R. Montanari

Characterisation of plasma sprayed W coatings on a CuCrZr alloy for nuclear fusion reactor applications

Abstract Mock-ups of thermal shields for international thermonuclear experimental reactor (ITER) have been manufactured by depositing 5-mm-thick W coatings on a CuCrZr alloy (ELBRODUR®). Coatings, made by controlled atmosphere plasma spray technique, have low porosity and excellent thermal fatigue resistance under high heat flux. Their structure has been investigated by scanning electron microscopy (SEM), scanning tunnel microscopy (STM), energy dispersion spectroscopy (EDS) and inductively coupled plasma (ICP) analysis. In spite of the remarkable thermo-mechanical properties of coatings, the present work evidenced some flaws (small cracks, unmolten W particles and presence of Ni in the external layers). Quality of coatings may be improved by strictly controlling purity and granulometry of sprayed W powders and some process parameters.

Synthesis and Structural Characterization of Trimetallic Perovskite-Type Oxides, LaFexCo1−xO3, by the Thermal Decomposition of Cyano Complexes, La[FexCo1−x(CN6)]·nH2O

Abstract Heteronuclear complexes in the series La[FexCo1−x(CN6)]·nH2O, with x = 0 ∼ 1 were prepared. Some complexes consisted of a mixture of hexagonal and orthorhombic phases, due to the presence of different amounts of crystallization water molecules. The lattice constants estimated for the orthorhombic structures increased linearly with increasing Fe content, x. The formation of the perovskite-type oxides by the thermal decomposition of the complexes has been studied by simultaneous thermogravimetric and differential thermal analysis (TG/DTA), X-ray fluorescence (XRF), and X-ray diffraction (XRD). The thermal decomposition of the complexes caused the formation of a single perovskitic phase at different temperatures, depending on the Fe content. Orthorhombic LaFeO3 and hexagonal LaCoO3 were already obtained at about 600°C from the decomposition of the corresponding complexes. For the complexes containing both Fe and Co, the elemental analysis showed that the Fe content was always lower than the prescribed ratios. The single perovskitic phase was formed with increasing temperature as the prescribed ratios of Fe and Co became more similar, the maximum being for x = 0.5; at 1000°C, a single hexagonal phase was formed for x ≤ 0.35, and a single orthorhombic phase was formed for x ≥ 0.44, both being the measured values.

A remotely operated FIMEC apparatus for the mechanical characterization of neutron irradiated materials

Abstract From Flat-top Cylinder Indenter for Mechanical Characterization (FIMEC) test yield stress and tensile strength can be obtained. Results regarding different materials, most of them of fusion interest, show the general validity of the method. Moreover, indication on the ductile to brittle transition temperature (DBTT) of martensitic steels has been drawn by performing tests at different temperatures. FIMEC offers the possibility to perform several tests on a small volume of material, e.g. it has been estimated that 18 indentations can be made on a single disk (Φ=25 mm, h=5 mm). For this reason, its application is of interest to characterize materials irradiated in the future IFMIF or in other sources with a limited irradiation volume. A remotely operated apparatus has been designed and costructed to work in hot cell on irradiated samples. It employs a WC punch (Φ=1 mm) and can operate at temperatures in the range between −180°C and +200°C. Details of the apparatus are presented.

Artificial neural networks to optimize the extrusion of an aluminium alloy

Extrusion of aluminium alloys is a complex process which depends on the characteristics of the material and on the process parameters (initial billet temperature, extrusion ratio, friction at the interfaces, die geometry etc.). The temperature profile at the die exit, largely influences microstructure, mechanical properties, and surface quality of an extruded product, consequently it is the most important parameter for controlling the process. In turn the temperature profile depends on other process variables whose right choice is fundamental to avoid surface damage of the extruded product. In the present work, two neural networks were implemented to optimize the aluminium extrusion process determining the temperature profile of an Al 6060 alloy (UNI 9006/1) at the exit of induction heater (ANN1) and at the exit of the die (ANN2). The three-layer neural networks with Levemberg Marquardt algorithm were trained with the experimental data from the industrial process. The temperature profiles, predicted by the neural network, closely agree with experimental values.

Structural and mechanical properties of welded joints of reduced activation martensitic steels

Abstract Gas tungsten arc welding and electron beam welding methods were used to realise welding pools on plates of reduced activation martensitic steels. Structural and mechanical features of these simulated joints have been investigated in as-welded and post-welding heat-treated conditions. The research allowed to assess how each welding technique affects the original mechanical properties of materials and to find suitable post-welding heat treatments. This paper reports results from experimental activities on BATMAN II and F82H mod. steels carried out in the frame of the European Blanket Project – Structural Materials Program.

