Structural analysis and corrosion studies on an ISO 5832-9 biomedical alloy with TiO2 sol–gel layers

The aim of this study was to demonstrate the relationship between the structural and corrosion properties of an ISO 5832-9 biomedical alloy modified with titanium dioxide (TiO2) layers. These layers were obtained via the sol–gel method by acid-catalyzed hydrolysis of titanium isopropoxide in isopropanol solution. To obtain TiO2 layers with different structural properties, the coated samples were annealed at temperatures of 200, 300, 400, 450, 500, 600 and 800 C for 2 h. For all the prepared samples, accelerated corrosion measurements were performed in Tyrode’s physiological solution using electrochemical methods. The most important corrosion parameters were determined: corrosion potential, polarization resistance, corrosion rate, breakdown and repassivation potentials. Corrosion damage was analyzed using scanning electron microscopy. Structural analysis was carried out for selected TiO2 coatings annealed at 200, 400, 600 and 800 C. In addition, the morphology, chemical composition, crystallinity, thickness and density of the deposited TiO2 layers were determined using suitable electron and X-ray measurement methods. It was shown that the structure and character of interactions between substrate and deposited TiO2 layers depended on annealing temperature. All the obtained TiO2 coatings exhibit anticorrosion properties, but these properties are related to the crystalline structure and character of substrate–layer interaction. From the point of view of corrosion, the best TiO2 sol–gel coatings for stainless steel intended for biomedical applications seem to be those obtained at 400 C.This study was supported by Grant No. N N507 501339 of the National Science Centre. The authors wish to express their thanks to J. Borowski (MEDGAL, Poland) for the Rex 734 alloy

Evolution of Microstructure in Two Austenitic Alloys at High Temperatures

A niobium stabilised AISI 347 austenitic stainless steel was investigated after static ageing and creep deformation at temperatures between 500°C and 800°C for times up to 88 000 h. Coarse and fine NbX precipitates and also s-phase precipitates were formed in the temperature range 500-700°C. A new method for accurate measurement of &#963;-phase volume fraction using scanning electron microscopy and backscattered electrons (SEM/BSE) was developed. <p />The volume fraction and precipitate size of coarse NbX precipitates and of s-phase particles were determined using different SEM techniques. Energy filtered transmission electron microscopy (EFTEM) was used to determine the volume fraction and precipitate size of secondary NbX precipitates. Atom probe field ion microscopy (APFIM) was used to follow the changes in matrix composition and to measure NbX composition. The material was affected by nitrogen uptake when aged at 800°C leading to the formation of Z-phase, Cr<font size="-1">₂</font>N and M<font size="-1">₂₃</font>C<font size="-1">₆</font>. <p />The iron-rich austenitic Alloy 800 was investigated for statically aged and creep deformed conditions between 600°C and 1000°C for up to 84 000 h. Coarse TiX precipitates were present in all investigated conditions. M<font size="-1">₂₃</font>C<font size="-1">₆</font> and &#947;´-precipitates were found in material creep tested at 600, 650 and 700°C. M<font size="-1">₂₃</font>C<font size="-1">₆</font> was also found at 800°C but not at 1000°C. The volume fraction and precipitate size of &#947;´ particles were determined using EFTEM at 600, 650 and at 700°C. At 1000°C AlN precipitated due to nitrogen uptake from the atmosphere. <p />Thermodynamical simulations of growth and coarsening of precipitates have been performed using Thermo-Calc and DICTRA and the results were compared to the experimental results. The growth and coarsening of primary and secondary NbX particles in AISI 347 was modelled during manufacturing and creep at 700°C, and the results showed good agreement with experimental results. &#963;-phase growth at 700°C was also simulated, and it was shown that for good agreement with experimental results additions are needed to the thermodynamic description of &#963;-phase accounting for the stabilising effect of silicon. The evolution of the microstructure due to nitrogen uptake at 1000°C was also predicted

Evolution of Microstructure in Two Austenitic Alloys at High Temperatures

A niobium stabilised AISI 347 austenitic stainless steel was investigated after static ageing and creep deformation at temperatures between 500\ub0C and 800\ub0C for times up to 88 000 h. Coarse and fine NbX precipitates and also s-phase precipitates were formed in the temperature range 500-700\ub0C. A new method for accurate measurement of σ-phase volume fraction using scanning electron microscopy and backscattered electrons (SEM/BSE) was developed. The volume fraction and precipitate size of coarse NbX precipitates and of s-phase particles were determined using different SEM techniques. Energy filtered transmission electron microscopy (EFTEM) was used to determine the volume fraction and precipitate size of secondary NbX precipitates. Atom probe field ion microscopy (APFIM) was used to follow the changes in matrix composition and to measure NbX composition. The material was affected by nitrogen uptake when aged at 800\ub0C leading to the formation of Z-phase, Cr2N and M23C6. The iron-rich austenitic Alloy 800 was investigated for statically aged and creep deformed conditions between 600\ub0C and 1000\ub0C for up to 84 000 h. Coarse TiX precipitates were present in all investigated conditions. M23C6 and γ\ub4-precipitates were found in material creep tested at 600, 650 and 700\ub0C. M23C6 was also found at 800\ub0C but not at 1000\ub0C. The volume fraction and precipitate size of γ\ub4 particles were determined using EFTEM at 600, 650 and at 700\ub0C. At 1000\ub0C AlN precipitated due to nitrogen uptake from the atmosphere. Thermodynamical simulations of growth and coarsening of precipitates have been performed using Thermo-Calc and DICTRA and the results were compared to the experimental results. The growth and coarsening of primary and secondary NbX particles in AISI 347 was modelled during manufacturing and creep at 700\ub0C, and the results showed good agreement with experimental results. σ-phase growth at 700\ub0C was also simulated, and it was shown that for good agreement with experimental results additions are needed to the thermodynamic description of σ-phase accounting for the stabilising effect of silicon. The evolution of the microstructure due to nitrogen uptake at 1000\ub0C was also predicted

Microstructural Aspect of Long Term Service of the Austenitic TP347HFG Steel / Wpływ Długotrwałej Eksploatacji Na Mikrostrukturę Stali TP347HFG

The paper presents the results of research on the microstructure of austenitic creep-resisting TP347HFG steel after longterm service. The tests of the microstructure were performed using SEM and TEM. The identification of precipitates was carried out using selective electron diffraction. It has been shown that long-term service mostly contributes to the processes of precipitation of secondary NbC carbides inside grains and M23C6 carbides on grain boundaries. In the microstructure of the examined steel, also the processes of polygonization and recrystallization of the matrix were revealed.W pracy przedstawiono wyniki badań mikrostruktury austenitycznej, żarowytrzymałej stali TP347HFG po długotrwałej eksploatacji. Badania mikrostruktury przeprowadzono przy użyciu techniki SEM i TEM. Identyfikację wydzieleń wykonano wykorzystując selektywną dyfrakcję elektronów. Wykazano, że długotrwała eksploatacja badanej stali przyczyniła się głównie do procesów wydzielania: węglików wtórnych NbC wewnątrz ziaren oraz węglików M23C6 po granicach ziaren. Ujawniono również w mikrostrukturze badanej stali procesy poligonizacji i rekrystalizacji osnowy