No description available.

No description available.

This work deals with issues of preparation of intermetallics based on iron, nickel and titanium aluminides. It works with an idea of preparation of bulk material by reaction synthe-sis from kinetic spraying deposits by cold spray. Theoretical part is concerned with phases and compounds of these aluminides for structural applications, their characteristics and present fabrication. In experimental part there are studied microstructures created by annealing of deposits.

Single crystals of iron with 20, 28 and 40 at. % aluminium were deformed in compression at room temperature. The later two alloys were deformed also at temperatures in the range of yield stress anomaly. Room temperature deformation was carried under the atomic force microscope (AFM) and the evolution of surface was recorded in-situ. Samples deformed at elevated temperatures were investigated by AFM after the deformation. Dislocation structures in deformed samples were then investigated in transmission electron microscope (TEM). Observations of surface (AFM) and bulk (TEM) are compared. Results of both techniques mutually agree and support the interpretation of observed phenomena. Several original analysis methods were developed. Most notably the stereographic reconstruction, which was applied to dislocation structures and carbide particles present in investigated alloys. Model explaining the distribution of carbide particle axes is presented. Powered by TCPDF (www.tcpdf.org)

Aluminetos de ferro pertencem a uma classe de materiais interessantes por combinarem excelente resistência à oxidação com boas propriedades mecânicas em temperaturas moderadas a altas (até 500°C). Estes materiais, contudo, em temperatura ambiente, possuem baixa ductilidade (menos de 5% de alongamento à tração), característica correlacionada a efeitos ambientais, o que dificulta seu processo de conformação. A fundição é uma via de processamento tradicionalmente aplicada às ligas frágeis (por exemplo, ferro fundido cinzento), sendo necessário conhecer as propriedades termoquímicas da liga para que as peças fundidas sejam livres de defeitos atribuídos ao processo, como a formação de poros e rechupes. Neste trabalho foram investigadas três ligas distintas de intermetálicos, Fe28Al, Fe28Al6Cr e Fe28Al6Cr1Ti, sob a influência de diferentes condições de solidificação. Para estas três composições de liga, observou-se a redução do tamanho de grão em até 60% pela adição de Al-5Ti-1B como inoculante ao metal fundido. As ligas foram produzidas em condições laboratorias (forno de indução com capacidade máxima de 2 kg e proteção sob fluxo de Ar), bem como em escala industrial (forno de indução com capacidade de 100 kg). O processo em escala industrial resultou em peças de fundição com pequena quantidade de defeitos, sendo este um indicativo da capacidade de produzir peças destes aluminetos de ferro diretamente pelo processo de fundição. A técnica de tomografia de raios X auxiliou na mensuração da contração linear (~6%) e tendência à formação de defeitos. Além da caracterização microestrutural das ligas produzidas, este trabalho, ao relatar processo de fundição em escala industrial, inclui uma discussão sobre a reatividade do metal fundido e o material refratário que reveste o forno de indução. Dessa forma, os dados obtidos permitem averiguar a fundibilidade de ligas da família FeAl pelo uso de forno de indução. / Iron aluminides are good candidates for applications at moderate to high temperatures (up to 500 °C) because they combine excellent oxidation resistance with good mechanical properties. However, these materials have low ductility at room temperature (less than 5% traction elongation), a characteristic correlated to environmental effects, which hinders their conformation process. Casting is a processing route traditionally applied to brittle alloys (eg, gray cast iron), it is necessary to know the thermochemical properties of the alloys to reduce the formation of defects attributed to the casting process, such as the formation of pores, scar and blows. In this work, three different alloys of intermetallic (Fe28Al, Fe28Al6Cr and Fe28Al6Cr1Ti) were investigated under the influence of different solidification conditions. For these three alloy compositions, grain size reduction by up to 60% was observed by the addition of Al-5Ti-1B as an inoculant to the molten metal. The alloys were produced under laboratory conditions (induction furnace with a maximum capacity of 2 kg and protection under Air flow), as well as industrial scale (induction furnace with capacity of 100 kg). The industrial scale process resulted in castings with a small number of defects, which is an indication of the ability to produce mechanical parts of these iron aluminides directly by the casting process. The X-ray tomography technique assisted in the measurement of linear contraction (~ 6%) and tendency to defect formation. In addition to the microstructural characterization of the alloys, this work includes a discussion about the reactivity of the molten metal and the refractory material coating the induction furnace. In this way, the obtained data allow to investigate the FeAl alloys castability by the use of induction furnace.