Martensite formation during heat treatments of AISI 304 steel with biphasic structure

Abstract X-ray diffraction (XRD) and Mossbauer spectroscopy (MS) measurements were used to evaluate the content of α′ ferromagnetic phase in AISI 304 steel cold rolled and then heat treated for increasing times (up to 7.2×104 s) at 673 K. Both analytical techniques show the same trend of α′ content versus annealing time, characterized by two maxima. The first maximum has been attributed to the growth of preexisting martensitic zones due a different microstress relaxation in the two phases, which induces a stress gradient through the α′/γ interface forcing it to move. The second maximum was explained assuming that local composition variations were induced by carbide precipitation and that a new martensitic phase may form during cooling to room temperature. MS data, i.e. the increase of average hyperfine field of magnetic fraction and the constancy of austenite isomer shift, and the analysis of precision XRD peak profiles support this assumption. Furthermore, MS measurements evidenced in cold rolled samples the presence of layers with decreasing martensite content towards the inside.

Small-scale nondestructive stress-strain and creep tests feasible during irradiation

Abstract By the use of suitable cylindrical indenters with flat surface and constant penetration rates, load versus penetration depth curves are obtained which show correspondence with the stress-strain curves of the tensile tests for some materials examined. After an initial linear stage, a limit load of σU is reached after which a stage of work-hardening occurs with a trend comparable to the one of the tensile test. The limit load σ U ⋟ 3σ y , σ y being the yielding load of the tensile test. For penetration rates = 100 μm/min, the loads of the work-hadening stage above σU tend to saturation values comparable to the loads of hardness only for some of the materials examined. With larger penetration rates, loads are reached well in excess above those of hardness. With metal-matrix composite alloys a stage with drops of load has been observed at the level of the loads of hardness. Besides those with constant penetration rates, tests with constant load have been also made on the same materials, with the results interpreted on the basis of the penetration rate effects.

Mechanical characterisation of fusion materials by indentation test

FIMEC is an indentation test, which permits the evaluation of yield and ultimate tensile stress and to draw indication about the ductile to brittle transition temperature. The apparatus has been recently implemented with a feedback system to maintain strictly constant the penetration speed of cylindrical punch during the test. So, experiments performed on several materials, as those for first wall and blanket, show that the scattering between data from FIMEC and from standard tensile test is reduced within the range ±0.06, i.e. it is comparable with the scattering obtained in different tensile tests on the same material. A numerical simulation has been carried out to understand the basic mechanism of the process.

A miniaturized test method for the mechanical characterization of structural materials for fusion reactors

Abstract This work deals with a non-destructive method for mechanical tests which is based on the indentation of materials at a constant rate by means of a cylinder with a small radius and penetrating flat surface. The load versus penetration depth curves obtained using this method have shown correspondences with those of tensile tests and have given indications about the mechanical properties on a reduced scale. In this work penetration tests have been carried out on various kinds of Cr martensitic steels (MANET-2, BATMAN and modified F82H) which are of interest for first wall and structural applications in future fusion reactors. The load versus penetration depth curves have been examined with reference to data obtained in tensile tests and to microhardness measurements. Penetration tests have been performed at various temperature (from −180 to 100°C). Conclusions, which can be drawn for the ductile to brittle transition, are discussed for MANET-2 steel. Preliminary results obtained on BATMAN and modified F82H steels are reported. The characteristics of the indenter imprints have been studied by scanning electron microscopy.

Effect of powder mix composition on Al foam morphology

The effect of mix composition on foam morphology has been examined by image analysis carried out on metallographic sections of Al foams prepared by powder metallurgy. Two sets of samples have been prepared by using SiC particles with mean sizes of 37 and 60 µm. Each set consists of 16 groups of samples with different amounts of TiH2 (0.1, 0.2, 0.4, and 0.6 wt%) and SiC (0.8, 2.8, 6, and 9 wt%). The distribution of SiC particles on the internal walls of the bubbles has been investigated by scanning electron microscopy observations, which evidenced also the presence of particles of another phase, identified as Ti3Al by energy dispersion spectroscopy. Some tests, performed without SiC particles, showed that the Al foaming occurs also under these conditions, however foams exhibit few bubbles of very large size and irregular shape. Experimental data have been used for training two multi-layer feedforward artificial neural networks. The models represent useful tools for predicting morphologic features of foams for any given mix composition in the training range.