Papers by Author: Marcello Cabibbo

Abstract: The properties of anodized aluminum, and wear resistance in particular, are of high interest for the scientific community. In this study, discs of AA6082 were subjected to a peculiar hard anodizing process leading to anodized samples having different thicknesses. In order to investigate the wear mechanism of samples, unidirectional tribological tests were performed against alumina balls (corundum) under different loading conditions. Surface and microstructure of all the samples were characterized before and after the tribological tests, using different characterization techniques. The tribological tests showed remarkable differences in the friction coefficient and wear behavior of the anodized AA6082 samples, related to the microstructure modifications and to the specific applied sliding conditions.

Abstract: Magnesium alloys containing rare earth elements are known to have high specific strength and corrosion resistance. The addition of SiC ceramic particles makes the metal matrix composite stronger with better wear and creep resistance and a still good machinability. The role of the reinforcement particles to the enhanced strength can be quantitatively evaluated using transmission electron microscopy (TEM). This paper presents a quantitative strengthening evaluation in a SiC Mg-RE composite alloy. The different contributions were determined by TEM inspections. The microstructure strengthening mechanism was studied after room temperature compression specimens. The way of combining the different contributions and the comparison to the measured yield stress, is also discussed and justified.

Abstract: This paper is a review of the thermal stability of nanostructured nitride coatings synthesised by reactive magnetron sputtering technique. In the last three decade, nitride based coatings have been widely applied as hard wear-protective coatings in mechanical components. More recently, a larger interest has been addressed to evaluate the thermal stability of such coatings, as their mechanical and tribological properties are deteriorated at high working temperatures. This study describes the microstructural, mechanical and compositional stability of nano-crystalline Cr-N and nano-composited Ti-N based coatings (Ti-Al-Si-B-N and Ti-Cr-B-N) after air and vacuum annealing. For Cr-N coatings annealing in vacuum induces phase transformation from CrN to Cr2N, while after annealing in air only Cr2O3 phase is present. For Ti-N based coatings, a well-definite multilayered structure was shown after air annealing. Degradation of mechanical properties was observed for all the nitride coatings after thermal annealing in air.

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: This paper presents a transmission electron microscopy study of two Fe-1.5wt.%Mo steels stabilized with 1.5wt.%SiO2 and 1.5wt.%TiO2,respectively, and compacted through spark plasma sintering. The microstructure inspections revealed that sintered FeMo+SiO2 is able to maintain a nanometric scale grained structure up to a sintering temperature of 815°C, while the steel modified with TiO2 retained its nanometric scale microstructure up to 900°C. The ultra-fine grained structure (within 100-150 nm) was also directly correlated to the grain boundary stability through systematic extinction contours survey. Local nano-welding phenomena shows the effective compacting process of the Fe-Mo powders during sintering. Residual nano-porosity was found to decorate most of the grain boundaries and the triple grain junctions in all the sintering conditions examined, although this nano-porosity accounted for values within 0.26% in volume fraction.

Abstract: Thin nanostructured Ti-B-N-based films are known for their excellent mechanical properties and their stability at high temperature in oxidizing atmosphere. In the present work, nanostructured Ti-Al-Si-B-N and Ti-Cr-B-N coatings deposited on AISI 304 stainless steel by ion implantation have been characterized. To evaluate the oxidation resistance and thermal stability, the coatings were annealed at 900°C in vacuum and in air for 4 hours. The mechanical properties, phase composition and micro-structure of as-deposited and annealed coatings were addressed. The coatings showed a complex multilayered structure and a substantial change of their mechanical properties, due to the new structure and to the phases formed after air annealing.

Abstract: Microstructure evolution with equal channel angular pressing (ECAP) using route Bc, that is a 90° axial rotation of the billet between passes, up to 8 passes, was investigated by transmission electron microscopy. The study has been focused on the induced development of boundary misorientation and spacing toward microstructure refinement. Cell (low-angle) and grain (high-angle) misorientation and spacing were determined from about 250 boundaries per pass of ECAP, systematically using whether Kikuchi patterns or Moiré fringes, these latter where possible. The average cell size and misorientation saturate within the first two passes. Misorientation and spacing of high-angle boundaries decrease with the number of passes. After 8 passes, mean cell size is ≈ 1.3 µm and the fraction of high-angle boundaries is ≈ 0.7. Differences in rate of grain structure evolution per pass are linked to differences in ability of dislocations introduced in new passes to recombine with the existing ones. As ECAP strain rises, the misorientation distribution develops strong deviations from the MacKenzie distribution for statistical grain orientation. This is interpreted as a result of the tendency to form equiaxed grains in a textured grain structure.

Abstract: The tensile properties and the microstructure of an Al-7%Si-0.6%Mg-0.5%Cu rheo-cast component were investigated. The material underwent a T5 treatment, consisting in ageing at 160, 175 and 190°C for durations ranging from 0.5 to 48h. Tensile testing indicated that the T5 treatment resulted in a relatively good level of strength and in a comparatively low ductility. In order to improve ductility, maintaining as low as possible the cost of the final component, a single solution treatment at 500°C for 4h was subsequently applied. The tensile strength and ductility of the solution treated and aged material were higher than in the T5 condition. These differences were attributed to the microstructural evolution occurring during exposure at 500°C, in particular to the spheroidization of eutectic-Si and to a more homogeneous distribution of the precipitates.

Abstract: The microstructural evolution with strain was investigated either in a Zr-modified 6082 Al-Mg-Si alloy and in the same alloy added with 0.117wt.% Sc, subjected to severe plastic deformations. Materials were deformed by equal-channel angular pressing using route BC, up to a true strain of ∼12. A strain of ~4 produced a sub-micrometer scale microstructure with very fine cells (nanometer scale) in the grain interior. The role of fine dispersoids (Al3(Sc1-x,Zrx)) was investigated by transmission electron microscopy techniques and discussed. Dispersoids were responsible for a more complex dislocation substructure with strain. Compared to the commercial parent alloy, block wall formation and propagation were favored by the presence of Sc-Zr containing dispersoids, while cell boundary evolution was less affected, compared to the commercial parent alloy. Mean misorientation across block walls increased with strain much more in the Sc-Zr containing alloy, reaching a plateau, starting from a true strain of ∼8. Misorientation across cell boundaries continuously increased to ∼8° and ∼5° for the Sc-Zr and Zr containing alloy, respectively.

Abstract: Mechanical spectroscopy was employed to investigate the microstructure evolution of a Zr-modified 6082 Al-Mg-Si alloy and the same alloy with Sc addition after ageing and following severe plastic deformation through equal channel angular pressing. Measurements of the internal friction and dynamic young modulus have been performed in isothermal and isochronal runs in the frequency range 0.1 - 104 Hz. The anelasticity spectra reveal in the temperature range 470-870 K both structural and anelastic relaxation processes. Two structural damping maxima connected with inverse temperature trend of the modulus occur in the alloys submitted to equal channel angular pressing, the first one is strongly suppresed by Sc and Zr addition. An anelastic relaxation peak whose strength depends on the nature and morphology of precipitates and dispersoids and on the deformation and ageing condition was observed in all samples investigated. The high background damping occurring before the first structural damping maximum is analyzed with reference to a superplastic behavior of the equal channel angular pressing processed alloys.