Titanium matrix composites have been extensively evaluated for their potential to replace conventional superalloys in high temperature structural applications, with significant weight-savings while maintaining comparable mechanical properties. The purpose of this investigation is the evaluation of a gamma titanium aluminide alloy with nominal composition Ti-46.5Al-4(Cr,Nb,Ta,B)at.% as a matrix material for use in intermediate temperature applications (400-800㩠in future aerospace transportation systems, as very light-weight structures are needed for cost and weight reduction goals. Mechanical characterization testing was performed over the potential usable temperature range (21-800㩮 Thermal expansion behavior was evaluated, as thermal mismatch of the constituents is an expected problem in composites employing this matrix material. Monotonic testing was conducted on rolled sheet material samples to obtain material properties. The alloy exhibited good strength and stiffness retention at elevated temperatures, as well as improved toughness. Monotonic testing was also conducted on specimens exposed to elevated temperatures to determine the degradation effects of high temperature exposure and oxidation. The exposure did not significantly degrade the alloy properties at elevated temperatures; however, room temperature ductility decreased. Analytical modeling using AGLPLY software was conducted to predict the residual stress state after composite consolidation as well as the potential mechanical behavior of [0]4 laminates with a 㭍ET matrix. Silicon carbide (Ultra-SCS) and alumina (Nextel 610) fibers were selected as potential reinforcing materials for the analysis. High residual stresses were predicted due to the thermal mismatch in the materials. Laminates with Nextel 610 fibers were found to offer the better potential for a composite in this comparison as they provide a better thermal match. Coupons of SCS-6/㭍ET were manufactured with different volume fractions (10% and 20%). Both manufacturing attempts resulted in transverse cracking in the matrix from the residual thermal stress.

Point defects control many properties of technological importance in intermetallic compounds such as atomic diffusion, creep, hardness, mechanical properties and sintering. Farther, since intermetallic compounds are characterized by long range atomic order, the point defects in these compounds can be qualitatively different from those in pure metals and disordered alloys. In the present study, we have chosen β-NiAl for our point defect studies since it is a potential candidate for high temperature applications and a model system for the study of basic phenomena in ordered alloys. We have used a mean field formulation for studying point defect concentrations. The outline of the formulation is as follows: We divide the rigid, body centred cubic lattice into two interpenetrating cubic sublattices called α and j3 which are made up of the cube corners and body centres respectively. We write a generic free energy function (G) that involves the temperature T and the six sublattice occupancies viz., the A (Ni), B (Al) and vacancies (V) on the two sublattices α andβ. We use the constraints on the number of α and β sublattice sites viz., the number of α sublattice sites is equal to the number of β sublattice sites, to write G as a function of four of the six sublattice occupancies and T. We define three auxiliary parameters η1, η2 and η3 which correspond to the vacancy concentration, the differential B species population on the two sublatices (the chemical or atomic order), and the differential vacancy population on the two sublattices, respectively. We then rewrite G as a function of T, xB and ηi. The G can now be minimized with respect to the three auxiliary variables so that we recover the free energy (G) as a function of XB and T only. The formulation requires as inputs the Ni-Ni, Al-Al, Ni-Al, Ni-V and Al-V interaction energies in the nn and nnn shells. We have obtained the Ni-Ni, Al-Al and Ni-Al interaction energies from the effective pair potentials reported in the literature. For the Ni-V and Al-V interaction energies we have used a bond breaking model in which we have assumed that the Ni-V and Al-V interaction energies in the nnn shell to be zero. Using the above interaction parameters in our mean field formulation we have determined the concentrations of various types of point defects in β-NiAL We have specifically chosen the temperature range of 800 - 2000 K and the composition range of 45 - 55 atomic% Al. Our results can be summarised as follows: 1.The predominant defect in the stoichiometric alloy is a combination of an Ni-antisite defect and two vacancies on the Ni sublattice. 2.The Al-rich alloys of composition (50 + ∆) atomic% contain 2∆% vacancies;since the alloys are almost perfectly ordered, these vacancies predominantly occupy the Ni sublattice. Similarly, the Ni-rich alloys of composition (50 — ∆)atomic% contain ∆% Ni antisites. 3.Both the vacancies on the Ni sublattice (in Al-rich alloys) and Ni-antisites (in Ni-rich alloys) show negligible temperature dependence, and hence owe their origin to the off-stoichiometry. 4.In all the alloys, the Al-antisites have the lowest concentration (of the order 10-6 even at 2000 K) and the concentration of the vacancies on the β sublattice is the next lowest. Thus, our results support the view that β-NiAl is a triple defect B2 and, if we consider constitutional vacancies as those which have a little or no temperature dependence, there exist constitutional vacancies in Al-rich β-NiAl. This conclusion is in agreement with some of the experimental results. However, it must be pointed out that there is considerable disagreement among experimental results from different groups.

