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Preparation and preliminary study of the properties of electrophoretically depositied nc-TiO2 coatings on 316L steel DOI:10.15199/28.2016.6.4

  TiO2 coatings on 316L steel were obtained by use of electrophoretic deposition (EPD) method. Potential zeta of nc-TiO2 particles in suspensions containing water and ethanol in different ratios was measured. Suspensions’ pH was stabilized by addition of acetic or citric acid and ammonia solution. Addition of citric acid in small amount decreased the zeta potential. Optimal suspensions’ parameters for cathodophoretic and anodophoretic deposition were selected based on the results of zeta potential measurements versus pH for suspensions with different water-ethanol concentration. For the chosen suspensions the rate of TiO2 deposition was measured. Coatings’ cohesion was improved by sintering or addition of biopolymer (chitosan) into suspension. The microstructure of coatings was examined by scanning electron microscopy. The roughness and thickness of the coatings were measured by optical profilometer. The corrosion resistance in Ringer’s solution was examined by use of polarization curves. The corrosion resistance of coated steel was higher than that of uncoated one. For sintered coatings the corrosion currents were lower, but the passive area was larger for not sintered ones. The contact angle of the coatings was measured using a sitting drop method and superhydrophilic properties of TiO2 coatings were confirmed. Manufactured coatings may be potentially used as self-cleaning materials. Additionally, TiO2 coatings improve corrosion resistance of steel and exhibit good bactericidal properties. These characteristics may make this sort of materials potentially useful also for medical purposes. Key words: TiO2 coatings, electrophoretic deposition, 316L steel, zeta potential measurements, superhydrophilic coating.1. INTRODUCTION Stainless steel is commonly used in many areas, e.g. marine systems, nuclear, chemical, food, construction industries and biomedical purposes [1÷3]. A wide range of industrial applications is possible [...]

TEM analysis of creep mechanisms of single-crystalline nickel-base superalloys

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Single crystal (SC) superalloys are especially designed for gas turbine blades in jet aircraft engines and stationary turbines generating electricity. The basis criterion in material selection for gas turbine blades is a high creep strength, because they have to withstand centrifugal forces acting during operation. The main creep mechanisms in polycrystalline superalloys are: -- dislocation creep by combined slip and climb of dislocations, -- diffusion creep by Nabarro-Herring or Coble mechanism [1, 2]. As it was shown for many superalloys [3÷5], in the range of temperature and stresses characteristic for service conditions of turbine blades, grain boundary diffusion by Coble mechanism is the main factor controlling the creep rate. Therefore the elimination of grain boundaries is necessary for improvement of creep resistance of superalloys. It can be achieved by a directional solidification with a grain selector, as was demonstrated first by B. Piearcey from Pratt & Whitney in 1970 [6]. Single crystal superalloys exhibit the highest creep resistance among metallic materials. The development of chemical composition of SC superalloys resulted in elaboration of four generations of these alloys [7, 8] and progress continues already towards the fifths generation [9]. The as-cast single crystal superalloys exhibit a dendritic structure with the eutectic areas between dendrite arms, which can be homogenized by heat treatment. The typical microstructure consists of about 70% of cuboid γʹ precipitates, about 400 nm in size, separated by γ matrix channels about 30 nm wide. Residual eutectic areas contain large irregular γʹ precipitates. Such two-phase microstructure guarantees the high creep resistance of single-crystalline turbine blades. The γʹ precipitates are an ordered Ni3Al-base intermetallic phase with the lattice parameter only slightly smaller than the Ni-base γ solid solution. This produc[...]

