The influence of the powder material and the main processing parameters on geometrical features and dilution of the clads is investigated and discussed. Physical and analytical model that allow the explanation of the process and the prediction of the clad geometry and dilution is discussed. Using these models, useful tools for cladding operators and engineers are proposed. The energetic balance of the process is presented. Energetic redistribution in laser cladding process is analysed in detail, and quantification of process efficiency and energy losses is given. The influence of the processing parameters and the chemical/physical properties of the materials is considered throughout the various experiments performed. Some selected properties of the coating produced in different processing conditions are analysed. In specific, the variation of the chemical composition of the clad due to substrate dilution is considered, and its effect on the characteristics of the coatings is discussed.

The standard powder metallurgy process is composed by three main step, the powder production, the compaction and the sintering, and the possible secondary operation that allow to improve the mechanical properties and/or the dimensional and geometrical precision. The present work aims at investigating the influence of processing variables on the dimensional and geometrical precision of parts produced by Powder Metallurgy.

Cemented carbides are composite materials formed by high amount of WC bonded by a soft phase, usually Co. They are used in many applications, such as drawing dies, cutting tools and hot rolls due to theirs remarkable properties of high hardness and wear resistance. Mechanical properties are strongly related to microstructure, namely the binder amount and the carbide grain size. Increasing the binder content and the carbide grain size, the hardness decreases ad the fracture toughness increases. In this PhD, the correlations between the mechanical properties of WC-Co and the microstructural characteristics, in parts taken from industrial production, were defined. After that, the influence of the residual microporosity on the mechanical properties was evaluated. Considering the production process, another important modification of the final microstructure of WC-Co occurs due to the liquid cobalt migration phenomenon. Based on this, also the liquid cobalt migration that occurs during sintering was investigated. At the end of the thesis, since a few data are available in literature, Thermal Fatigue and oxidation damage in WC-Co were studied. The main results of this PhD thesis show that the hardness and fracture toughness of WC-Co are defined by the mean binder free path and not by the contiguity since the high standard deviations, the microstructural fineness and also the high carbide grain size scatter. Differently, in case of mechanical strength, also the residual microporosity that depends on the dewaxing stage must be defined. Furthermore, the dewaxing stage acts on the liquid cobalt migration that affects the surface properties and also the final microstructure of the WC-Co part in industrial production. At the end, considering the damages that occur during high temperature applications, the TF and oxidation resistance of WC-Co results affected by the Co content: high cobalt content leads to a better condition of TF damage and s higher oxidation resistance.

Rheocasting process integrated with the high pressure die casting method (Rheo-HPDC) has established itself as a new promising technology to produce high-quality components. However, different types of microstructural segregation induced by the semi-solid process influence the properties of the final component. The semi-solid microstructural features and the new compositions require detailed corrosion studies and verification. The first part of this thesis deals with microstructural and corrosion studies of the conventional and Rheo-HPDC Al-Si alloys. In this part, corrosion properties of two Al–Si alloys containing 2.5 and 4.5 wt % silicon cast by Rheo-HPDC method were examined in the diluted Harrison solution using polarization and electrochemical impedance spectroscopy (EIS) techniques on as-cast and ground surfaces. The microstructural studies revealed that samples taken from different positions (with respect to the feeding gate) contain different fractions of solid and liquid parts of the initial slurry. It was shown that the Rheo-HPDC Al-Si alloys are prone to the localized form of corrosion inside the eutectic region at the interface of aluminum with silicon phase and intermetallic particles. Electrochemical behavior of as-cast, ground surface, and bulk material was shown to be different due to the presence of a segregated skin layer and the surface quality. Corrosion properties of the two Al-Si alloys cast by the conventional and Rheo-HPDC process were also evaluated and compared in 0.01, 0.05, 0.1 and 0.6 M NaCl solutions. The conventional HPDC and semi-solid alloys presented similar EIS responses. However, the semi-solid samples with a lower fraction of the eutectic phase showed slightly higher impedance values in the more diluted sodium chloride solutions. Corrosion morphological features, including localized corrosion, trenching and co-operative corrosion rings were comparable for both types of alloys. However, in the anodic polarization test, the semi-solid alloys presented a higher resistance to pitting corrosion. To protect aluminum alloys from corrosion, chromium-based