Nickel based superalloys are currently being investigated for high temperature applications in advanced steam power plant operating at temperatures of 700˚C and above. Three nickel-based superalloys Inconel 617, Inconel 625 and Nimonic 263 alloys, which are of primary interest for boiler technology components such as furnace walls, superheater tubes, header and steam pipes, etc and for steam turbine technology components such as HP &IP cylinders, rotor forgings, casing and valve chest, blading, etc., have been evaluated for long and short term creep performance. Creep deformation processes occurring at high temperatures and stresses lead to the evolution of microstructures in the form of precipitation, precipitate coarsening and recovery effects. The deterioration in mechanical properties as a result of this microstructural change has been evaluated by hardness testing. This work discusses the microstructural evolution occurring in alloys in samples that have been creep exposed at a series of temperatures from 650°C to 775°C and for durations from 1000 to 45,000 hours using advanced FEGSEM, TEM, XRD and phase extraction techniques. The fractions and morphology of different phases, their locations during exposure to higher temperatures and probable creep fracture mechanism in these alloys are illustrated and discussed.

No description available.

The CdTe family members (in particular CdZnTe) remain the substrate of choice for epitaxial growth of HgCdTe for use in high performance infrared (IR) detectors and focal plane arrays. This is the case despite advances in the use of alternate substrate technologies such as buffered GaAs and GaAs on Si; these technologies, to date, have not reproducibly demonstrated device performance comparable to the arrays made in HgCdTe grown on CdZnTe and CdTe. The quality of CdTe family materials has improved significantly over the past several years and so the quality and reproducibility of IR detectors has improved along with them. It is clear, however, that CdTe family substrates still have a significant impact on the performance of HgCdTe devices and that further research is required to reduce the effects of substrate on these devices. Unlike silicon or gallium arsenide, it is very difficult to grow the large area single crystals of CdZnTe due to thermodynamic limitations. It has the lowest thermal conductivity among all semiconductors that makes it difficult to obtain planar solid-liquid interface, which is desirable for the growth of large area single crystals of CdZnTe. Due to its high ionicity and weak bonding, defects are easily incorporated during the growth. Also, it is well established that both the structural defects and impurity content of Hg1-xCdxTe epitaxial layers are strongly influenced by the quality of the substrates used in the epitaxial growth process. A substrate of poor structural quality will result in a poor substrate/layer interface from which defects will propagate into the epilayer. It is known that our focal plane arrays (FPAs) are backside illuminated, with the device connected to underlying silicon multiplexer, using a matrix of indium bumps. Thus the substrate should have high IR transmission to pass the radiation on to the detector for collection. High IR transmission requires chemically and electrically homogeneous crystals free from extraneous second phase particles. This objective is one of the most difficult thermodynamic and technological problems in the growth of CdTe and related alloys. The bulk CdZnTe crystals grown from melt suffer from the inherent disadvantage of accommodating tellurium precipitates because of high growth temperature and phase diagram limitations. These tellurium (Te) precipitates condense as cadmium vacancies and Te interstitials during the cooling process, which contribute to intrinsic point defects. Although extensive efforts have been made in the area of purification of the CdZnTe crystals by using 6N pure starting materials, still the high temperature melt growth leads to impurity pickup during the crystal growth process. This deviation in the stoichiometry, especially due to free carriers, impurities and second phase tellurium precipitates, play the major role in reducing the infrared transmission through the CdZnTe substrate material. Also they affect the device performance when used for detector applications. In this context a thorough investigation of the non-stoichiometry of the CdZnTe material is mandatory to improve the material quality. It is my endeavor in this respect to present in this thesis “optical and