Zmiany mikrostruktury podczas eksploatacji łopatki turbiny gazowej z monokrystalicznego nadstopu niklu CMSX-4

  Metodami skaningowej i transmisyjnej mikroskopii elektronowej zbadano zmiany mikrostruktury w .opatce z monokrystalicznego nadstopu niklu CMSX-4 po 12 700 godzinach pracy w energetycznej turbinie gazowej. Zaobserwowano wyra.ne ro.nice w mi- krostrukturze u podstawy, w .rodku wysoko.ci i przy wierzcho.ku piora .opatki. Szczego.owa analiza podstruktury dyslokacyjnej umo.liwi.a okre.lenie ro.nych mechanizmow pe.zania dzia.aj.cych w poszczegolnych strefach .opatki. Scanning and transmission electron microscopy investigations of microstructural changes in CMSX-4 single crystal blade after exploitation for 12 700 hours in stationary gas turbine have been performed. The pronounced differences in microstructure at the bottom, in the middle of the height and at the blade tip have been observed. The detailed analysis of the dislocation substructure enabled determination of different creep mechanisms acting in particular zones of the blade. S.owa kluczowe: monokrystaliczne nadstopy niklu, pe.zanie, .opatki turbin gazowych Key words: single crystal nickel-base superalloys, creep, gas turbine blades S. 307 UHTNIK-WIADOMO.CI HUTNICZE Nr 4 Stereoscan oraz transmisyjnego mikroskopu elektro- nowego JEOL JEM-2010 ARP. 3. Wyniki bada.. 3.1. Zmiany kszta.tu wydziele. fazy ƒÁf. Obserwacje mikrostruktury na przekrojach rownoleg.ych do kierunku dzia.ania si.y od.rodkowej ujawni.y zro.nicowanie kszta.- tu i wielko.ci wydziele. fazy ƒÁf przy podstawie, w .rodku i na wierzcho.ku piora .opatki (rys. 2). U podstawy .opatki, gdzie podczas pracy tempera- tura by.a najni.sza (807 ‹C), a napr..enia najwy.- sze (189 MPa), mikrostruktura nadstopu CMSX-4 obserwowana na obrazach SEM by.a podobna, jak w stanie obrobionym cieplnie. Na obrazach SEM nie zaobserwowano zmian w mikrostrukturze spowodo- wanych dzia.aniem napr..e. w wysokiej temperatu- rze. Wzrost temperatury w .rodku piora .opatki do 927 ‹C i jednoczesny spadek [...]

Characterisation of microstructure evolution during high temperature creep of CMSX-4 superalloy by TEM, HRSTEM and high spatial resolution EDX mapping

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CMSX-4 is a single crystal nickel-base superalloy, applied for aircraft and stationary gas turbine blades. Its microstructure consists on the cuboidal precipitates of ?Á?Ś phase (Ni3Al-based), coherent with ?Á solid solution matrix. Chemical composition of CMSX-4 superalloy contain s more than 10 chemical elements and is especially designed to achieve around 70% volume fraction of ?Á?Ś phase. The ?Á.?Á?Ś interfaces, separating the disordered ?Á solid solution and ordered ?Á?Ś phase precipitates, are the strong obstacles for dislocation movement, what allows to obtain the high temperature strength. During exploitation, the turbine blades are subjected to the load by centrifugal force at high temperature in the range from 700 to 1100?‹C, thus undergo creep deformation. The parts of the turbine blade work at different temperatures and stresses, therefore microstructural changes caused by creep must be investigated over a wide temperature.stress range. Depending on the temperature and stress, three creep regimes with distinct modes of predominant microstructural changes in single crystal superalloys can be distunguished [1]: .. at intermediate temperature (700?€850?‹C) and high stress (550?€900 MPa) a pronounced primary creep deformation up to even 5% occurs at the very short time of several hours, .. at high temperature (850?€950?‹C) and medium stress (150?€550 MPa) the tertiary creep is dominant with the creep strain increasing monotonically with creep strain, .. at high temperature (900?€1200?‹C) and low stress (50?€185 MPa) the creep curves display small creep-hardening and a distinct plateau, during which the creep strain is almost not varied with time, followed by the pronounced increase of creep strain leading rupture. The microstructural changes caused by creep of CMSX-4 superalloy at the above mentioned creep regimes have been investigated by many researchers, e.[...]

