In response to the extensive energy demands on national and global levels, concentrated solar power (CSP) plants are designed to harness and convert solar energy to electricity. For such green energy application, robust, reliable and durable materials for CSP constructions are required. The corrosion resistance is among many parameters to consider in these thermalelectrical stations such as for pipes and storage tanks in CSP. In this investigation, the corrosion behavior of AISI 304 stainless steel (18 wt. % Cr, 8 wt. % Ni) with the heat transfer fluid, also known as solar salt, has been examined. The ternary eutectic salt mixture with the composition, 53 wt. % KNO3, 40 wt. % NaNO2, and 7 wt. % NaNO3, that melts at 142°C, has a potential use in CSP as a heat transfer fluid. The solar salt was prepared for this corrosion study from reagent grades of high purity nitrites and nitrates. Samples of AISI 304 stainless steel were sectioned from a sheet stock of the alloy and exposed to solar salt at 530°C in air at 1 atmospheric pressure. After test intervals of 250, 500, and 750 hours in total immersion condition, AISI 304 stainless steel samples have developed a scale of corrosion products made up of multiple oxides. X-ray diffraction and scanning electron microscopy with X-ray energy-dispersive spectroscopy were employed to examine the extent of corrosion and identify the corrosion products. Transmission electron microscopy was used to verify the corrosion products identity via electron diffraction patterns. Oxides of iron were found to be the primary corrosion products in the presence of the molten alkali nitrates-nitrite salt mixture because of the dissolution of the protective chromium oxide (Cr2O3) scale formed on AISI 304 stainless steel coupons. The corrosion scale was uniform in thickness and made up of sodium iron oxide (NaFeO2), iron oxide, hematite (Fe2O3), and chromium-iron oxide (Cr,Fe)2O3 solid solution. The latter was iv found near the AISI 304 stainless steel. This indicates that the scale formed, particularly on the upper layers with presence of sodium iron oxide and iron oxide, hematite, is protective, and forms an effective barrier against penetration of fused solar salt. At the alloy interface with the bulk corrosion scale, the corrosion process induced a compositional modification in the grains located at the interface. There are iron rich and iron depleted grains at the interface if compared to the nominal iron content of the alloy. The mode of attack is identified as uniform at the test temperature of 530°C, showing a parabolic behavior with a parabolic rate constant (Kp) equals to (m2 /sec). By extrapolation, annual corrosion rate is estimated to reach 0.784 mils per year. Corrosion behavior of AISI 304 stainless steel is discussed in terms of thermodynamics and reaction paths.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20703号 / 工博第4400号 / 新制||工||1684(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 乾 晴行, 教授 安田 秀幸 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

In the combustion zone of industrial- and aero- gas turbines, thermomechanical fatigue (TMF) is the dominant damage mechanism. Thermomechanical fatigue is a coupling of independent creep, fatigue, and oxidation damage mechanisms that interact and accelerate microstructural degradation. A mixture of intergranular cracking due to creep, transgranular cracking due to fatigue, and surface embrittlement due to oxidation is often observed in gas turbine components removed from service. The current maintenance scheme for gas turbines is to remove components from service when any criteria (elongation, stress-rupture, crack length, etc.) exceed the designed maximum allowable. Experimental, theoretical, and numerical analyses are performed to determine the state of the component as it relates to each criterion (a time consuming process). While calculating these metrics individually has been successful in the past, a better approach would be to develop a unified mechanical modeling that incorporates the constitutive response, microstructural degradation, and rupture of the subject material via a damage variable used to predict the cumulative “damage state” within a component. This would allow for a priori predictions of microstructural degradation, crack propagation/arrest, and component-level lifing. In this study, a unified mechanical model for creep-fatigue (deformation, cracking, and rupture) is proposed. It is hypothesized that damage quantification techniques can be used to develop accurate creep, fatigue, and plastic/ductile cumulative- nonlinear- damage laws within the continuum damage mechanics principle. These damage laws when coupled with appropriate constitutive equations and a degrading stiffness tensor can be used to predict the mechanical state of a component. A series of monotonic, creep, fatigue, and tensile-hold creepfatigue tests are obtained from literature for 304 stainless steel at 600°C (1112°F) in an air. iv Cumulative- nonlinear- creep, fatigue, and a coupled creep-fatigue damage laws are developed. The individual damage variables are incorporated as an internal state variable within a novel unified viscoplasticity constitutive model (zero yield surface) and degrading stiffness tensor. These equations are implemented as a custom material model within a custom FORTRAN onedimensional finite element code. The radial return mapping technique is used with the updated stress vector solved by Newton-Raphson iteration. A consistent tangent stiffness matrix is derived based on the inelastic strain increment. All available experimental data is compared to finite element results to determine the ability of the unified mechanical model to predict deformation, damage evolution, crack growth, and rupture under a creep-fatigue environment.

