study [1] carried out to characterize the electron-beam cladding and alloying of plain carbon steel containing about 0.4wt% C with AISI 316 stainless steel showed that a surface duplex structure can be obtained from an initially fully austenitic clad material. The interface between the overlayer material and the substrate was characterized by a steep concentration gradient of the stainless steel alloy elements when the electron-beam parameters had been adjusted to clad the surface. In this case the chemical composition of the surface layer as well as the electrochemical corrosion behaviour were found to be practically identical to that of the original AISI 316. It seemed of interest to study the microstructural changes after thermal treatments in the carbide precipitation range and to investigate their effect on the intergranular corrosion resistance of the clad layer. It is well known that thermal ageing of FeCrNi alloys in the temperature range 400-850 °C produces carbide precipitation at grain boundaries [2]. The chromium carbides (mainly Cr23C6) act as a sink for Cr and, as a consequence of the low diffusion coefficient of Cr in austenite, a zone with Cr content lower than the matrix content is created adjacent to the carbide. This Cr-impoverished zone is retained and is responsible for the intergranular corrosion of FeCrNi alloy by the theory of Cr depletion [3]. The generally superior resistance of duplex steels to intergranular corrosion has been known for over 50 years [4]. A series of investigations has demonstrated that the resistance of two-phase austenitic ferritic steels is a strong function of the alloy microstructure [5]. With these premises, the purpose of this letter is to describe the results of a preliminary study on the susceptibility to intergranular corrosion of the AISI 316 clad layer ageing treatments in the caribide precipitation range. The chemical composition of the AISI 316 sheets cladded on C40 carbon steel (0.4 wt % carbon) is shown in Table I. The processing conditions have been described elsewhere [6]. Two heating treatments were performed at 650 °C for 100 and 500 h in a vacuum furnace, followed by air-cooling. Microstructural observations were carried out after the thermal treatments and after the electrochemical tests by means of scanning electron microscopy. Energy-dispersive X-ray (EDX) microanalysis was performed on some precipitates. The susceptibility to intergranular corrosion was evaluated by means of the electrochemical potentiokinetic reactivation (EPR) test following the procedure reported in [7]. The microstructural observations on the clad layer thermally treated at 650 °C for 100 h revealed the presence of Cr-, Tiand Mo-rich carbides, precipitated intragranularly. Fig. 1, obtained after diamond polishing, shows the alpha-phase finely distributed and the discrete intragranular precipitates. Fig. 2a, obtained after nitric acid etching, shows the preferential etching around the intragranular precipitates, most of which have been detached as well as some slight attack on the ferritic phase. Fig. 2b shows some of the precipitates whose composition has been determined by EDX and is reported in Table II.