structural investigations of defects in CdZnTe (Zn~4%) crystals”. The present thesis has been organized into six chapters. Chapter 1: It presents an up to date comprehensive review of the defects in CdTe binary and CdZnTe ternary compound semiconductors. It includes an introduction to the ternary II-VI cadmium zinc telluride with potential device applications. Issues related to CdTe based substrates for infrared (IR) applications have been discussed. Growth as well as several material aspects like crystal structure, band structure, mechanical, thermal, optical and dielectric properties have been discussed in details. The chapter ends with the motivation and scope for the present thesis. Chapter 2 : Te precipitates were identified and characterized in CdZnTe (Zn ~ 4%) crystals using various physical characterization techniques and the results are presented in Chapter 2. X-ray diffraction rocking curve measurements were carried out on a series of samples to assess the overall crystalline quality of the as grown CdZnTe crystals, in conjunction with Fourier transform infrared (FTIR) absorption spectroscopy measurements to identify the presence of Te precipitates. Further, the CdZnTe samples having Te precipitates were systematically characterized using micro-Raman imaging technique. CdZnTe wafers grown in three and six zone furnaces using quartz and/or pyrolytic boron nitride (PBN) crucibles have been subjected to micro-Raman imaging to quantify and understand the nature of Te precipitates. It is well known that for the normal phase of Te precipitates, the Raman modes appear centered around 121 (A1), 141(E) /TO (CdTe) cm -1and a weak mode around 92 (E) cm -1 in CdZnTe indicating the presence of trigonal lattice of Te. Using the micro-Raman maps and taking the spatial distribution of the area ratio of 121 to 141 cm-1 Raman modes, the size and distribution of Te precipitates were estimated. A substantial reduction in Te precipitate size and an improvement in the IR transmission in the 2.2 – 5 µm IR window was observed in the CdZnTe crystals subjected to post growth annealing under Cd+Zn vapors at 650 oC for 6 hrs. Also it is shown that the samples grown in pyrolytic boron nitride (PBN) crucibles have shown an overall improvement in the crystalline quality and reduction in the Te precipitate size as compared to the samples grown in quartz crucibles. The possible reasons for these observations have been discussed in chapter 2. The presence of Te precipitates under high pressure phase was detected by the blueshift of the Raman bands that appear at 121 (A1) cm-1for a normal Te phase, indicating that these micro-Raman maps are basically the distribution of Te precipitates in different phases. NIR microscopy imaging has been carried out to further substantiate the presence of Te precipitates under high pressure phase and that of larger Te precipitates. The significance of micro-Raman imaging lies in quantifying and demonstrating the high pressure phase of Te precipitates in CdZnTe crystals in a non-destructive way. Also it is shown that the presence of Te precipitates lead to loss of useful signal in the 2.2 – 6 µm wavelength regions and hence are “deleterious” for substrate applications of CdZnTe crystals required for the growth and fabrication of HgCdTe detectors. Chapter 3: The effects of annealing and hydrogenation on the low temperature photoluminescence (PL) spectra of CdZnTe (Zn ~ 4%) crystals are reported in this chapter. It is shown that annealing at 600 oC for 12 hrs under Cd vapors has resulted in the disappearance of both C-A and DAP recombination features (attributed to singly ionized cadmium vacancy acceptors) observed in the 1.5 – 1.6 eV band edge region in the low temperature PL spectra of CdZnTe, confirming the origination of these bands from Cd vacancy defects. The presence of copper impurity has been identified by the appearance of the 1.616 (AoX) eV energy peak attributed to exciton bound to the neutral copper acceptor and the 1.469 eV band attributed to copper acceptor in the donor acceptor pair (DAP) recombinations. It is shown that, only annealing under Cd+Zn vapors at 650 oC for 6 hrs has resulted in the passivation of the 1.469 eV band and the mechanism has been explained invoking the Hume-Rothery rule. Passivation of the 1.469 eV band is significant, since CdZnTe substrate copper contamination was found to degrade HgCdTe epitaxial layer and hence the performance of HgCdTe infrared (IR) detectors. Also it shown that vacuum annealing has resulted in the introduction of a new defect band around 0.85 eV in