Microstructural analysis of as-cast single-crystalline CMSX-4 gas turbine blade

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Turbine blades are critical components in both aircraft and stationary gas turbines. They must be capable to withstand extreme conditions of temperature and stress during operation. The demand for increase of the operating temperature above 1000°C resulted in a development of directionally solidified single crystal (SC) superalloys for gas turbine blades [1÷3]. Single crystal solidification was achieved by application of helical grain selector between the casting and chill plate, enabling selection of a single grain, which fills the whole mould by dendritic growth. Although such superalloys are called “single crystals", they consist of two phases: γ solid solution matrix and about 70% volume fraction of γʹ precipitates. Segregation of chemical elements, which occurs during solidification, leads to the differences in the microstructure inside dendrite arms and between them. Dendritic regions consist of γʹ precipitates surrounded by narrow γ channels, while interdendritic spaces contain coarse irregular γ-γʹ eutectic islands [3÷6]. Casting of turbine blades in single crystal form is very complicated and requires definition of optimum parameters of technological process. Beside the careful inspection of the presence of casting defects, the quality control of as-cast blades involves checking of the deviation from the desired crystallographic orientation, which should be less than 15° [1]. However, to check in more detail if the solidification parameters of the casting are properly chosen, the assessment of the microstructure in microand nanoscale should be performed. The present paper reports the results of the analysis of the crystallographic orientation and microstructure of trial casting of CMSX-4 single crystal gas turbine blade performed within the project on “Dire[...]

Influence of polyethylenimine on the electrophoretic deposition of SiO2 and Ni/SiO2 coatings on 316L stainless steel DOI:10.15199/28.2016.4.6

  The aim of the present work was to investigate the influence of polyethylenimine, a cationic polymer surfactant, on the microstructure and corrosion resistance of SiO2 and Ni/SiO2 coatings electrophoretically deposited on 316L stainless steel. The relationship between zeta potential and pH of the ethanol-based suspensions of SiO2 and Ni powder particles with addition of polyethylenimine was determined. The parameters of electrophoretic deposition process (applied voltage, time, distance between electrodes) were developed to prepare good quality coatings. Cathodophoresis from suspensions with polyethylenimine addition was performed with slightly lower applied voltage and time as compared to anodic deposition of coatings without surfactant. The microstructure of the coatings, their surface roughness and adhesion to the substrate were investigated. The protective behaviour of the coatings was studied by potentiodynamic measurements in 3.5% NaCl water solution. The microstructure and properties of the coatings were compared with those obtained without polyethylenimine addition. It was determined that the microstructure of SiO2 and Ni/SiO2 coatings deposited from suspensions containing polyethylenimine was more uniform and contained smaller amount of cracks and voids than the coatings achieved without polyelectrolyte. It was also observed that the quality improvement of the coatings deposited on 316L steel due to addition of polyelectrolyte with binding properties leads to increase of their corrosion resistance. Key words: electrophoretic deposition (EPD), SiO2 coatings, Ni/SiO2 composite coatings, polyelectrolyte, polyethylenimine.1. INTRODUCTION 316L stainless steel is widely used in applications involving severe corrosive conditions. Due to the formation of a thin chromium oxide film the steel exhibits good corrosion resistance in oxidising media [1]. However, in Cl- containing environment, deterioration of the passive film occurs and the stee[...]

Development of single crystal Ni-based superalloys for advanced aircraft turbine blades

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Development of single crystal (SX) nickel-based superalloys from polycrystalline to current SX 5th generation is discussed. The properties, and microstructural changes during creep tests under laboratory and service conditions are described. Microstructural stability of 2nd and 4th generation SX superalloys are presented and discussed. INTRODUCTION Single crystal nickel-base superalloys have[...]

Microstructure and magnetic properties of the Cu-1% Co single crystal

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The Cu-Co alloys has been found to be ideal candidates to study the giant magnetoresistance phenomenon (GMR), originally observed in thin magnetic films [1], that have focused great research interest. Such materials can be applied in micro-switching devices and detection heads in magnetic recording. The GMR phenomenon, a big change of the electrical resistivity caused by external magnetic field, is attributed to the spin-dependent scattering that takes place in the interfaces between magnetic particles and the metallic matrix in which they are embedded [2÷5]. The films made of the alloys based on non-magnetic elements (Cu, Ag) with precipitated ferromagnetic particles (Fe, Co) appear to posses GMR properties [6, 7]. However, besides the GMR phenomenon, such alloys provide a unique opportunity to study their basic properties, such their microstructures and magnetic properties. The purpose of this work is to present the results of the transmission electron microscopy (TEM) analysis which provides the real size distribution of Co particles in the Cu-1% Co single crystal. We report also the results of the magnetic measurements in connection with the real distribution of Co particles which allows us to explain the relation between the size of the Co particle and the magnetic properties. EXPERIMENTAL DETAILS In the present study the Cu-1% Co (wt. %) single crystal of the origin reported in [8] was used. The investigations were performed on a sample aged at 600°C for 24 hours. The samples for TEM and magnetic investigations were cut out from the single crystal. TEM investigations were performed using a JEOL JEM-2010 ARP microscope on thin foils prepared by the electropolishing method. Image analyses were carried out using the ImageJ software. The magnetic properties were measured using a standard vibrating sample magnetometer in the external[...]

