Papers by Author: Nadia Ucciardello

Abstract: Tungsten is a promising armour material for plasma facing components of nuclear fusion reactors. Two materials with different density and purity have been examined by optical microscopy, X-ray diffraction (XRD), instrumented indentation tests (FIMEC) and mechanical spectroscopy. For both the materials yield stress and elastic modulus strictly depend on the residual porosity. Moreover, the material with higher porosity (≈ 9%) is not stable and remarkable modulus variations are observed during heating. The IF spectrum exhibits a relaxation Q^-1 peak superimposed to an exponentially increasing background. The peak is a single Debye peak with activation energy H = 74.86 kJ mol^-1 and pre-exponential factor τ₀ = 1.76 x 10^-9 s that has been ascribed to dislocation interaction with intrinsic point defects (autointerstitial and substitutional).

Abstract: The paper reports the results of an extensive characterization of the Ti6Al4V-SiC_f composite produced by hot isostatic pressing (HIP) to assess its capability to withstand the in-service conditions of turbine blades operating at middle temperatures in aeronautical engines. The microstructure of composite, in as-fabricated condition and after long-term heat treatments (up to 1,000 hours) in the temperature range 673-873 K, has been investigated by means of different techniques. Particular attention was paid to the micro-chemical evolution of fibre-matrix interface which is scarcely affected also by the most severe heat treatments examined here. This leads to stable mechanical properties as evidenced by hardness, tensile and FIMEC instrumented indentation tests. Therefore, the composite can operate at the maximum temperature (873 K) foreseen for its aeronautical applications without remarkable modifications of its microstructure and degradation of mechanical properties. The mechanical characterization has been completed by internal friction and dynamic modulus measurements carried out both at constant and increasing temperature, from 80 to 1173 K.

Abstract: The discontinuous precipitation of a high-nitrogen (0.8 wt%) austenitic steel has been investigated after successive steps of heat treatment at two different temperatures (800 and 850 °C). After each step of heating the material has been examined by X-ray diffraction (XRD), optical microscopy (OM), transmission electron microscopy (TEM), Auger electron spectroscopy (AES) and microhardness tests. The precipitation of Cr2N induces the formation of a secondary austenitic phase, leads to the redistribution of N between transformed and untransformed zones and to local variations of mechanical properties.

Abstract: The paper briefly examines the metallurgical effects of shock waves on different metals. Two points are then specifically addressed. The first one regards how X-ray diffraction (XRD) can be usefully employed to get exhaustive information about the microstructure of a shock loaded metal. The second point concerns the mechanisms of martensitic transformation in alloys such as AISI 304 when submitted to repeated explosive deformations.

Abstract: Roll Diffusion Bonding (RDB) is a new process, developed at C.S.M., for producing Ti composites reinforced by long fibres. The prototypal “diffusion bonding” plant permits to co-roll at high temperature in superplastic rolling field (under temperature and strain rate control) foils of titanium alloy and fabrics made of SiC monofilaments. This study evidenced that the Ti6Al4V-SiCf composite produced by roll-bonding exhibits superior mechanical properties with respect the same material prepared by Hot Isostatic Pressing (HIP) owing to the smaller grain size and the higher dislocation density.

Abstract: Internal friction (IF) and dynamic modulus measurements on a high nitrogen (0.8 wt%) austenitic steel in the temperature range from room temperature to 800 °C have been carried out by using a vibrating reed technique with electrostatic excitation and frequency modulation detection of flexural vibrations in the frequency range of kHz. The IF spectrum of the as-prepared material shows a broad peak superimposed to an exponentially increasing background. The discontinuous precipitation of Cr2N phase changes the characteristics of the peak. The results have been discussed by considering interstitial-substitutional (i-s) interactions.

Abstract: The composite, consisting of Ti6Al4V matrix reinforced by unidirectional SiC fibres (SCS-6), has been investigated by mechanical spectroscopy at temperatures up to 1,173 K. For comparison, the same experiments have been performed on the corresponding monolithic alloy. The internal friction (IF) spectrum of the composite exhibits a new relaxation peak superimposed to an exponentially increasing background. This peak, which is not present in the monolithic alloy, has an activation energy H = 186 kJ mol-1 and a relaxation time 0 = 2.3 x 10-15 s. The phenomenon has been attributed to a reorientation of interstitial-substitutional pairs in the  phase of Ti6Al4V matrix around the fibres. This explanation is supported by the results of micro-chemical characterization carried out by X-ray photoelectron spectroscopy (XPS) combined with Ar ion sputtering.

Abstract: Spark Plasma Sintering (SPS) of nanostructured FeMo powder produces samples with satisfactory density, however the final grain size critically depends on the sintering temperature. Two groups (sets A and B) of samples have been examined by means of internal friction (IF) and dynamic modulus measurements carried out in successive test runs on the same samples to assess their structural stability. Set A and B had been sintered at 1113 and 1128 K and had an average grain size of 100 nm and 1 µm, respectively. TEM and XRD have been performed on the samples in as-prepared condition and after IF measurements cycles. The samples with smaller grains are more stable and substantially are not affected by grain coarsening which, on the contrary, occurs in those with grains of larger size. The heating up to 923 K during the tests diminishes dislocation density in both the groups. An anomalous trend of resonance frequency during the first test run in samples of set A has been ascribed to the formation of small cracks relaxing internal stresses.

Abstract: In this paper a neural network approach is used to model the diode laser assisted forming process. In particular thin sheets of Aluminum alloy AA 6082 were bended in the elastic range and then treated with a diode laser with the aim to reduce the spring back phenomenon. Experimental tests were performed to study the influence of the process parameters such as laser power, laser speed and starting elastic deformation on the evolution of forming process. In particular the heating effects on the elastic properties of the material was studied. A statistical approach is used to define the experimental plan and discuss the experimental results. Interesting trend of the effects of the diode laser on the forming process were found. Subsequently in order to predict the residual inflexion, during the laser forming, a multilayer feedforward artificial neural network has been implemented. A sensitivity analysis on the artificial neural network model is used to show the significance of all the input data employed. As a result of sensitivity analysis, a check between experimental and calculated trends for each investigated variables was performed, which revealed an appreciable fit between data displayed.