the low temperature PL spectra of CdZnTe possibly due to the loss of Cd and/or Zn. Further, the effects of hydrogenation in passivating the defect bands observed in the low temperature PL spectra of the control CdZnTe crystals are discussed. Using micro-Raman imaging technique, it is shown that hydrogenation has resulted in the reduction in size and restoration of normal phase for Te precipitates, which otherwise were present under high pressure phase in CdZnTe crystals. It is shown that the net effect of hydrogenation is to improve the quality of CdZnTe crystals at low temperature (50 oC) as compared to the high Cd+Zn annealing temperature (650 oC) whose effect is only to reduce the size of Te precipitates. To further substantiate this an analysis of the temperature dependent resonance micro-Raman spectra recorded with 633 and 488 nm lasers has been made and it is shown that appearance of the multiple orders (up to 4 orders) of the CdTe like LO phonon modes and emergence of the ZnTe like LO phonon mode are clear indications of the improved quality of the hydrogenated CdZnTe crystals. Chapter 4: Manifestation of Fe2+and Fe3+charge states of Fe in undoped CdZnTe (Zn ~ 4 %) crystals grown in quartz crucibles by asymmetrical Bridgemann method and their respective optical and magnetic behaviors have been discussed in this chapter. Fe2+being optically active shows absorption around 2295 cm-1in the low temperature (T = 3 K) FTIR spectra, while Fe3+being magnetically active exhibits coexistence of para and ferromagnetic phases, as identified by low temperature electron spin resonance and supported independently by low temperature SQUID and AC susceptibility measurements. In the paramagnetic phase (TC ~ 4.8 K) the inverse of ac susceptibility follows the Curie-Weiss law. In the ferromagnetic phase (TC ~ 4.8 K) the thermal evolution of magnetization follows the well known Bloch’s T3/2 law. This is further supported by the appearance of hysteresis in the SQUID measurements at 2K below TC. Small coercive field of 10 Oe as estimated in the hysteresis suggests that the magnetic anisotropy is very small in these systems. Chapter 5: In this chapter, details of the indigenously developed laser beam induced current (LBIC) instrumentation have been presented. These include instrumental arrangement of the micro-mechanical system for raster scanning of defects in semicoductors and fabrication details of continuous flow liquid helium cryostat for low temperature LBIC measurements. Preliminary LBIC data recorded using this system have been shown to demonstrate the operability of the system. Chapter 6: This chapter includes a brief write-up summarizing the results and draws the attention for the possible future work. Appendix A: Here C++ programs for LBIC measurements are presented. Appendix B: Here the CAD diagrams for the full cross sectional view of the liquid helium cryostat consisting of “assembly liquid helium cryostat” and “part liquid helium cryostat” are attached.

Les aciers TWIP se déforment par maclage et par glissement de dislocations, avec pour conséquence de forts taux d’écrouissage. Les mécanismes de déformation sont contrôlés par l’énergie de faute d’empilement (EFE). Un modèle de prévision de l’EFE et une régression de TNéel (transition antiferro/paramagnétique) de l’austénite sont proposés pour les systèmes FeMnXC (X = Cu, Cr, Al, Si et Ti). Les nuances FeMnCuC étudiées ont une EFE plus faible que la nuance de référence Fe22Mn0,6C. La formation de martensite [epsilon]?se substitue au maclage, sans dégradation des caractéristiques mécaniques en traction. La contrainte d'écoulement diminue avec la teneur en carbone et la formation de martensite [alpha]' aux plus basses EFE réduit l'allongement à rupture. La substitution d'une partie du manganèse par du cuivre permet un gain de 20% sur le module d'Young à température ambiante, en abaissant TNéel en dessous de 0ºC. La précipitation intragranulaire de carbures de vanadium augmente la limite d’élasticité mais n’influence pas le taux d’écrouissage. Aucune interaction entre précipités et macles n'a été observée en microscopie. Les calculs de cohérence et les mesures au MET montrent que les carbures ont une relation d'orientation avec l'austénite et sont semi-cohérents avec une faible cohérence résiduelle. Les contraintes induites ne semblent pas suffisantes pour piéger de grandes quantités d'hydrogène. Les alliages FeMnC + TiC présentent un fort durcissement par effet composite en début de déformation, tandis que l'écrouissage par effet TWIP n'est pas modifié par la présence des