High temperature corrosion of some selected stainless steels and Ni-base alloys – an advanced microscopy study

The thesis deals with high temperature corrosion behavior of some selective stainless steels and Ni-base alloys with applications in power generation technologies, e.g. boilers fired by biomass and waste. The initial stages of KCl-induced oxidation behavior of two alumina formers (alloys Kanthal APMT and TH1) and one chromia former (alloy Sanicro 25) were analyzed in an O2/H2O environment using in-situ ESEM method. Besides, the effects of thermal cycling on the oxidation behaviour of a Ni-base alloy HR-214 was studied in air at 1200\ub0C. The in-situ oxidation experiments provided an opportunity to view dynamic processes occurring during the oxidation process ′′live′′. The in-situ results were validated by ex-situ exposures, i.e. reference tube furnace. The alloys were corroded in the matter of minutes in the studied environment. Quite evidently, the severest oxidation attack occurred locally in the vicinity of KCl particles, where oxide crusts and oxide shells/rims (consisting of Fe-, Cr- and Al- oxides) were formed. STEM studies showed that all the three alloys formed a thin base oxide scale. Chlorine-induced oxidation caused chlorination of the alloys as evidenced by detection of chlorine below the protective scales.In the case of the Ni-base alloy HR-214, both isothermal and cyclic exposures led to the formation of a duplex oxide morphology, composed of a columnar alumina layer overlaid by a complex Ni(Al,Cr)2O4 spinel. It was evident that thermal cycling resulted in the formation of vertical cracks in the multi-layered scale. Additionally, STEM/EDX revealed outwards transport of Cr through the cracks/ alumina grain boundaries, which caused thickening of the outer spinel layer. Moreover, an attempt was made to develop the newly introduced TKD method to study nano-sized oxide scales. This was conducted by (a) designing a dedicated sample holder, (b) specimen preparation and (c) acquisition parameters. These efforts made it possible to achieve data-rich TKD orientation maps (with indexing rates > 85%). Thus, the technique was effectively employed to obtain useful information from the microstructure and microtexture of the fine-grained oxide scales. Besides, the technique provided information concerning the crystallographic orientation relationship at oxide/oxide and oxide/alloy interfaces. Keywords: high-temperature materials; oxidation; KCl; STEM; STEM/EDX; TKD

High temperature corrosion of some selected stainless steels and Ni-base alloys – an advanced microscopy study

The thesis deals with high temperature corrosion behavior of some selective stainless steels and Ni-base alloys with applications in power generation technologies, e.g. boilers fired by biomass and waste. The initial stages of KCl-induced oxidation behavior of two alumina formers (alloys Kanthal APMT and TH1) and one chromia former (alloy Sanicro 25) were analyzed in an O2/H2O environment using in-situ ESEM method. Besides, the effects of thermal cycling on the oxidation behaviour of a Ni-base alloy HR-214 was studied in air at 1200\ub0C. The in-situ oxidation experiments provided an opportunity to view dynamic processes occurring during the oxidation process ′′live′′. The in-situ results were validated by ex-situ exposures, i.e. reference tube furnace. The alloys were corroded in the matter of minutes in the studied environment. Quite evidently, the severest oxidation attack occurred locally in the vicinity of KCl particles, where oxide crusts and oxide shells/rims (consisting of Fe-, Cr- and Al- oxides) were formed. STEM studies showed that all the three alloys formed a thin base oxide scale. Chlorine-induced oxidation caused chlorination of the alloys as evidenced by detection of chlorine below the protective scales.In the case of the Ni-base alloy HR-214, both isothermal and cyclic exposures led to the formation of a duplex oxide morphology, composed of a columnar alumina layer overlaid by a complex Ni(Al,Cr)2O4 spinel. It was evident that thermal cycling resulted in the formation of vertical cracks in the multi-layered scale. Additionally, STEM/EDX revealed outwards transport of Cr through the cracks/ alumina grain boundaries, which caused thickening of the outer spinel layer. Moreover, an attempt was made to develop the newly introduced TKD method to study nano-sized oxide scales. This was conducted by (a) designing a dedicated sample holder, (b) specimen preparation and (c) acquisition parameters. These efforts made it possible to achieve data-rich TKD orientation maps (with indexing rates > 85%). Thus, the technique was effectively employed to obtain useful information from the microstructure and microtexture of the fine-grained oxide scales. Besides, the technique provided information concerning the crystallographic orientation relationship at oxide/oxide and oxide/alloy interfaces. Keywords: high-temperature materials; oxidation; KCl; STEM; STEM/EDX; TKD

The capability of Transmission Kikuchi Diffraction technique for characterizing nano-grained oxide scales formed on a FeCrAl stainless steel