conversion coating has been successfully used for decades, due to its extensive protection. However, rising concerns and new restrictions on the environmental hazards of Cr (VI) compounds have led to intensive efforts to develop alternative coatings. The second and third parts of this thesis address the effort to investigate two alternatives in this field. Cerium-based conversion coatings were deposited on the conventional and Rheo-HPDC Al-Si alloys by immersion in cerium nitrate aqueous solutions. Different parameters were studied to optimize the conversion coating, and NaCl or H2O2 were also added to the solution to modify or accelerate the deposition process. The results revealed that applying cerium-based conversion coating on Al-Si alloys, is possible and a selective deposition is obtained due to the presence of iron-rich intermetallic particles inside the eutectic region. Under the accelerated conditions, the deposition mechanism includes dissolution of the aluminum matrix, selective dissolution of aluminum from the noble intermetallic particles, oxidation of iron from these particles, and the deposition of cerium hydroxide/oxide layer. The results revealed that the improvement in corrosion resistance in the presence of selectively deposited cerium-based conversion coating is more significant compared to the homogenous coating obtained from the conversion solution containing H2O2. The aluminum alloy with a higher amount of silicon showed more active surface during the conversion process which reduces the required concentration of Ce(NO3)3 but also makes it difficult to work with more aggressive solutions. In the third part of this thesis, the possible protective effect of polypyrrole coating on pure aluminum and Rheo-HPDC Al-Si alloys was investigated. Different electropolymerization solutions containing the Py monomer, SDS, DHBDS (Tiron), C6H8O7 and NaNO3 were used. The presence of nitrate anions led to the passivation of the aluminum electrode (both pure and alloy) during the electropolymerization and to the deposition of a thicker/more conductive coating. These facts resulted in longer and more efficient corrosion protection in NaCl solutions. This polypyrrole coating was able to keep the alloys’ surface potential noble for at least 168 hours. Which can be attributed to the anodic protection provided by the reduction of the polymer. It was shown that the presence of silicon phase or intermetallic particles has a positive effect on the electropolymerization of polypyrrole film. Therefore, the coatings deposited on the alloys possess higher thicknesses compared to those deposited on the pure aluminum. In the presence of chloride ions, all coatings suffered from the formation of blisters as a result of severe (localized) galvanic interaction of polypyrrole with aluminum. This may question the application of polypyrrole coating in concentrated NaCl solutions. However, it is shown that the protection efficiency can be improved by altering the solution chemistry which affects the polymer/metal interface and the conductivity and the barrier properties of the coating. Therefore, the application of polypyrrole in corrosion protection is not totally ruled out but needs specific considerations.

The contact fatigue and surface damage of prealloyed (Fe-0.85Mo, Fe-1.5Mo) and diffusion bonded (Ni-free, low-Ni, high-Ni) powder metallurgy (PM) steels were investigated. Materials subjected to contact stress fail due to the nucleation of subsurface cracks (contact fatigue cracks), nucleation of brittle surface cracks, and surface plastic deformation. The occurrence of these contact damage mechanisms was predicted using theoretical models, which were developed by assuming that crack nucleation is preceded either by local plastic deformation (contact fatigue and surface plastic deformation) or local brittleness (brittle surface cracks ) of the metallic matrix. With reference to the mean yield strength of the matrix (mean approach) or the yield strength of soft constituents (local approach), the models predict the theoretical resistance of materials to the formation of damage mechanisms. The models were then verified using experimental evidence from lubricated rolling-sliding contact tests. In addition, the effect of compact density and microstructures of materials on the resistance to contact damage mechanisms was investigated. Density and microstructure were modified by varying green density, alloying elements, sintering temperature and time, and applying strengthening treatments: carburizing and shot peening on prealloyed (homogenous microstructure) and carburizing, sinterhardening and through hardening on diffusion bonded (heterogeneous microstructure) steels. The theoretical resistance to subsurface and surface crack nucleation in prealloyed materials was predicted using the mean approach since the microstructure is homogeneous. But the local approach is applied for diffusion bonded materials (Ni-free and low-Ni); exceptionally, the mean approach was applied for some homogeneous microstructure of Ni-free material sintered at a prolonged time. However, the models have a limitation in predicting the contact damage mechanisms in a high-Ni material. This issue may require further investigation to modify the model. Shot peening provides higher resistance to the nucleation of surface cracks. High compact density, high sintering temperature and time, and sinterhardening improve the resistance to contact damage mechanisms for Ni-free and low-Ni materials.