particules TiC. Cependant, le clivage des précipités primaires de grande taille réduit l'allongement à rupture / TWIP steels deformation occurs by twinning and by dislocations gliding which leads to high a strain hardening. The deformation mechanisms are controlled by the stacking fault energy (SFE). A model for the prediction of the SFE and a law for TNéel (antiferro to paramagnetic transition) for austenite are proposed in FeMnXC systems (X = Cu, Cr, Al, Si et Ti). The studied FeMnCuC grades have a lower SFE than the Fe22Mn0,6C reference. The formation of [epsilon]-martensite replaces twinning without any deterioration of the mechanical properties. The flow stress decreases with the carbon content and the formation of [alpha]'-martensite at the lowest SFEs reduces the elongation to fracture. Substituting a part of the manganese content by copper leads to a 20% increase of the Young's Modulus at room temperature by decreasing TNéel below 0ºC. The precipitation of intragranular vanadium carbide increases the yield stress but does not influence the strain hardening rate. No interaction between precipitates and twins has been observed by microscopy. The coherency calculations and the TEM observations show that the carbides have an orientation relation with the austenite and are semi-coherent with a low residual coherency. The resulting stresses do not seem to be high enough to trap large quantities of hydrogen. The FeMnC + TiC alloys exhibit a strong hardening by composite effect at the beginning of deformation, while the strain hardening due to TWIP effect is not modified by the presence of the TiC particles. Meanwhile, cleavage occurs in the largest primary precipitates, which reduces the elongation to fracture

L’alliage d’aluminium 6061-T6 a été choisi comme matériau de structure du casier et du caisson du cœur de réacteur Jules Horowitz (RJH). Transparent aux neutrons, il doit ses bonnes propriétés mécaniques à la précipitation de fines aiguilles nanométriques appelées béta'' contenant Mg et Si et à la présence de dispersoïdes Al(Cr,Fe,Mn)Si jouant un rôle important dans la résistance à la recristallisation. Le caisson et le casier seront soumis à de forts flux neutroniques à une température avoisinant les 50°C. L’objectif de cette thèse est d’étudier les évolutions microstructurales de l’alliage sous irradiation et plus particulièrement la stabilité des précipités. Pour cela, des études analytiques par irradiations in-situ et ex-situ aux électrons et aux ions à température ambiante et forte dose ont été réalisées ainsi qu’une étude du comportement des précipités sous irradiations aux neutrons à faible dose. La caractérisation fine des précipités par Microscopie Electronique en Transmission a montré que les dispersoïdes sont stables sous irradiation, cependant ils présentent une structure cœur/coquille avec un cœur riche en (Fe, Mn) et une coquille riche en Cr qui s’accentue sous irradiation par accélération de la diffusion. En revanche, les nano-phases type béta’’ sont déstabilisées par l’irradiation. Elles sont dissoutes par irradiation aux ions au profit de l’apparition d’amas riches en Mg, Si, Al, Cu et Cr participant à l’augmentation du durcissement de l’alliage, tandis qu’elles tendent à se transformer en précipités cubiques sous irradiation aux neutrons. / The 6061-T6 Aluminium alloy, whose microstructure contains Al(Fe,Mn,Cr)Si dispersoids and hardening needle-shaped beta” precipitates (Mg, Si), has been chosen as the structural material for the core vessel of the Material Testing Jules Horowitz Nuclear Reactor. Because it will be submitted to high neutron fluxes at a temperature around 50°C, it is necessary to study microstructural evolutions induced by irradiation and especially the stability of the second phase particles. In this work, analytical studies by in-situ and ex-situ electron and ion irradiations have been performed, as well as a study under neutron irradiation. The precipitates characterization by Transmission Electron Microscopy demonstrates that Al(Fe,Mn,Cr)Si dispersoids are driven under irradiation towards their equilibrium configuration, consisting of a core/shell structure, enhanced by irradiation, with a (Fe, Mn) enriched core surrounded by a Cr-enriched shell. In contrast, the (Mg,Si) beta” precipitates are destabilized by irradiation. They dissolve under ion irradiation in favor of a new precipitation of (Mg,Si,Cu,Cr,Al) rich clusters resulting in an increase of the alloy’s hardness. beta’’ precipitates tend towards a transformation to cubic precipitates under neutron irradiation.

FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico / Ligas Fe-Cr-Mo sÃo amplamente utilizadas na fabricaÃÃo de tubulaÃÃes para a indÃstria petroquÃmica nos processos de transporte e refino do petrÃleo devido a uma combinaÃÃo de propriedades como dureza, tenacidade e resistÃncia à fluÃncia e à corrosÃo em altas temperaturas. Atualmente, a liga Fe-9Cr-1Mo à a mais comumente utilizada. Contudo, em refinarias de petrÃleo que apresentam altos Ãndices de acidez, tem-se observado, de maneira acentuada, processos de corrosÃo nessas ligas, principalmente processos relacionados à acidez naftÃnica. Um aumento no teor de molibdÃnio da liga tem sido apontado como uma possÃvel soluÃÃo para este problema, visto que a adiÃÃo desse elemento na liga resulta em uma melhor resistÃncia à corrosÃo. Nesse contexto, foram desenvolvidas no LACAM - LaboratÃrio de CaracterizaÃÃo de Materiais da UFC, ligas Fe-9Cr-XMo com trÃs diferentes percentuais em peso de molibdÃnio (X = 5, 7, 9 wt%). A caracterizaÃÃo dessas ligas, bem como dos precipitados que nelas surgem devido aos tratamentos tÃrmicos, e o estudo da sua resistÃncia à corrosÃo serÃo feitos atravÃs de vÃrias tÃcnicas. Neste trabalho, especificamente, foi utilizada a Espectroscopia MÃssbauer e a DifraÃÃo de Raios-x para uma melhor compreensÃo das mudanÃas na estrutura das ligas quando submetidas Ãs diferentes condiÃÃes de serviÃo dentro da faixa de temperatura de 450ÂC a 650ÂC. Os resultados obtidos permitem concluir que a distribuiÃÃo dos elementos de liga na matriz à influenciada pelo teor de molibdÃnio, pela temperatura e o pelo tempo de tratamento. TambÃm foram extraÃdos e caracterizados os precipitados da liga comercial ASTM A213 da classe T91 utilizada para efeito de comparaÃÃo com as ligas E1, E2 e E3. A presenÃa destes precipitados altera a distribuiÃÃo dos elementos de liga na matriz. A anÃlise da influÃncia da concentraÃÃo dos elementos de impureza na estrutura das ligas estudadas foi investigada atravÃs de um modelo de primeiros e segundos vizinhos. O valor do campo magnÃtico hiperfino varia proporcionalmente ao nÃmero de vizinhos de impureza nos sÃtios de ferro. A distribuiÃÃo de elementos de liga na matriz està diretamente relacionada à distribuiÃÃo de campo magnÃtico hiperfino. As probabilidades associadas aos valores de campo hiperfino permitiram uma melhor compreensÃo da estrutura das ligas. / Fe-Cr-Mo alloys are widely used in the manufacture of piping for the petrochemical industry in oil transport and refining processes due to a combination of properties such as hardness, toughness, creep resistance and corrosion resistance at high temperatures. Nowadays the Fe-9Cr-1Mo alloy is the most common alloy in use. However, in some refineries with higher acidity levels, strong corrosion processes have been observed, specially in processes related to naphthenic acid corrosion. An increasing in the Molybdenum content has been pointed out as a possible solution to this problem, since the addition of this alloying element results in a better corrosion resistance. In this context, Fe-9Cr-XMo alloys with three different Molydbenum contents (X = 5, 7, 9 wt%) have been developed at LACAM - LaboratÃrio de CaracterizaÃÃo de Materiais at the Universidade Federal do CearÃ. The characterization of these alloys, as well as the precipitates that appear on them due to heat treatment and studies of their corrosion resistance were made through various techniques. Specifically in this work, Transmission MÃssbauer Spectroscopy and X-Ray Diffraction were used in order to achieve a better understanding of the structure changes of the alloys when they are subjected to different service conditions in the temperature range from 450ÂC to 650ÂC. The results led to the conclusion that the distribuition of the alloy elements in the matrix is influenced by the molibdenum content, the temperature and the treatment time. The precipitates present in the ASTM A213 grade T91, used for comparation with E1, E2 and E3 alloys, have been extracted and characterized as well. The presence of this precipitates changes the distribuition of the alloy elements in the matrix. The analysis of the influence of the impurity elements in the structure of the alloys was investigated by a model of first and second neighbors. The changes in the magnetic hyperfine field is proportional to the number of impurity atoms in the neighborhood of the iron sites. The distribuition of the alloy elements in the matrix is direct related to the distribution of the magnetic hyperfine field. The probabilities associated to the values of hiperfine field allow for a better understanding of the structure of the alloys.