Study of the metastable phases obtained by non-equilibrium processing techniques has come a long way during the past five decades. New metastable phases have often given new perspectives to the research on synthesis of novel materials systems. Metastable materials produced by two non-equilibrium processing methods were studied for this dissertation- 304-type austenitic stainless steel (SS304 or Fe-18Cr-8Ni)+aluminum coatings produced by plasma enhanced magnetron sputter-deposition (PEMS) and nanocrystalline Ti, Zr and Hf powders processed by mechanical milling (MM). The objective of the study was to understand the crystallographic and microstructural aspects of these materials. Four SS304+Al coatings with a nominal Al percentages of 0, 4, 7 and 10 wt.% in the coatings were deposited on an SS304 substrate by PEMS using SS304 and Al targets. The as-deposited coatings were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and three-dimensional atom probe microscopy (3DAP). Surface morphology and chemical analysis were studied by SEM. Phase identification was carried out by XRD and TEM. The microstructural features of all the coatings, as observed in the TEM, consisted of columnar grains with the columnar grain width (a measure of grain size) increasing with an increase in the Al content. The coatings had grains with average grain sizes of about 100, 290, 320 and 980 nm, respectively for 0, 4, 7 and 10 wt.% Al. The observed grain structures and increase in grain size were related to substrate temperature during deposition. XRD results indicated that the Al-free coating consisted of the non-equilibrium ferrite and sigma phases. In the 4Al, 7Al and 10Al coatings, equilibrium ferrite and B2 phases were observed but no sigma phase was found. In 10Al coating, we were able to demonstrate experimentally using 3DAP studies that NiAl phase formation is preferred over the FeAl phase at nano scale. During mechanical milling of the hexagonal close packed (HCP) metals Hf, Ti and Zr powders, unknown nanocrystalline phases with face centered cubic (FCC) structure were found. The FCC phases could be either allotropes of the respective metals or impurity stabilized phases. However, upon MM under high purity conditions, it was revealed that the FCC phases were impurity stabilized. The decrease in crystallite size down to nanometer levels, an increase in atomic volume, lattice strain, and possible contamination were the factors responsible for the transformation.

"Laser micromachining is essential in today’s advanced manufacturing, of e.g., printed circuit boards and electronic components, especially laser microdrilling. Continued demands for miniaturization, in particular of high-performance MEMS components, have generated a need for smaller holes and microvias as well as smaller and more controllable spot-welds than ever before. All these neeeds require smaller taper of the microholes and more stable and controlled laser micromachining process than currently available. Therefore considerable attention must be focused on the laser process parameters that control critical specifications such as accuracy of the hole size as well as its shape and taper angle, all of which highly influence quality of the laser micromachining processes. Determination of process parameters in laser micromachining, however, is expensive because it is done mostly by trial and error. This Dissertation attempts to reduce the experimental time and cost associated with establishing the process parameters in laser micromachining by employing analytical, computational, and experimental solutions (ACES) methodology."