This letter focuses on the capability of Transmission Kikuchi Diffraction (TKD) in Scanning Electron Microscope (SEM) for obtaining microstructural and micro-textural information from nano-grained oxide scales formed on a FeCrAl alloy. Orientation maps, with an indexing rate of 85%, showed the formation of grains in the range 20-300 nm. TKD revealed the existence of an orientation relationship at the alloy/oxide interface as well as the presence of a single grain (40 nm) with spinel structure in the alumina scale. A pre-tilted sample holder was designed for 110 investigations at short working distances with minimized mechanical drift of the thin foils. (C) 2015 The Authors. Published by Elsevier B.V

Development of advanced hybrid materials with the help of pulse electrodeposition

Pulse-electrodeposition has been applied to enhance properties of two different types of lightweight construction materials, a periodic cellular material (PCM) and a micro-sandwich. For the PCM, the deformation behaviour of the nanocrystalline Ni-18wt.%Fe sleeve material (bulk samples) has been investigated up to 548 K. The material exhibits plasticity of >30% fracture strain at higher temperatures compared to <15 % at room temperature. TEM characterization shows that coarser grains are present which enable strain hardening by intra-granular dislocation accumulation. This leads to larger fracture strains at higher temperatures. Hence, for allowing application of the PCM at elevated temperatures, the sleeve material has to be stabilized against deformation-induced grain growth. For the micro-sandwich, the pulse-electrodeposited Nickel coating on the face sheets or polymer fibres of the sandwich core can provide extra strength. With respect to the fibres, the plating process needs to be improved further to achieve a continuous and homogeneous coating

Deformation and texture evolution of OFHC copper during dynamic tensile extrusion

During dynamic tensile extrusion (DTE) the material is subjected to a complex deformation history, including high strain rates, large strains and elevated temperatures. This technique provides unique means to explore material performance under extreme conditions. In this work, the microstructural evolution of 99.98% commercially pure copper during the DTE test was investigated by means of electron backscatter diffraction (EBSD). The investigation was focused on the segment of the extruded jet that remained in the die, since numerical simulation showed that material points along the longitudinal axis of such segment correspond to different stages of a common temperature compensated deformation history. Therefore, post mortem microstructure information extracted at different locations along the center line is equivalent to in situ real-time measurement during the deformation process. EBSD investigations along the center line showed a progressive elongation of the grains, and an accompanying development of a strong 〈0 0 1〉 + 〈1 1 1〉 dual fiber texture. Meta-dynamic discontinuous dynamic recrystallization (DRX) occurred at larger strains, and it was demonstrated that nucleation occurred during straining, while subsequent grain growth took place during post-deformation cooling in the die. According to strain energy minimization arguments, the recrystallization resulted in an increased 〈0 0 1〉 texture component. The critical strain for recrystallization was well predicted from a power-law dependence on the Zener–Hollomon parameter, including grain size dependence and a temperature dependent activation energy. In addition, it was shown that 〈0 0 1〉 and 〈1 1 1〉oriented grains develop different dislocation substructures during straining, exhibiting elongated cells/micro-bands and typical cell structures, respectively. The present results also confirm that dynamic tensile ductility increases with decreasing initial grain size as a result of grain refinement and lowering of dislocation and twin densities during DRX

Modelling and simulation of dynamic recrystallization (DRX) in OFHC copper at very high strain rates

At high strain rates, deformation processes are essentially adiabatic and if the plastic work is large enough dynamic recrystallization can occur. In this work, an examination on microstructure evolution of OFHC copper in Dynamic Tensile Extrusion (DTE) test, performed at 400 m/s, was carried out. EBSD investigations, along the center line of the fragment remaining in the extrusion die, showed a progressive elongation of the grains, and an accompanying development of a strong < 001 > + < 111 > dual fiber texture. Discontinuous dynamic recrystallization (DRX) occurred at larger strains, and it was showed that nucleation occurred during straining. A criterion for DRX to occur, based on the evolution of Zener-Hollomon parameter during the dynamic deformation process, is proposed. Finally, DTE test was simulated using the modified Rusinek-Klepaczko constitutive model incorporating a model for the prediction of DRX initiation

On the capability of in-situ exposure in an environmental scanning electron microscope for investigating the atmospheric corrosion of magnesium