L'alliage N19 est un superalliage base nickel, élaboré par métallurgie des poudres, qui a été développé récemment en vue d'une application pour disques de turbine aéronautique. L'objectif de cette étude est d'optimiser la microstructure de cet alliage en agissant sur les paramètres de traitements thermiques pour améliorer les propriétés mécaniques de l'alliage. Une bonne compréhension des relations entre les paramètres de traitements thermiques et la microstructure, d'une part, et, des relations entre la microstructure et les propriétés mécaniques, d'autre part, est donc nécessaire. De nombreux traitements thermiques ont été appliqués à l'alliage N19 pour évaluer l'effet de la température de mise en solution, des conditions de refroidissement et de la température de revenu sur la taille de grains, et sur la taille et la distribution des précipités gamma prime. L'observation des microstructures en microscopie électronique à balayage et en transmission a permis d'évaluer l'effet des différentes étapes du traitement thermique sur les caractéristiques microstructurales de l'alliage. L'effet de ces modifications microstructurales sur la vitesse de propagation de fissure en fatigue-fluage à 650°C a été étudié. Les résultats de ces essais de propagation de fissure en fatigue-fluage ont été analysés à l'aide d'essais de comportement en fatigue-relaxation. Une synthèse des différentes propriétés mécaniques de l'alliage en fonction des paramètres de traitements thermiques et des caractéristiques microstructurales a été proposée. / The N19 alloy is a powder metallurgy nickel-based superalloy which has recently been developed for aircraft engine turbine disks. The aim of this study is to optimize the N19 microstructure through the adjustment of the heat treatment parameters in order to enhance the mechanical properties of this alloy. A good understanding of the relationships between the heat treatment parameters and the microstructure, and between the microstructure and the mechanical properties is required. Numerous heat treatments were applied to the alloy to investigate the effect of the solutionizing temperature, the cooling path, and the ageing temperature on the grain size, and on the gamma prime precipitates size and distribution. The observation of the microstructures using scanning electron microscopy and transmission electron microscopy supports the analysis of the heat treatment parameters effects on the microstructural features of the alloy. The effect of the microstructural modifications on the creep fatigue crack growth rate at 650°C was studied. The results of the creep fatigue crack growth tests were analyzed using cyclic stress-relaxation tests. A synthesis of the various mechanical properties of the alloy according to the heat treatment parameters and microstructural features is proposed.

The thesis deals with the study and analysis of crystallographic defects on the surface of silicon wafers produced by Czochralski method. It focuses primarily on growth defects and oxygen precipitates, which play an important role in the development of appropriate nucleation centers for growth of stacking faults. The growth of stacking faults near the surface of silicon wafers is supported by their oxidation and selective etching. Such a highlighted stacking faults are known as the OISF (Oxidation Induced Stacking Fault). Spatial distribution of OISF on the wafer gives feedback to the process of pulling silicon single crystal and wafers surface quality. Moreover the work describes the device for automatic detection and analysis of OISF, which was developed for ON Semiconductor company in Rožnov Radhoštěm.