Esta tese apresenta o estudo sobre microconformação, no caso microextrusão de quatro materiais diferentes, um aço SAE 1020, um aço inoxidável AISI 304, um alumínio AA6531 e um latão ASTM C34000. Para avaliar o efeito do tamanho da peça sobre o processo de microextrusão, dois tamanhos de corpos de prova foram utilizados, um com ∅ 4 mm e outro com ∅ 1 mm. Para cada tamanho de corpo de prova, três ângulos de extrusão foram utilizados, 30°, 45° e 60°. A primeira parte do trabalho se resume a caracterização dos materiais, com analise química e metalográfica. Após a caracterização dos materiais, as curvas de escoamento através do ensaio de compressão utilizando dois tamanhos de corpos de prova foram feitos em todos os materiais. O atrito foi determinado utilizando o ensaio de anel de atrito com três tamanhos diferentes de corpos de prova. As curvas de calibração foram feitas utilizando o software SIMUFACT. Os ensaios de extrusão foram feitos em uma máquina de ensaio universal com capacidade de captura de dados como força e deslocamento. Um modelo matemático foi utilizado para comparar a força de extrusão calculada com o medido no processo. Os resultados indicam que as curvas de escoamento de tamanho macro podem ser aplicadas a peças de tamanho meso. O atrito medido pelo ensaio de anel de atrito mostrou que os valores de atrito de tamanho micro tem um pequeno valor maior que para tamanho macro. Os valores de força de extrusão calculada e medido no ensaio para peças de tamanho meso tem boa aproximação com diferença de 3,2% para o aço inoxidável. Para peça de tamanho micro, a diferença entre o valor medido e o calculado aumenta chegando a diferença de 995% para o corpos de prova de aço comum. / This thesis presents the study about microforming, in this case microextrusion of the four different materials, an SAE 1020 steel, an AISI 304 stainless steel, an AA6531 aluminum and a C34000 brass. To evaluate the size effect about microextrusion process, two sizes of specimens were used, with a ∅ 4 mm and another with ∅ 1 mm. For each size of specimen, three extrusion angles were used, 30 °, 45 ° and 60 °. The first part of the work was to materials characterizations with chemical and metallographic analysis. Following the materials characterization, the flow stress curves was made using the compression test with two sizes of specimens. The friction was determined using the friction ring test with the three different size. Calibration curves were performed using the software SIMUFACT . The extrusion tests were performed in a universal testing machine capable of capturing such as force and displacement data. A mathematical model was used to compare the extrusion force it was calculated and the force measured in microextrusion. The results indicate that the flow stress curves of macro size can be applied to meso sizes. The ring friction test indicate that the friction values of the micro size has a small value greater than macro size. The extrusion force calculated and measured in the test to meso size has good approximation with a difference of 3.2% in the stainless steel. To pieces of the micro size, the difference between the measured and calculated force increases 995% for the samples of SAE 1020 steel.

Esta tese apresenta o estudo sobre microconformação, no caso microextrusão de quatro materiais diferentes, um aço SAE 1020, um aço inoxidável AISI 304, um alumínio AA6531 e um latão ASTM C34000. Para avaliar o efeito do tamanho da peça sobre o processo de microextrusão, dois tamanhos de corpos de prova foram utilizados, um com ∅ 4 mm e outro com ∅ 1 mm. Para cada tamanho de corpo de prova, três ângulos de extrusão foram utilizados, 30°, 45° e 60°. A primeira parte do trabalho se resume a caracterização dos materiais, com analise química e metalográfica. Após a caracterização dos materiais, as curvas de escoamento através do ensaio de compressão utilizando dois tamanhos de corpos de prova foram feitos em todos os materiais. O atrito foi determinado utilizando o ensaio de anel de atrito com três tamanhos diferentes de corpos de prova. As curvas de calibração foram feitas utilizando o software SIMUFACT. Os ensaios de extrusão foram feitos em uma máquina de ensaio universal com capacidade de captura de dados como força e deslocamento. Um modelo matemático foi utilizado para comparar a força de extrusão calculada com o medido no processo. Os resultados indicam que as curvas de escoamento de tamanho macro podem ser aplicadas a peças de tamanho meso. O atrito medido pelo ensaio de anel de atrito mostrou que os valores de atrito de tamanho micro tem um pequeno valor maior que para tamanho macro. Os valores de força de extrusão calculada e medido no ensaio para peças de tamanho meso tem boa aproximação com diferença de 3,2% para o aço inoxidável. Para peça de tamanho micro, a diferença entre o valor medido e o calculado aumenta chegando a diferença de 995% para o corpos de prova de aço comum. / This thesis presents the study about microforming, in this case microextrusion of the four different materials, an SAE 1020 steel, an AISI 304 stainless steel, an AA6531 aluminum and a C34000 brass. To evaluate the size effect about microextrusion process, two sizes of specimens were used, with a ∅ 4 mm and another with ∅ 1 mm. For each size of specimen, three extrusion angles were used, 30 °, 45 ° and 60 °. The first part of the work was to materials characterizations with chemical and metallographic analysis. Following the materials characterization, the flow stress curves was made using the compression test with two sizes of specimens. The friction was determined using the friction ring test with the three different size. Calibration curves were performed using the software SIMUFACT . The extrusion tests were performed in a universal testing machine capable of capturing such as force and displacement data. A mathematical model was used to compare the extrusion force it was calculated and the force measured in microextrusion. The results indicate that the flow stress curves of macro size can be applied to meso sizes. The ring friction test indicate that the friction values of the micro size has a small value greater than macro size. The extrusion force calculated and measured in the test to meso size has good approximation with a difference of 3.2% in the stainless steel. To pieces of the micro size, the difference between the measured and calculated force increases 995% for the samples of SAE 1020 steel.