Ultrafine grained (~ 1μm) steels have been the subject of extensive research work during the past years. These steels generally offer interesting perspectives looking for improved mechanical properties. UFG Powder Metallurgy hot work tool steels (HWTS) can be fabricated by high energy mechanical milling (MM) followed by spark plasma sintering (SPS). However, similarly to most UFG and Nano-Crystalline (NC) metals, reduced ductility and toughness result from the early plastic instabilities in these steels. Industrialization of UFG PM Tool Steels requires the application of specific metallurgical tailoring to produce tools with sound mechanical properties or in a more optimistic way, to break the Strength-Toughness “trade-off†in these materials. Among the possible ways proposed to restore ductility and toughness without losing the high strength, “Harmonic microstructure†design seems to be a very promising endeavor in this regard. Harmonic microstructure materials consist of a tunable volume fraction of evenly spaced “isolated†coarse-grained particles (CG) surrounded by a 3D interconnected network of UFG particles. CGs provide ductility and toughness, while high strength is guaranteed by the interconnected network of UFGs. This peculiar design offers an extra work hardening due to the generation of geometrically necessary dislocations at the interfaces of UFGs and confined CGs that are essentially present to accommodate the strain gradient imposed by the inhomogeneous (bimodal grained) microstructure. The first part of this work is devoted to the development of PM tool steels with harmonic microstructure. Due to the difficulties of processing hard tool steel particles according to the methods reported in the literature, an economical, simple alternative approach is also proposed. Near full density “Harmonic structure“ AISI H13 samples were produced using different volume fractions of UFG/NC mechanically milled (MM) and CG as-atomized particles followed by short time (30 min) low-temperature (1100°C) SPS. A combination of high hardness and significantly improved fracture toughness was achieved for the blends containing more that 50% UFG particles. The optimized mechanical properties was achieved by the mixture of 60% UFG particles where the sample showed a hardness near to the value predicted by the rule of mixtures (i.e. 405 HV10 vs. 406 HV10) while apparent fracture toughness (Kapp) was about 10% higher than that of predicted by the same rule (i.e. 52.0 MPa*m1/2 vs. ~47.0 MPa*m1/2). A toughening effect was evidenced for the samples essentially showing harmonic microstructure. Toughening was interpreted to be the result of the deviatory effect of coarse-grained round atomized particles together with energy dissipation by decohesion at the CG/UFG or UFG/UFG interfaces leading to a local drop of the driving force for the crack propagation. The design allowed to easily adjust the strength and toughness to meet the specific application-oriented requirements. The harmonic steel was also subjected to Thermal Fatigue (TF) testing. The preliminary results confirmed that this microstructure combined the beneficial effects of both of its constituents, i.e., the low crack nucleation rate of CG H13 and the low crack propagation rate of UFG H13, thus showing the lowest pyrocracking factor. Moreover, TF crack deflection as an extrinsic toughening mechanism was evidenced in Harmonic Microstructure. The second part of this work deals with the production and characterization of a PM HWTS reinforced with partially stabilized zirconia (PSZ). HWTS composites show improved hardness and remarkable wear resistance but generally also a systematic lower fracture toughness than the base material. Deteriorated toughness in metal matrix composites (MMCs) with a high strength matrix is mainly interpreted as a result of early damage initiation at the hard particles (HPs) or Matrix-HP interface. This damage can be even anticipated in the presence of readily damaged HPs (i.e. processing related flaws). Selection of PSZ as reinforcement was aimed at improving the strength and fracture toughness of the composite by taking advantage of the transformation toughening effect of PSZ. Two different types of PSZ, different volume fractions (10 and 20 vol. %) and sizes of reinforcement were used. Mechanical Alloying (MA) was used to process the composite powders to refine the matrix microstructure and both the matrix and PSZ particle size hence increasing the strength of the PSZ particles according to the Griffith strength formalism, and also to overcome the aggregation problems. Powders were consolidated by (SPS). The influence of processing parameters on density and microstructure was investigated. Short time (30 min) low-temperature (1100°C) consolidation by SPS allowed preserving the refined microstructure while achieving a maximum relative density of 98.6%. Moreover, short time sintering did not allow the extensive formation of thermodynamically plausible reaction products at the PSZ-H13 interface. As a result of dispersion hardening, the hardness of the as-sintered composites (i.e. maximum hardness of ~ 920 HV10) was increased compared to the mechanically milled UFG H13 (i.e. ~ 755 HV10), while in comparison to the as-atomized H13 (i.e. ~ 640 HV10) the improved hardness was ascribed to the synergic effect of dispersion hardening, microstructural refinement and strain hardening induced by MA. In these composites, tempering resistance at 550°C and 650°C was significantly improved due to the dispersion hardening effect. The hot compressive yield strength of the composites at 650°C and 450°C was increased up to 1.8 times the unreinforced UFG H13. t to m transformation during hot compression was evidenced and contributed to the strengthening. The hardness of the composites in heat treated condition (i.e. ~ 600 HV10) was significantly improved compared to that of the unreinforced matrix (i.e. ~ 420 HV10) while the apparent fracture toughness was drastically decreased to half the Kapp of the base material (19 MPa*m1/2 vs. 36 MPa*m1/2). However, the fracture toughness was slightly higher than that of a TiC reinforced H13 (i.e. 17 MPa*m1/2) with the same hardness (i.e. ~ 600 HV10).