The feasibility of environmental scanning electron microscope (BEM) in studying the atmospheric corrosion behavior of 99.97% Mg was investigated. For reference, ex-situ exposure was performed. A model system was designed by spraying few salt particles on the metal surface and further promoting the corrosion process using platinum (Pt) deposition in the form of 1 x 1 x 1 pm(3) dots around the salt particles to create strong artificial cathodic sites. The results showed that the electron beam play a significant role in the corrosion process of scanned regions. This was attributed to the irradiation damage occurring on the metal surface during the BEM in-situ experiment. After achieving to a reliable process route, in a successful attempt, the morphology and composition of the corrosion products formed in-situ in the ESEM were in agreement with those of the sample exposed ex-situ

A new semi-solid casting technique for fabricating SiC-reinforced Mg alloys matrix composites

The capability of the newly developed rheocasting (RC) technique in combination with the RheoMetal process for producing SiC particulate-reinforced AM50 and AZ91D matrix composites (Mg-based MMCs) was investigated. The quality of the MMCs was studied by analyzing the fraction of casting pores, number density of SiC clusters and the uniformity of SiC particles. Solid fraction, particle size and oxidation of SiC particles had strong impacts on the overall quality of the MMCs. The MMCs produced by 40% solid fraction and oxidized micron-sized SiC particles exhibited an excellent casting quality. A low-quality MMC was obtained when non-oxidized sub-micron sized SiC particles were employed. The results showed the formation of various types intermetallic particles and carbides such as MgO, Mg2Si, Al2MgC2, Mg2C3, Al4C3 as the interfacial reaction products of SiC/Mg alloy's melts. Mg hydride (alpha-MgH2) was also identified in inter-dendritic regions of the MMCs for the first time

Novel Insights into the Oxidation of High Temperature Alloys - The Role of Environment, Microstructure and Reactive elements

The properties of materials can be deteriorated, when they react with an environment at high temperature and form various types of corrosion products, such as oxides and nitrides. In order to perform successfully at elevated temperatures, all these materials are required to be protected by a slow-growing, continuous and adherent surface oxide layer. Despite years of scientific study, there are still many technological and research questions in the field. In this thesis, the high-temperature corrosion behavior of a number of iron (Fe)- and nickel (Ni)-base alloys are studied and some of the long-standing scientific "mysteries" are addressed. In addition, the thesis presents the fruitful development of the newly introduced TKD method, which was extensively employed to study the microstructure and microtexture of fine-grained oxide scales. \ua0The initial stages of KCl-induced oxidation behavior of two alumina formers (alloys Kanthal APMT and TH1) and one chromia former (alloy Sanicro 25) were analyzed in an O2/H2O environment using an in-situ ESEM method, which were complemented by ex-situ exposures. The in-situ oxidation experiments provided an opportunity to view dynamic processes occurring during the oxidation process ′′live′′. Notably, chlorination of the alloys was evidenced by detection of chlorine below the oxide scales already after 1 hour of exposure. In addition, the effects of thermal cycling on the oxidation behavior of an alumina forming Ni-base alloy (HR-214) was studied in air at 1200\ub0C. Vertical cracks due to the thermal cycling caused consumption of Al for re-healing the cracks and sooner occurrence of transition from alumina to chromia scale. Moreover, nitridation resistance of an alumina-forming alloy (Kanthal APMT) was also studied in a mixture of 95% N2 + 5% H2 at 900\ub0C, where probable paths for nitrogen dissociation and diffusion into the alloy were suggested.\ua0The cornerstone of the thesis is indeed the unravelling of the connection between two long-standing enigmas in the field, i.e., the roles of water and the REs. The interplay between water and REs caused the formation of a previously unrecognized (″messy″) transient nanocrystalline alumina layer with yttrium-decorated grain boundaries. A new scenario for high temperature oxidation is presented, in which water diffuses along yttrium-decorated alumina grain boundaries and is cathodically reduced within the scale. This understanding is supported by identification of hydride in the oxide scale using low-loss EELS. The concept of ″critical″ size of the RE particles on the oxidation performance is explored

Microstructural characterization and formation of alpha \u27 martensite phase in Ti-6Al-4V alloy butt joints produced by friction stir and gas tungsten arc welding processes

The obtained microstructures of a Ti-6Al-4V alloy welded by Gas Tungsten Arc Welding (GTAW) and Friction Stir Welding (FSW) were investigated and evaluated quantitatively. In the GTAW method, the effect of current was examined so that the samples were subjected to various currents between 90 and 120 A. In the FSW process, samples were welded by different rotational speeds (450-850 rpm). Non-destructive tests including Visual and Radiography Tests (VT and RT) were used to identify defect-free samples. The microstructural studies by electron microscopes revealed formation of different phases in the weld area of the samples welded via mentioned methods. The recorded peak temperatures in the weld regions compared favorably with the expectations about the evolved microstructures. A bimodal microstructure was just obtained in the FSWed sample with a peak temperature below beta transus temperature (