Esta tese apresenta o estudo sobre microconformação, no caso microextrusão de quatro materiais diferentes, um aço SAE 1020, um aço inoxidável AISI 304, um alumínio AA6531 e um latão ASTM C34000. Para avaliar o efeito do tamanho da peça sobre o processo de microextrusão, dois tamanhos de corpos de prova foram utilizados, um com ∅ 4 mm e outro com ∅ 1 mm. Para cada tamanho de corpo de prova, três ângulos de extrusão foram utilizados, 30°, 45° e 60°. A primeira parte do trabalho se resume a caracterização dos materiais, com analise química e metalográfica. Após a caracterização dos materiais, as curvas de escoamento através do ensaio de compressão utilizando dois tamanhos de corpos de prova foram feitos em todos os materiais. O atrito foi determinado utilizando o ensaio de anel de atrito com três tamanhos diferentes de corpos de prova. As curvas de calibração foram feitas utilizando o software SIMUFACT. Os ensaios de extrusão foram feitos em uma máquina de ensaio universal com capacidade de captura de dados como força e deslocamento. Um modelo matemático foi utilizado para comparar a força de extrusão calculada com o medido no processo. Os resultados indicam que as curvas de escoamento de tamanho macro podem ser aplicadas a peças de tamanho meso. O atrito medido pelo ensaio de anel de atrito mostrou que os valores de atrito de tamanho micro tem um pequeno valor maior que para tamanho macro. Os valores de força de extrusão calculada e medido no ensaio para peças de tamanho meso tem boa aproximação com diferença de 3,2% para o aço inoxidável. Para peça de tamanho micro, a diferença entre o valor medido e o calculado aumenta chegando a diferença de 995% para o corpos de prova de aço comum. / This thesis presents the study about microforming, in this case microextrusion of the four different materials, an SAE 1020 steel, an AISI 304 stainless steel, an AA6531 aluminum and a C34000 brass. To evaluate the size effect about microextrusion process, two sizes of specimens were used, with a ∅ 4 mm and another with ∅ 1 mm. For each size of specimen, three extrusion angles were used, 30 °, 45 ° and 60 °. The first part of the work was to materials characterizations with chemical and metallographic analysis. Following the materials characterization, the flow stress curves was made using the compression test with two sizes of specimens. The friction was determined using the friction ring test with the three different size. Calibration curves were performed using the software SIMUFACT . The extrusion tests were performed in a universal testing machine capable of capturing such as force and displacement data. A mathematical model was used to compare the extrusion force it was calculated and the force measured in microextrusion. The results indicate that the flow stress curves of macro size can be applied to meso sizes. The ring friction test indicate that the friction values of the micro size has a small value greater than macro size. The extrusion force calculated and measured in the test to meso size has good approximation with a difference of 3.2% in the stainless steel. To pieces of the micro size, the difference between the measured and calculated force increases 995% for the samples of SAE 1020 steel.