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Magnesium alloy Mg-4Y-3Nd (WN43) was subjected to ECAP at T∼300 °C up to 8 passes, using route BC. The initial as-extruded material with average grain size of ∼10 μm, was effectively refined down to sub-micrometer scale, reaching a grain size of ∼380 nm, after 8 passes. At the same time, secondary phase particle precipitation of β′-Mg12Nd2Y, and β-Mg14Nd2Y was induced. At 8 passes, fragmentation and coarsening of these phases occurred. Both grain refinement and β′/β precipitation strengthened the alloy. The strengthening was detected by nanoindentation hardness and accounted of up to 30% increment at 8 ECAP pass, compared to the initial extruded condition.

The effects of postprocessing annealing at 225 &deg, C for 2 h on the creep properties of AlSi10Mg alloy were investigated through constant load experiments carried out at 150 &deg, C, 175 &deg, C and 225 &deg, C. In the range of the experimental conditions here considered, the annealing treatment resulted in an increase in minimum creep rate for a given stress. The reduction in creep strength was higher at the lowest temperature, while the effect progressively vanished as temperature increased and/or applied stress decreased. The minimum creep rate dependence on applied stress was modeled using a physically-based model which took into account the ripening of Si particles at high temperature and which had been previously applied to the as-deposited alloy. The model was successfully validated, since it gave an excellent description of the experimental data.

Co-Cr alloys such as L605 are widely applied for the manufacture of medical devices, including tiny cardiovascular stents. The presence of potentially toxic and allergenic release of Ni, Co, and Cr ions from these devices remains an unsolved concern. Surface modification by oxygen plasma immersion implantation (PIII) could be an excellent technique to create a dense and thin passive oxide layer on a relatively complex shape of a tiny device, such as a stent, thus reducing the potential release of metallic ions. The effect of oxygen PIII was investigated on L605 alloy specimens, from 5 to 50 mTorr gas pressures, and under pulsed bias voltages from −0.1 to −10 kV. The surface chemistry was investigated by x-ray photoelectron spectroscopy, while its morphology and surface energy were evaluated, respectively, by atomic force microscopy and scanning electron microscopy and by a sessile drop static contact angle. Electrochemical characterization was performed by potentiodynamic tests in the saline solution. Mechanical properties of the modified surface layer, specifically film adhesion and hardness (H), were assessed by scratch and nanoindentation tests. Results shown that the oxidized layers were composed of a mixture of Co and Cr oxides and hydroxides and were rich in Co. The corrosion rate was considerably reduced after O PIII, even for treatments using low bias voltage (−0.1 kV) and with consequent low oxygen implantation depth. Moreover, O PIII also improved surface hardness. The oxidized layers were found to have good adhesion and to be scratch resistant.

Bulk ultrafine grained metal matrix composites (MMC) have attracted much attention of many researchers due to their potential in terms of excellent mechanical properties for engineering applications, such as high strength, which can be two or more times of that of their coarse grained counterpart. Bulk ultrafine grained Cu-3wt.%C MMC samples were produced by Ball-Milling (BM) followed by Spark Plasma Sintering (SPS), at a temperature of 900 °C. The Cu-C MMC was compacted progressively by repeating the BM + SPS procedure without changing the weight ratio between Cu and graphite. The room temperature creep behavior, and the strain rate sensitivity (SRS) were inspected by using nanoindentation measurements. Strain rate ranged 0.0025-to-0.5 s−1, and the contact dwelling times ranged 5-to-300 s. A secondary steady-state regime was reached starting from a dwelling time of 120 s irrespective of the strain rate and Cu-C compaction level. A negative trend of the SRS exponent with Cu-C compaction was obtained, with creep stress exponent as high as 28. These results were discussed according to the microstructure features that differentiated the Cu-3wt.%C MMC obtained by the progressive BM + SPS compaction levels.

A basic model developed to describe the creep response of fcc metals has been used to obtain a new constitutive equation valid for alloys and composites reinforced with a dispersion of nanosized particles. The new model gives an excellent description of the minimum strain rate dependence of applied stress and temperature, and gives reason for the reduced creep rate observed in these composites when compared with conventional alloys. The reason for the different behaviour is that the dislocation mean free path becomes equivalent to the distance between nanosized particles. The volume fraction and size of the nanosized particulate thus assumes a key-role in determining the creep response of these materials. In addition, the strengthening effect due to the interaction between particles and dislocation has been described by introducing a back stress, which is in general proportional to the Orowan stress and has the nature of a true threshold stress.

A basic model was applied to pure Ni and then to a single-phase superalloy. The high temperature deformation of the superalloy, a solution treated IN718, was investigated by torsion testing in a high-temperature regime (1000–1100 °C) where no precipitation of secondary phases was expected. The material exhibited the classical behaviour of alloys which undergo dynamic recrystallization. The peak-flow stress dependence on strain-rate and temperature was described by a physically-based set of constitutive equations, which took into account both dislocation hardening and solid solution strengthening, and was previously successfully used for the description of creep deformation of Cu, Al alloys and austenitic steels. The model provided an excellent description of the experimental data and for this reason can be considered an excellent basis for further development, which should take into account the precipitation of secondary phases.