Microstructure of the complex metallic β-Al3Mg2 phase

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Aluminium and magnesium are fairly simple metals with close packed lattices: Mg is hcp, whereas Al is fcc. The phase diagram of Al-Mg system contains intermetallic compounds. The mixture of Al/Mg ratio 3:2 atoms yields an extremely complex crystal structure. The β-Al3Mg2, called also Samson phase (Fd-3mS: ICSD 57964, cF 1168), has cubic lattice with a = 28.242 Å and a unit cell containing 1168 atoms [1]. It is decorated by a number of large clusters of atoms, with near icosahedral atoms symmetry, leaving few atoms in between to fill-in space [2]. The Samson phase has hardness about 350 HV, Young’s modulus 68 GPa and low density 2.3 g/cm3. This phase is very brittle at room temperature (fracture toughness K1c is below 0.5 MPa⋅m1/2). Under compressive stress and temperatures higher than 250°C, β-phase become plastic [3]. Comparison with Al shows the best compromise between strength and plastic deformation - strength 3 times higher than pure Al and plastic strain of about 40%. It means that application fields for this phase are in aeronautic and automotive industry. Different stability range of the β-Al3Mg2 in Al-Mg phase diagram is proposed in literature [4, 5]. According to the phase diagram published by Murray [4], the β-phase has a congruent point at 451°C and 38.5% Mg. The stability range of this phase extends from 38.5 up to 40.3% Mg in room temperature. On the Al-rich side, β-Al3Mg2 coexists with the α-phase (which is a solid solution of Mg in Al). On the Mg-rich side, the β-phase coexists with γ-Al12Mg17 and with R-phase that forms below 350°C by [...]

Application of EFTEM and FIB electron tomography to 3D visualization and metrology of nanoparticles in Inconel 718 superalloy

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Inconel 718 (IN718) is one of the most common precipitation-hardened Ni-base superalloys. It belongs to a class of Ni-Fe superalloys, containing a relatively high content of Fe as well as Ti and Al to provide γ′, Ni3(Ti, Al) precipitates. It also contains Nb which leads to precipitation of the tetragonal Ni3Nb phase (γ") [1]. A disadvantage is that the γ" precipitates have a worse thermal stability than γ′ leading to a faster coarsening rate and to a gradual transformation to an orthorhombic phase of Ni3Nb (δ) above about 800°C. This limits the use of the alloy to below about 650°C. The γ" phase occurs in the form of as elongated, oriented particles (disc-shaped) within the γ matrix, while the γ′ precipitates are spherical [1, 2]. Transmission electron microscopy (TEM) images give valuable information about the microstructure and chemical composition of materials. However, they “only" provide a 2D projection of a 3D object. Electron tomography was developed to reconstruct objects in three dimensions (3D) from a tilt series of TEM images. This technique is well accepted in the life sciences as a method used to study viruses or cells. The resolution in the reconstructions, however, is limited to a few nanometers. Electron tomography techniques have recently been adopted by researchers in materials science [2]. In practice, however, the resolution is still of the order of one to two nanometers because of the limited stability of the sample holders and the presence of dynamic diffraction in crystalline solids. Electron tomography is a technique that uses a transmission electron microscope (TEM) to determine a 3D structure from any given asymmetric object [3]. This process can be simply broken down into 4 steps. First, a series of 2D projection images of the specimen are recorded and systematically tilted to different angles in the microscope. Second, these individual im[...]

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