The aim of this work is to find out the most suitable method for detecting and analyzing accurately the formation conditions of secondary phases, particularly Sigma-phase (σ-phase) and Chiphase (χ-phase) in duplex stainless steels (UNS S32205 and UNS S32750). The microstructure was characterized after a solution annealing at 1080oC followed by an isothermal heating at 830oC for different time ranges, ranging from 1 minute to 9 hours, in order to enlighten the controversial point concerning the mechanism of χ-phase nucleation in relation with the σ-phase. Etched samples were observed using optical microscopy (MO), and scanning electron microscopy (FESEM) with a backscattered electron detector (BSE) was used on unetched samples. Compositional microanalysis (EDS) was carried out for identifying the different phases present in the steels. Sigma phase was easily observed using different etching procedures, whereas χ-phase was only clearly detected with FESEM-BSE on unetched samples. The compositional analyses showed that the molybdenum content in χphase almost doubles the content of this element in σ-phase, and as a result the kinetics of nucleation and growth were also found to be remarkably faster when the alloy content in the steel is higher. In addition, chromium nitrides and carbides were also observed to precipitate as a result of the heat treatments and, in the case of the chromium nitrides, they act as a favorable site for the nucleation of σphase and χ-phase.

The aim of this work is to study the precipitation mechanism of the intermetallic phases present in duplex stainless steels (UNS S32205 and UNS S32750), as well as to find out the most suitable method for detecting and analyzing accurately these secondary phases, particularly Sigma-phase, Chi-phase, nitrides and carbides. The samples were characterized after a solution annealing at 1080oC followed by an isothermal treatment at 830oC from 1 min to 9 h, with the purpose of figuring out the mechanism of chi-phase nucleation and nitrides formation in relation with the sigma-phase. The study has two main objectives: 1) to find out the most suitable technique for the detection, identification and quantification of the secondary phases, obtaining the best results with the combination of field emission scanning electron microscopy (FESEM) and backscattered electron detector (BSE) in comparison with the optical microscopy (MO); 2) to study the influence of the chemical composition on the nucleation mechanism of the intermetallic phases. It has been concluded that molybdenum balance content in chi-phase related to sigma phase is close to two, consequently the kinetics of nucleation and growth of these phases is remarkably faster when this alloying element content in the steel is higher. Chromium nitrides and carbides were also observed to precipitate as a result of the heat treatments carried out to the specimen wherein chromium nitrides role is a favorable site for the nucleation of sigma and chi phases.

Duplex and superduplex stainless steels are characterised by high corrosion resistance and high mechanical strength. However, these steels can suffer formation of secondary brittle phases when they reach temperatures between 600°C 600 and 950°C950 °C, which can lead to the catastrophic service failure of components. In order to understand the influence of the mechanical history of the steel part, equal-channel angular pressing was applied followed by different thermal treatments. Microstructural characterisation was carried out on the ECAPed samples before and after thermal treatment. The analysis of the hardness evolution of the same samples was also evaluated.

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Nanocrystalline structure of CuFeCo (50:25:25 wt%) alloy has been obtained by high energy mechanical milling from elemental metal powder mixture during large hours of work. Phase transformations and diffusion in the system subjected to heat treatment are discussed. Thermal stability at high temperatures is analysed and considered of importance for several applications. The nanostructure was studied by employing X-Ray diffraction and electron microscopy. It has been determined the reduction in crystallite size and the induced microstrain by the milling time. The solid solution achievement through the increment of defect density was confirmed by Mössbauer analysis. Magnetic behaviour was analysed through magnetization technique entailing their soft ferromagnetic behaviour related to the microstructural changes.