The Equal Channel Angular Processing or pressing, i.e. the ECAP, allows to modify the properties of materials at the microstructure level. It consists in the induction of a high amount of shear deformation in the material that leads in general to a grain size refinement, a precipitate dispersion and a redistribution of dislocations depending on experiment conditions. The objective of the present investigation is to understand how the ECAP can influence the surface and the bulk mechanical properties of double aluminium alloy specimens. Each specimen was composed of a tubular part of the series AA6026 as well as of a cylindrical part of the series AA6012 assembled together before ECAP. A negligible bonding effect was observed after ECAP and after uniaxial compression tests performed at constant temperatures varying between 200 and 300°C with different press ram velocities. The characterization of each ECAP condition was initially represented in terms of the stress versus deformation flow curves. The load versus stroke curves were preferred due to the friction acting at the forming tool- specimen interface. It was obtained a decrease in the load versus stroke levels with increasing the number of ECAP passes under the experiment conditions of the present investigation. The increase in the press ram velocity determined an increase in the load for a given stroke.

The present study aims at investigating the effect of the peculiar microstructure of additive manufactured samples on the creep behavior of an AlSi10Mg alloy. Constant load creep experiments were carried out between 150 and 205 ℃ on samples produced by powder bed fusion additive manufacturing (AM). The specimens were mostly strained up to rupture, although in some cases the tests were interrupted at the early onset of the tertiary region. By analyzing the time to rupture, in the different load and temperature conditions, as a function of the applied stress, it can be clearly seen that the alloy produced by AM is substantially comparable, in terms of time to rupture, with an alloy of similar composition, tested in the die-cast state. The high values of the stress exponent suggest that the creep behavior is strongly affected by the presence of secondary-phase particles.

Friction stir welds are considered reliable joints for their lack of voids, cracks and distortions. When compared to the base material, friction stir welding (FSW) joints typically exhibit finer grain structured (especially at the nugget zone, NZ). Similarly, refined grain structure can also be obtained by severe plastic deformation (SPD) techniques, such as equal channel angular pressing (ECAP). In fact, the fine grain structures produced within the NZ of FSW or friction stir processed (FSP) materials are usually coarser than the ones achieved by ECAP. The former is characterized by lower dislocation density, higher high-angle boundary fraction and different mechanical strength, compared to what can be obtained by ECAP. In this study, a dedicated cold-rolling (CR) set-up, specifically designed to simulate an ECAP-equivalent shear deformation, was used to further refine the grain structure of FSW AA5754 sheets. The effect of ECAP-equivalent deformation induced by CR in a 2 mm-thick AA5754-H111 FSW joint was investigated. FSW was carried out at two different rotational (&omega, ) and translational (v) welding speeds, 600 rpm, 200 mm/min and 1800 rpm, 75 mm/min, respectively. FSW sheets were then CR to obtain an equivalent shear strain of &epsilon, ~ 1.08, that is equivalent to 1-ECAP pass carried out with an internal die channels intersecting at an angle &phi, = 90&deg, with a curvature extending over an angle &Psi, = 20&deg, By CR, the sheet thickness reduced only by ~20%. The role of annealing on the FSW and CR plastically deformed AA5754 was also investigated. This was applied either prior or after FSW, and it resulted that whenever it follows the FSW, the mean volume fraction of dispersoids and Mg-rich particles is higher than the case of annealing preceding the FSW process. On the contrary, it was found that the annealing treatment had a minimal role on the dispersoids and particles mean size. The here reported post-FSW ECAP-simulated deformation, obtained by a customized CR process, showed sheet integrity and a significant concurrent grain size refinement.

AlSiMg alloys produced by additive manufacturing possess an extremely fine and complex microstructure that in many ways defies the most widely used phenomenological models, which, in fact, have turned out to be poorly suited for predicting their mechanical properties. The underlying rationale for the peculiar properties of these alloys has been qualitatively established, however the need for a constitutive model with better predictive capability is still strong. To this aim, the ultra-fine microstructure was described by using a model-material (MM) consisting of soft and hard zones deforming under a similar strain rate. A physically-based set of constitutive equations which took into account also the coarsening/ripening phenomena of the Si-particles was used to predict the creep behavior of the MM. In parallel, the creep response of an AlSi10Mg alloy produced by additive manufaturing and tested in the as-deposited condition was investigated at temperatures ranging from 150 to 225 °C. The minimum creep rate curves obtained for the MM by the constitutive model were then compared with the experimental data obtained by testing the real alloy under constant load in different initial states. The excellent correlation between model curves and experimental results was discussed, taking into account the evolution of the microstructure during creep.

Among different welding techniques currently available in the market, friction stir welding (FSW) is surely considered as an effective and reliable joining technique. Friction stir welds are characterized by lack of voids, cracks and distortions, as the technique does not involve any material fusion. The grained structure of the weld joint is generally finer than the base material. In particular, the nugget zone (NZ) experiences a dynamic recrystallization process during FSW that generally guarantees a stable very-fine grained structure. In the present study, the effect of cold rolling (CR) on the formability limits, the resulted microstructure, and mechanical response of FSW but joint was investigated. To this purpose, an AA5754 aluminium alloy was used. The FSW was performed with rotational and welding speeds equal to 1200 rpm and 100 mm/min, respectively, and an initial tool sinking of 0.1 mm. Strips extracted from the FSWed sheets were CR, with the rolling direction perpendicular to the welding line. Two setups were used in the CR experiments. One, conventional to determine the formability limit of the FSW AA5754 sheet; a second one performed in a CR set-up designed to induce an equivalent strain ε ∼ 1, in a single passage through the CR gage. This was aimed to make the post-FSW CR able to induce a further grained structure reduction within the NZ.