This thesis describes the work of my Ph.D studies in Industrial Engineering during past three years. It regards preparation of copper-tin alloys from green solvents for decorative purposes. Actual industrial process involves cyanide based complex bath in order to produce white bronze layers and they often contain lead as brightener and whitener. The aim of the thesis is to develop a more environmental friendly process for white bronze electrodeposition. Two different electrolytes were considered as eligible candidates: one involves a simple organic acid aqueous solution as bulk electrolyte, and the other one is a new deep eutectic solvent bath. The investigation of electrodeposition using methanesulfonic acid as complexing agent considered a commercial bath and its optimization in order to be used in the decorative industry. The focus of the study was on the optimization of deposition parameters and verification of bath and deposit stability which was very important from industrial point of view. Obvious improvements on deposits quality and bath stability can be realized by replacement on anode material and utilization of pulse current. Contrary to that, research on electrodeposition from deep eutectic system was quite new and few relevant studies can be referred especially in the field of alloys. As a result, this work started from deposition of single metal for better understanding of behaviors of mixtures between choline chloride and ethylene glycol or urea. Successful deposition of copper-tin alloys can be carried out under warmed conditions and variation on film composition can be controlled by changing concentrations of metallic salts in the bath. Pulse current acted as an effective tool to refine microstructure of deposits in a similar way as in aqueous solvents. Since no brightener was added, reduced luster was observed here and working mechanism of additives was found to be rather different from that in conventional baths. In summary, operation parameters including temperature, salt concentration, anode material and supporting electrolytes influence the resultant properties of deposits in great extent. Deposit quality from both solvents can be improved by using pulse current with proper frequency.

The European Union has undertaken several efforts to reduce the non-exhaust particulate matter emissions from road vehicles. One of the major sources of these emissions is the wear of brake pads and discs. The experimental work presented in this thesis has been performed within the European project called LOWBRASYS that aims at reducing the particulate matter emissions due to the wear of the components of braking systems. Different strategies have been identified for reaching this aim but the present work focuses on the investigation of the wear mechanisms at the disc-pad interface and on their role on the emission in the atmosphere and in the environment of wear debris. In order to achieve the reduction in the particulate matter emissions, the traditional gray cast iron discs have been coated with different cermet coatings deposited via high velocity oxygen fuel (HVOF) process. A further attempt for improving the wear resistance of the gray cast iron discs was performed by applying on them an industrial heat treatment. The cermet coatings have been widely employed in the field of the oil and gas industry for improving the wear resistance and the service life of valves and pipes but their use in the field of road-vehicles-braking-systems has never been explored so far. The present work focuses on the tribological characterization of HVOF coated and heat treated discs performed by means of laboratory-scale-pin-on-disc tribometers at both room and high temperatures (300°C). The sliding speed of 1.57 m/s and the nominal contact pressure of 1 MPa used for this characterization have been selected in order to replicate the actual contact conditions between brake pads and brake discs during an urban cycle braking action. Since the use of lab-scale experimental apparatuses, these tests have not been meant to reproduce real braking conditions for which specific dyno bench tests have been performed in further characterization tests. The pins used during the PoD characterization, made of three different commercially-available friction materials, had dimensions of 12 mm in height and 6 and 10 mm in diameter. The discs, 6 mm in thickness and 58.9 mm in diameter, were coated with a 70 µm thick layer of cermet materials. The heat treated discs had the same dimensions of the coated ones. The tested specimens have been machined from the real braking components, the pins have been extracted from the brake pads while the discs from the braking tracks of the rotors. Since the novelty of the application of the HVOF coatings in the field of braking systems, two preliminary studies of the surface parameters of the coated discs and of the running-in of the pin-disc system have been performed. The first one was conducted on WC-CoCr coated discs polished with different intensities in order to achieve four different average surface roughnesses: 5, 1, 0.1, 0.04 µm respectively. The results of the study highlighted that the wear of the friction material decreased by four order of magnitude by passing from an average surface roughness equal to 5 µm to 0.04 µm. The friction coefficient followed the opposite trend passing from 0.3 for the 5 µm rough coating to 0.7 for the 0.04 µm. The SEM observation of the wear tracks on the discs revealed a high material transfer from the pins in the case of the 5 µm rough