Dissertation (Ph.D.)--Auburn University, 2005. / Abstract. Vita. Includes bibliographic references.

Thesis (M. Sc. ( Physics)) -- University of Limpopo, 2020 / In this work, we present first-principles calculation on the structural, thermodynamic, mechanical and electronic stabilities of Fe-Al and FeAl-X (X: Pt, Pd, Ru and Ag) alloys at lower and high temperatures. These systems have recently attracted a lot of attention for both scientific and possible technological application in turbines, Steel-It coating, energy sector, boilers, pipes and automotive parts as a potential replacement of steel due to their excellent resistance to oxidation at high temperatures. However, they suffer limited room temperature ductility and a sharp drop in strength above 873 K. We determined the lattice parameters, heats of formation, elastic constants, bulk to shear moduli, density of states, phonon dispersion curve and X-ray diffraction pattern for binary and ternary system at various concentrations between 0 ≤ x ≤ 10. Furthermore, the lattice expansion, elastic constants, Gibbs free energy, X-ray diffraction pattern and radial distribution function were done on the most stable systems to determine the melting point of FeAl-X ternary systems. A systematic investigation was performed on the stability of the Fe-Al alloys at zero K. We employed CASTEP code to evaluate the thermodynamic, elastic and electronic stability. Virtual crystal approximation was used to determine various atomic concentrations (0 ≤ x ≤ 5) of both Pt and Ru; this allowed more precise predictions on the materials’ behaviour. Further analysis was done on the density of states to describe the behaviour of each phase near the Fermi level; these phases were observed at different percentage compositions. A supercell approach, DMol3 was also used to evaluate these systems at a larger scale (0 ≤ x ≤ 50). VASP and LAMMPS codes were used to determine the stability of these FeAl-X ternary systems at concentrations (0 ≤ x ≤ 10). It was found that the equilibrium lattice parameters of the binary systems are in good agreement to within 2% with the available experimental data. The heats of formation showed that β2 FeAl phase was the most energetically stable system since it displayed the lowest value compared to all other binary systems. This observation accord well with the experimental phase diagram. It was also confirmed from the corresponding electronic DOS behaviour near the Fermi level. Furthermore, the shear modulus (C’) of these Fe-Al binary systems, i.e. FeAl, Fe2Al5, Fe4Al13, Fe5Al8, Fe2Al and FeAl3 were found to be positive fulfilling the condition of stability. The Fe2Al5 system was found to be the second most stable phase, followed by the monoclinic structure Fe4Al13. This observation was confirmed from the total DOS (where the Fermi level falls in the pseudogap, condition of stability). We further employed virtual crystal approximation and supercell approaches to model various atomic compositions at 0 ≤ x ≤1 and 0 ≤ x ≤ 50 for Ag, Pt, Pd and Ru. The heats of formation, density of states and elastic constants were determined to describe the structural, thermodynamic and mechanical stability of these systems. It was found that the addition of Ag, Pt, Pd and Ru enhances the stability at lower atomic percentage composition below 0.5%. Interestingly, the addition of Pt and Ru was found to significantly improve the ductility of the ternary FeAl-X compound for 0.2 and 0.5 at. % compositions. These systems showed that the Fe-sublattice was the preferred doping site with promising improvement in strength on the properties. It was further deduced that Ag and Pd stabilize the FeAl-X system at atomic percentage compositions of 0.5 and 0.7 respectively. Furthermore, a molecular dynamics-based LAMMPS-EAM was employed to model Fe50-XXXAl doped systems with either Ag, Pt or Pd. The lattice site preferences of the dopant were deduced from their energy landscape. More importantly, Ag and Pd doped systems gave comparable transition temperatures to experimental findings of 1273 K and 1073 K, respectively. Their thermodynamic and mechanical stability trends showed promising properties for industrial applications, displaying stability at a high temperature below 1300 K. This was evident for Ag, Pt (0.5 at %) and Pd (0.7 at %) doping as was the most stable systems with respect to Cij, ΔG, and RDF’s which indicated to influence the elastic instability above 1200 K as well as the ductility of these systems. The XRD confirmed that the doped systems preserved the structural symmetry as expected.