Alloy 617 displays a ductility dip during straining at exactly 1000˚C, leading to brittle fracture. A sudden decrease in ductility appearing during Gleeble hot ductility tests of Ni-based superalloys is a well-known phenomenon, while its cause is unknown. Many mechanisms have been established as possible contributors to the issue, and in later years not one, but the simultaneous presence of several of these mechanisms were confirmed as the cause. The ductility dip leads to solid state cracking and a specific solid state cracking phenomenon known as ductility dip cracking is specifically common in Ni-based superalloys. Ductility dip cracking is identified by intergranular cracks and the occurrence of specific precipitates, among other things. This work investigates the possibility that the decreased ductility is due to ductility dip cracking. Furthermore, other possible explanations are investigated. Visual examination was conducted through LOM, SEM and chemical analysis using EDS technique. Combined with thermodynamic calculations, the existence of Cr-rich M23C6 carbides, Ti(N,C) and Mo-rich particles, most likely M3B2, were confirmed. Further, it is established that the ductility dip is related to the lack of dynamic recrystallization at 1000˚C. It is not confirmed that the ductility dip in alloy 617 is due to ductility dip cracking. / Nickelbaslegeringen 617 uppvisar en minskning i duktilitet under Gleeble-dragprovning vid exakt 1000˚C vilket leder till sprött brott. En plötslig sänkning av duktiliteten vid varmdragning av Ni-baserade superlegeringar är ett välkänt fenomen, dock är orsaken inte fastställd. Många mekanismer har bekräftats som bidrag till problemet och under de senaste åren har den simultana närvaron av fler av dessa mekanismer bekräftats som orsaken. Sänkningen i duktilitet leder till sprickbildning i fast fas och en specifik typ av sprickbildning känd som ”ductility dip cracking” är speciellt förekommande i Ni-bas legeringar. Denna identifieras bland annat genom intergranulära sprickor och närvaron av specifika utskiljningar. Detta arbete undersöker möjligheten att duktilitetssänkningen beror på  ”ductility dip cracking”. Dessutom undersöks fler tänkbara förklaringar. Visuell granskning genomfördes via LOM och SEM och analys av sammansättningar via EDS-analys. I kombination med termodynamiska simuleringar blev förekomsten av Cr-rika M23C6 karbider, Ti(N,C) och Mo-rika partiklar, troligtvis M3B2, bekräftad. Fortsatt är det bekräftat att duktilitetssänkningen är relaterat till avsaknaden av rekristallisation vid 1000˚C. Det är inte bekräftat i detta arbete att duktilitetssänkningen i legering 617 beror av ”ductility dip cracking”.

Duplex stainless steels can be very attractive alternatives to austenitic grades due to their almost double strength at equal pitting corrosion resistance. When welding, the duplex alloys normally require addition of filler metal, while the commodity austenitic grades can often be welded autogenously. Over-alloyed consumables are used to counteract segregation of important alloying elements and to balance the two phases, ferrite and austenite, in the duplex weld metal. This work focuses on the weldability of the recently-developed lean duplex stainless steel LDX 2101® (EN 1.4162, UNS S32101). The pitting corrosion resistance of this grade is better than that of austenitic AISI 304 (EN 1.4307) and can reach the level of AISI 316L (EN 1.4404). The austenite formation is rapid in LDX 2101 compared to older duplex grades. Pitting resistance tests performed show that 1-2.5 mm thick laser and gas tungsten arc (GTA) welded LDX 2101 can have good corrosion properties even when welding autogenously. Additions of filler metal, nitrogen in the shielding gas, nitrogen-based backing gas and use of laser hybrid welding methods, however, increase the austenite formation. The pitting resistance may also be increased by suppressing formation of chromium nitrides in the weld metal and heat affected zone (HAZ). After thorough post-weld cleaning (pickling), pitting primarily occurred 1-3 mm from the fusion line, in the parent metal rather than in the HAZ. Neither the chromium nitride precipitates found in the HAZ, nor the element depletion along the fusion line that was revealed by electron probe microanalysis (EPMA) were found to locally decrease the pitting resistance. The preferential pitting location is suggested to be controlled by the residual weld oxide composition that varies over the surface. The composition and thickness of weld oxide formed on LDX 2101 and 2304 (EN 1.4362, UNS S32304) were determined using X-ray photoelectron spectroscopy (XPS). The heat tint on these lean duplex grades proved to contain significantly more manganese than what has been reported for standard austenitic stainless steels in the AISI 300 series. A new approach to heat tint formation is presented; whereby evaporation of material from the weld metal and subsequent deposition on the already-formed weld oxide are suggested to contribute to weld oxide formation. This is consistent with manganese loss from the weld metal, and nitrogen additions to the GTA shielding gas enhance the evaporation. The segregation of all elements apart from nitrogen is low in autogenously welded LDX 2101. This means that filler wire additions may not be required as for other duplex grades assuming that there is no large nitrogen loss that could cause excessive ferrite contents. As the nitrogen appears to be controlling the austenite formation, it becomes essential to avoid losing nitrogen during welding by choosing nitrogen-containing shielding and backing gas. / QC 20101213