coatings due to the abrasive interaction exerted by the hard coating asperities on the relatively soft friction material. The compactness of the material transferred onto the disc surface increased as the average surface roughness of the coating decreased leading to the formation of contact patches also on the coated disc surfaces. From the profilometric analysis of the worn disc their wear resulted negligible. The EDXS analysis of the secondary plateaus of pins detected an increasingly concentration of tungsten and cobalt with the decrease in the surface roughness of the coating meaning that, although the wear of the coating could not be detected from the profilometer, some minor transfer of material occurred during the sliding action. From all the considerations mentioned above the most promising surface roughness, in terms of frictional performances and industrial feasibility, was equal to 1 µm. The second study aimed at the investigation of the running-in stage of the WC-CoCr and Cr3C2-NiCr coated discs in the as-sprayed (Ra ≈ 5 µm) and polished conditions (Ra ≈ 1 µm). From this former investigation the polishing procedure of the coated discs was found fundamental in order to reach the best frictional and wear performances; the spontaneous surface modifications occurring during pin-on-disc tests were not as efficient as the controlled polishing procedure in reducing the surface parameters of the coated discs and so in improving their performances. On the basis of the results of the two former studies presented above, the tribological characterization at both room and high temperature (300°C) of the WC-CoCr, Cr3C2-NiCr, WC-Cr3C2-CoCr, WC-FeCrAlY coated discs and of the heat treated one was performed on specimens with an average surface roughness at around 1 µm. Three different friction material formulations were used in order to optimize their frictional and wear performances with the disc counterface. In all cases the EDXS analysis detected the presence of the coating elements, i.e. tungsten, cobalt and nickel, inside the secondary plateaus of the friction material. Depending on the abrasive content of the friction materials and on the testing temperature the amount of the transferred elements varied, i.e. a low amount of abrasives and a high testing temperature gave rise to a reduction in the elements transferred on the pin surfaces. The best tribological combination was the coupling between the friction material FMB, i.e. the one with the lowest amount of abrasives, and the WC-FeCrAlY coated disc. The results attained with the pin-on-disc tribological characterization of the friction materials were validated during full-scale dyno bench tests. These tests were performed on the most promising materials and they had a twofold aim: as mentioned above they were used to validate the results of the tribometer tests and to collect the particulate matter emitted during braking. The analysis of the wear debris highlighted that, in the case of the friction material FM4 slid against the WC-CoCr coated disc, some cobalt was present. From the deeper analysis of the debris resulted that traces of tungsten and cobalt were found only in the coarser fraction of wear debris, collected on the PM1 filter. The coupled SEM+EDXS analysis of the finer particulate matter, with an average aerodynamic diameter varying from 0.25 to 0.054 µm, did not detected the presence of the coating elements. This was consistent with the observations of the PM1 wear debris that identified WC particles with dimensions comparable to that of the initial carbide particle size inside the coatings; the wear mechanism of the coatings seemed to be the carbide pullout from the metallic matrix without the further fragmentation of the particles due to sliding. Nevertheless, during one of the TEM observation of the particles with an average aerodynamic diameter of 0.094 µm the SAED analysis detected the presence of W2C. On the basis of this last observation and considering that the presence of cobalt inside the finest fraction of debris could not be completely disregarded since it could remain stuck on the W2C particles, the selected coating material for the application in braking systems was the WC-FeCrAlY. This conclusion was consistent with the frictional and wear data attained from the PoD tribological characterization that identified the FMB friction material and the WC-FeCrAlY coated discs as the best coupling in terms of frictional and wear behavior. The study of the thermal behavior of the novel developed braking materials during a pin-on-disc room temperature test was performed by means of a finite element simulation based on the perfect contact approach. The heat flux applied as the thermal input of the model was calculated from the experimental data acquired during the tests. The calibration of the boundary conditions was performed by comparing the experimental curves of the temperatures acquired during the tests with the temperature curves calculated from the FE analysis. The results of the simulations showed that the temperature field during the pin-on-disc tests was influenced from both the friction coefficient and from the thermal properties of the coated discs, i.e. the lower thermal conductivity of the coatings gave rise to a higher average contact temperature with respect the uncoated cast iron discs. The results of FE analysis were then used to propose an analytical relationship that could be used for describing the raise in temperature during a pin-on-disc test without performing further thermal simulations.