There has been an urge for increasing the efficiency in advanced gas turbine engines. To fulfill these needs the inlet gas temperatures should be increased in the gas turbine engines, thermal barrier coatings (TBCs) have gained significant applications in increasing the gas inlet temperatures. Insulating characteristics of ceramic TBCs allow the operation at up to 150~250 ˚C higher gas temperatures. Because of the severe turbine engine operating conditions that include high temperature, steep temperature gradient, thermal cycling, oxidation and hot-corrosion, TBCs can fail by spallation at the interface between the metal and ceramic. The lack of understanding in failure mechanisms and their prediction warrant a development of non-destructive evaluation technique that can monitor the quality and degradation of TBCs. In addition, the development of NDE technique must be based on a robust correlation to the characteristics of TBC failure. The objective of this study is to develop electrochemical impedance spectroscopy (EIS) as a Non-destructive evaluation (NDE) technology for application to TBCs. To have a better understanding of the multilayer TBCs using EIS they were divided into individual layers and EIS were performed on them. The individual layers included polycrystalline ZrO2-7~8 wt.%Y2O3 (YSZ) (topcoat) of two different densities were subjected to sintering by varying the sintering temperature and holding time for three different thickness and hot extruded NiAl alloy buttons which were subjected to isothermal oxidation with varying temperature and time. NiAl is as similar to the available commercial bondcoats used in TBCs. Then degradation monitoring with electrolyte penetration was carried out on electron beam physical vapor deposited (EB PVD) TBCs as a function of isothermal exposure. Quality control for air plasma sprayed TBCs were carried out as a function of density, thickness and microstructure. Dense vertically cracked TBCs were tested as a function of vertical crack density and thickness. Electrochemical impedance response was acquired from all specimens at room temperature and analyzed with an AC equivalent circuit based on the impedance response as well as multi-layered structure and micro-constituents of specimens. Physical and microstructural features of these specimens were also examined by optical and electron microscopy. The EIS measurement was carried out in a three-electrode system using a standard Flat Cell (K0235) from Princeton Applied Research™ and IM6e BAS ZAHNER™ frequency response analyzer. The electrolyte employed in this investigation was 0.01M (molar) potassium Ferri/Ferro Cyanide {(K3Fe(CN)6/K4Fe(CN)6·3H2O)}. The thickness and density were directly related to the resistance and capacitance of the polycrystalline YSZ with varying thickness and open pores. As the effective thickness of the YSZ increased with sintering time and temperature, the resistance of the YSZ (RYSZ) increased proportionally. The variation in capacitance of YSZ (CYSZ) with respect to the change in porosity/density and thickness was clearly detected by EIS. The samples with high porosity (less dense) exhibited large capacitance, CYSZ, compared to those with less porosity (high density), given similar thickness. Cracking in the YSZ monoliths resulted in decrease of resistance and increase in capacitance and this was related to the electrolyte penetration. Growth and spallation of TGO scale on NiAl alloys during isothermal oxidation at various temperatures and holding time was also correlated with resistance and capacitance of the TGO scale. With an increase in the TGO thickness, the resistance of the TGO (RTGO) increased and capacitance of the TGO (CTGO) decreased. This trend in the resistance and capacitance of the TGO changed after prolonged heat treatment. This is because of the spallation of the TGO scale from the metal surface. The parabolic growth of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO, excluding the abrupt changes associated with the failure. As a function of isothermal exposure for EB-PVD TBCs, initial increase in the resistance of YSZ with thermal exposure was observed perhaps due to the high temperature sintering of YSZ. The parabolic growth of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO. An explanation based on electrolyte penetration into sub-critical damage is proposed for the gradual decrease in the resistances of YSZ and TGO with prolonged thermal exposure. Observation of exposed metallic bond coat surface on the fracture surface, which readily provides conduction, was related to the abrupt and large increase in the capacitance of YSZ and TGO. A direct relation between the resistance of the YSZ (RYSZ) and density of the YSZ was observed for APS TBCs with varying topcoat density. APS TBCs with varying topcoat chemistry and thickness were tested and directly related to resistance of topcoat. With the increase in the topcoat thickness, the capacitance decreased and the resistance increased. The higher values of CCAT and RCAT compared to that of CYSZ and RYSZ were related to the higher dielectric constant and resistivity of CaTiO3. Dense vertically cracked TBCs were tested with varying crack density were tested and the variation in the resistance was related indirectly to the cracks and directly to the difference in the thickness of the topcoat. EB-PVD TBCs with varying density (dense and columnar) were tested and the variation in resistance was attributed to the dense structure and columnar structure of the topcoat with columnar structure having lower resistance because of more electrolyte penetration through the columnar structure. From this study, EIS showed a potential as a NDE technique for quality assurance and lifetime remain assessment of TBCs. Future work should continue on developing a mathematical model to study the impedance curves and come up with a model for individual layers of TBC and then sum them up to get the multilayered TBC response. The flexible instrument probe of EIS needs to be designed and tested for field evaluation of TBCs.