P. Gondi

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.

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.

X-ray characterization of indium during melting

During melting of Indium the structure of solid and liquid phases have been investigated by X-ray diffractometry (XRD) in 1-g conditions. At the melting point TM a re-orientation of crystalline grains occurs in the solid phase. The texture change, unusually rapid for a thermally activated process, is attributed to an abnormal increase of vacancy concentration. This explanation is in agreement with the observed shifts of XRD peaks towards lower angles. As a consequence of the texture change, the lattice planes facing the first formed liquid are (002) and (101) planes, i.e. those planes allocating 1st and 2nd neighbours around a given atom with shell radii very close to the mean distance of nearest neighbours in liquid as obtained from the radial distribution function (RDF). n nConvective motions in the liquid can be eliminated by repeating the same XRD measurements in μ-g. To get the best experimental conditions it is discussed the possibility to use thin oxide films grown on the external surface of samples as containers during melting. This technique was already successfully tested by present investigators in the experiment ES 311 A-B carried out during the mission SPACELAB-1.

Distribution of C–Cr associates and mechanical stability of Cr martensitic steels

Structural and mechanical stability of two martensitic steels with different Cr content (MANET and modified F82H) has been studied by means of internal friction (IF) and dynamic modulus (Md) measurements, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) observations with EDS microanalysis and mechanical tests (hardness, Charpy). Following thermal treatments at 700°C, MANET samples cooled from the austenitic field at a rate of 150°C/min, exhibit Cr segregation both inside the grains and in the zones near grain boundaries. The Cr segregation induces internal stresses, which influence the mechanical properties, in particular the fracture mode, ductile–brittle transition temperature (DBTT) and upper shelf energy (USE). The material is not stable: DBTT changes depending on the time of the treatment and after 20 h at 700°C a mixed fracture mode (quasi-cleavage plus intercrystalline) is observed. Cr segregation is very weak in modified F82H steel submitted to the same treatments and a greater mechanical stability has been observed. The different behaviour of MANET and modified F82H is discussed on the basis of IF and Md results, which show that the stability of the distribution of C–Cr associates in as-quenched materials is a factor of great importance to avoid the Cr segregation.

Internal strains after recovery of hardness in tempered martensitic steels for fusion reactors

Abstract After tempering, with recovery of hardness, MANET steels present internal strains; these residual strains increase with quenching rate prior to tempering, and they remain after prolonged tempering times. On account of their persistence, after thermal treatments which lead to low dislocation and sub-boundary densities, the possibility has been considered that the high swelling resistance of MANET is connected with these centres of strain, probably connected with the formation, in ferrite, of Cr-enriched and contiguous Cr-depleted zones which may act as sinks for interstitials. Comparative observations on the internal strain behaviour of cold worked 316L stainless steel appear consistent with this possibility.

Structure evolution during heat treatments of 12% Cr martensitic steel for net

Abstract The structure evolution of martensitic 12% Cr steel has been followed in the range from room temperature up to 600°C by means of X-ray diffraction with analyses of the grain textures and of the diffraction line profiles, together with observations in optical and scanning electron microscopy and with hardness measurements. The behaviour of hardness appears referable to two stages in the transformation of martensite to ferrite, involving subsequent disappearance of martensite laths of different thickness and characterized by specific texture variations. Carbide precipitation in the ferritic volumes appears determining for the hardness change above 400°C, with concomitant effects on the hardness drop above 500°C depending on internal stress, dislocation density decreases. Some effects referable to elements with large diffusivities are also indicated.