The implantation of stents is an effective procedure to unblock the arteries of patients with serious heart problems. Traditionally, stents are made of inert materials such as stainless steel and titanium alloys. It has been shown that the traditional stents can cause restenosis or thrombosis. In recent years the proposal of biodegradable stents is attracting the interest of the industry and the research, since the stent is mechanically needed only in the first year, eliminating the problems caused by the long duration of the implant. Magnesium (Mg) alloys are of increasing interest because of their engineering properties, including the high strength to density ratio. Recently, they have been also proposed as biomaterials for the production of bioabsorbable stents and for other medical devices due to its harmless effect to human body when compared with other structural materials. In this work, the possibility to produce biodegradable stents made of magnesium starting from the powder is investigated. Pure Mg and the AZ91 Mg powders were used in the present study. Pure Mg powder was sintered by Spark Plasma Sintering at 400 and 470 °C, and the AZ91 powder was sintered at 400 °C without homogenization and at 470 °C after homogenization. The preforms produced by sintering were then submitted to hot compression, rod extrusion and tube extrusion at 330 and 380 °C with different strain rates. During all the process was not possible to obtain recrystallization or grain refinement on pure Mg, and after the tube extrusion it has shown a high brittleness and sever defects, which led to the decision of proceed only with AZ91 alloy. The AZ91 presented good recrystallization in al process, always following the Zener-Hollomon relation. The grain size obtained was as small as 1.5 μm. The AZ91 tube was then submitted to manual machining and laser cutting and it was possible to obtain the stent precursors. The results of the present investigation have demonstrated the suitability of the proposed route for producing Mg-based stents. It is clear, however, that the process has to be further optimized, investigating also the possibility of using different types of powder with a tailored composition.

The application of powder metallurgy (PM) technologies to the manufacturing of Oil & Gas turbomachineries’ components was investigated in the course of research collaboration with the Material and Processes Engineering Department of General Electric Oil & Gas (Italy). The thesis focused on the study of the pressure-assisted Hot Isostatic Pressing technology for the processing of the corrosion resistant Ni-base alloy N07626. The densification behaviour of the N07626 metal powder in condition of pressure assisted sintering was investigated by experiments conducted on a small scale by uniaxial hot pressing condition using a Spark Plasma Sintering (SPS) machine in the aim of extending the result to the initial stage of densification of HIP. The SPS exepriments demonstrated that the densification rate is strongly affected by the process temperature and it is less sensitive to the variation of applied pressure. The microstructure and mechanical properties of full-dense HIPped N07626 alloy, produced according to a fixed proprietary cycle and several experimental deviations were analyzed. The microstructure was studied by Optical Metallography, Scanning Electron Microscopy, Energy Dispersed X-Ray Spectroscopy and Electron Backscatter Diffraction. The mechanical properties of the alloy were assessed by tensile testing, conventional and instrumented Charpy V-Notch testing, JIC fracture toughness tests and fatigue crack growth rate testing. The tensile and impact toughness properties resulted sensitive to the local accumulation of oxygen in Oxygen Affected Zones (OAZs), that leads to a ductile to brittle transition in the impact toughness of the material. Two models for formation of OAZs were proposed based on the phase transformation and the oxidation/reduction reactions taking place in the HIP. The mechanical properties were discussed on the base of the microstructure of the Prior Particle Boundaries (PPBs) interface, focusing of the phase transformation products, represented by a thin layer of submicrometric oxides and carbides. The fracture mode was explained by the analogy with models of ductile micro-mechanisms of void nucleation and coalescence and with fracture models of particulate reinforced metal-matrix-composite. The Charpy impact toughness and the fracture toughness were correlated to the oxygen concentration and to the density of inclusions. The fatigue crack propagation behavior was discussed focusing on the effect of clustering of inclusions on the crack propagation path. A relation between the Paris slope with the impact toughness was found. Finally the increase of processing temperature (HIP and heat treatment) was found significanty beneficial for the toughness. This effect was investigate by grain-size analysis and was proposed to be related to a reduction of density of PPBs inclusions.