Deformation at very high strain rates of Al and ERGAL 7075

SummaryComparative examinations have been carried out on Al and ERGAL specimens deformed by compression either fast or slow. The high-strain-rate compressions were obtained by explosive. The dislocation structures have been examined by transmission electron microscopy and by Fourier analysis of the X-ray diffraction lines. Crystalline textures were also determined. After fast deformation smaller interaction energies appear among the dislocations with comparable total densities because of the accumulation in grain boundaries. The results have been interpreted by assuming that the loads which are reached in the shock and reflected wave fronts give rise to generations or multiplications of the dislocations on levels more differentiated than by slow compression.RiassuntoSono stati condotti esami comparativi su campioni di Al e di ERGAL deformati mediante compressione lenta o veloce. Le compressioni ad elevate velocità sono state ottenute per mezzo di esplosive. Le strutture delle dislocazioni sono state osservate con microscopia elettronica in trasmissione e analisi di Fourier delle righe di diffrazione dei raggi X. Sono state inoltre determinate le tessiture cristalline. Con densità totali di dislocazioni comparabili, dopo deformazione rapida appaiono minori energie dinterazione tra le dislocazioni stesse a causa di fenomeni di accumulo nei bordi di grano. I risultati sono stati interpretati ammettendo che i carichi raggiunti in corrispondenza delle onde durto, diretta e riflesse, diano luogo a generazione e moltiplicazione di dislocazioni su livelli piú differenziati che con compressione lenta.РезюмеПроводится сравнительный анализ образцов Al и ERGAL, подвергнутых деформации сжатия либо быстро, либо, медленно. Высокая скорость деформации сжатия получается в результате взрыва. Исследуется структура дислокаций с помощью трансмисионной электронной микроскопии и с помощью Фурье-анализа линий дифракции рентгеновских лучей. Определяются кристаллические структуры. После быстрой деформации возникают небольшие энергии взаимодействия между дислокациями с соизмеримыми полными плотностями, вследствие явления аккумуляции на границах зерен. Проводится интерпретация полученных результатов, предполагая, что нагрузки приводят к образованию или размножению дислокаций на уровнях более дифференцированных, чем в случае медленного сжатия.

Effects of thermal treatments on the ductile to brittle transition of MANET steel

The effect of the cooling rate T from austenitic field on the microstructural evolution and the mechanical stability of MANET steel for increasing times of tempering at 700 degrees C has been studied by means of Charpy tests, SEM observations with EDS microanalysis and X-ray diffraction analysis. The investigated rates T range from 150 to 3600 degrees C/min. The ductile to brittle transition temperature, the upper shelf energy and the slope a Delta E/Delta T in the transition region depend on the time of treatment with variations more pronounced in samples slowly cooled. After tempering times of about 20 h at 700 degrees C the samples cooled with T = 150 degrees C/min show a mixed mode of brittle fracture (quasi-cleavage and intercrystalline) with good correspondence between Cr-enriched zones and surfaces fractured in intercrystalline mode. Only quasi-cleavage mechanism is operative in samples quenched with T = 3600 degrees C/min, which present fracture surfaces with uniform Cr distribution. The results are discussed on the basis of C-Cr associate distributions determined by internal friction tests.

MANET steel : thermal treatments and Q-1 spectrum evolution

Abstract The internal friction spectrum of as-quenched MANET steel (10.5 Cr, 0.17 C, 0.003 N wt%) and its evolution following thermal treatments at increasing temperatures up to 773 K has been studied on the basis of the processes involving interactions between interstitial atoms (C and N) and substitutional Cr atoms in solid solution. The analysis of Q−1 curves shows that the successive stages of heat treatment cause complex redistributions of C atoms between interstices with a different number n of neighbour Cr atoms (0 ≤ n ≤ 6) and variations of the total amount of C in solid solution. The relaxation processes involving N have also been examined.

On the statistical distribution of Cr atoms in FeCr alloys with high swelling resistance in NFR

Abstract The distribution of chromium and carbon atoms in 9% and 11% Cr martensitic steels, quenched and then submitted to different annealing treatments, has been investigated by means of X-ray diffraction (XRD) analysis, microhardness tests, internal friction (IF) and dynamic modulus (Md) measurements. Results are analyzed on the basis of the concentrations of carbon atoms remaining in solution after the various stages of carbide precipitation.