Band 31, Heft 6

In this study, ultra fine-grained microstructure was produced in Al–Zn–Mg–Cu alloy by equal channel angular pressing (ECAP) at 200 °C and a pressing speed of 2 mm s −1 using route C. The microstructure of the specimens was characterized by transmission and scanning electron microscopes. The results showed that the mean grain size of the specimens effectively decreased with increasing pass number. That is, while the grain size of unECAPed specimen was 10 µm, this value decreased to 300 nm after 14 passes. High temperature tensile tests were carried out at a strain rate of 1 × 10 − 2 and temperature of 250 °C in the specimens with different pass number. It was seen that the flow stress decreased and the elongation to failure significantly increased with increasing pass number. The highest elongation to failure value of 90% was obtained in the specimen after 14 passes.

This experimental study aims at investigating the optimization of hard finish turning operation parameters for martensitic stainless steel using the analysis of variance (ANOVA) and back propagation neural network (BPNN). Experimental trials are carried out based on an orthogonal array of Taguchi method. The recorded data are assessed and evaluated using ANOVA and artificial neural network (ANN) techniques. A multilayer perception model is constructed with back-propagation algorithm using the input parameters of depth of cut, cutting speed and feed rate. Crater wear and surface roughness of the machined component are the output parameters. On completion of the experimental test, ANOVA and ANN models are employed to validate the results obtained and further to predict the behavior of the system under conditions within the prevailing operating range.

High cycle and force-controlled (HCF) as well as low cycle fatigue (LCF) behaviour of titanium alloy IMI 834 was comparatively studied in bare condition and with titanium aluminide (Al 2 Ti) coating, at 873 K in air, for compressor disc of aero engines. HCF testing of bare and Al 2 Ti coated IMI 834 alloy at 873 K in air revealed that the coated materials has higher lifetime than the bare alloy. However, for low cycle, high stress lifetimes, the unprotected bare alloy is stronger, possibly due to early crack formation and growth in coated layer. In the case of LCF tests at higher values of total strain amplitudes Al 2 Ti coated IMI 834 alloy has longer fatigue life ( N f ) as compared to the bare IMI 834 alloy which appears to be due to load shifting to the coat, but at lower values of total strain amplitude, Al 2 Ti coating on IMI 834 alloy seems to have no effect on N f . Higher values of cyclic strength coefficient ( K ′) and cyclic hardening exponent ( n ′) for coated IMI 834 alloy indicate that coating offers resistance to cyclic plastic deformation at 873 K in air, and hence is beneficial. The coating adheres well to the substrate and is also fairly resistant to high temperature corrosion in sea water atmosphere at 873 K.

In this study, the effect of erbium oxide (Er 2 O 3 ) addition on the microstructure, sinterability and mechanical properties of cubic zirconia (c-ZrO 2 ) was investigated. 0–15 wt% Er 2 O 3 and c-ZrO 2 powders were mixed by means of colloidal processing and the specimens were sintered at different temperatures for 1 hour. The XRD results showed the existence of cubic crystal structure in all specimens. The lattice parameter of the specimens decreased with increasing Er 2 O 3 addition up to 10 wt% and remained unchanged after this amount. The grain size of the Er 2 O 3 doped c-ZrO 2 specimens dropped up to 5 wt% Er 2 O 3 addition and stayed almost unchanged at higher Er 2 O 3 contents. Both hardness and fracture toughness measurements were done using Vickers hardness tester. The fracture toughness values of the undoped, 1, 5, 10 and 15 wt% Er 2 O 3 doped c-ZrO 2 specimens were found to be 1.64 MPam 1/2 , 1.77 MPam 1/2 , 2.06 MPam 1/2 , 2.57 MPam 1/2 and 2.31 MPam 1/2 respectively.

Lead sulfide nanostructures with different morphologies and particle sizes were obtained via a simple hydrothermal reaction between lead salicylate (Pb(Hsal) 2 ) and thioglycolic acid The novelty in this synthesis is using of thioglycolic acid as a sulfur source and stability agent simultaneously. Depending on the pH of the solution, morphologies of the PbS varied and crystals with shapes of nanoparticles, flower-like nanostructures and microrods were obtained. The effect of the reaction time on the morphology of the nanostructures has been investigated. Products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) ultraviolet-visible (UV-vis) and Fourier transform infrared (FT-IR) spectroscopy.

CoSO 4 ċH 2 O nanoparticles and barite (BaSO 4 ) microcubes have been prepared by hydrothermal decomposition of new precursors [Co(tsc) 2 ]Cl 2 and [Ba(tsc) 2 ]Cl 2 (tsc =  thiosemicarbazide), respectively. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) spectroscopy. As a comparison between two methods, thermal decomposition of novel precursors in high temperature boiling organic solvents were examined.

Hot corrosion behavior of two Fe-Cr alloys coated with the salt film of 25 NaCl + 75 Na 2 SO 4 (mass fraction) in air at 1173 K was studied. Results show that for both alloys of Fe-10Cr and Fe-20Cr, the mass gain of the alloy coated with 40–45 g/m 2 of 75 Na 2 SO 4  + 25 NaCl is greater than that of the alloy coated with 20–25 g/m 2 of the film with the same composition. The corrosion products of Fe-10Cr alloy can be divided into three layers, of which the outermost scale is Fe 2 O 3 , the intermediate layer is mixed oxides of Fe 2 O 3 and Cr 2 O 3 , and the innermost layer is mainly chromium sulfide. However, the corrosion products of Fe-20Cr only consist of two layers: the external scale of Fe 2 O 3 and Cr 2 O 3 , and the internal layer of chromium sulfide. Compared to Fe-10Cr, Fe-20Cr has better hot corrosion resistance. The hot corrosion mechanism was proposed and the influence of the different coating mass of 75 Na 2 SO 4  + 25 NaCl is also discussed.

Lead sulfide nanoparticles were synthesized at room temperature via a simple chemical reaction. In this synthesis, 2-mercaptoethanolwas used as the capping agent and sodium sulfide was used as a sulfur source. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible (UV-vis) spectroscopy and Fourier transform infrared (FT-IR) spectroscopy Electron microscopy study showed that without using a capping agent the bulk PbS is obtained, while adding the mercaptoethanol leads to production of nanoparticles. We found that the electronic absorption spectra as well as the particle sizes depend on the used capping agents. Two exitonic peaks with a large blue shift were observed when mercaptoethanol was used.

The investigation was primarily aimed to study the influence of grain size on high temperature corrosion behavior of 2.25 Cr-1 Mo steel in SO 2  + O 2 (ratio 2:1) environment. The various grain sizes (18 µm, 26 µm, 48 µm, and 72 µm) were obtained by different annealing treatment. Isothermal corrosion studies were carried out in different grain size specimens at 923 K for 8 hours. The corrosion film growth rate and the reaction kinetics were studied by weight gain method. The external scales of the post corroded specimen were characterized in Scanning Electron Microscope (SEM), Energy dispersive Spectroscopy (EDS) and X-Ray Diffraction analysis (XRD). Finally, it is concluded that the corrosion rate of 2.25 Cr-1 Mo steel strongly depends on grain sizes of the specimens. The corrosion rate increases with the decreases in grain size. The finer grains (18 µm) show higher corrosion rate than the coarse grains (72 µm). The reaction kinetics follows the parabolic growth rate. The scale analysis of finer grain size specimen (18 µm) shows the thicker scale and extensive scale cracking and spallations, whereas the coarse grain specimen (72 µm) shows adherent and compact scale without any scale cracking and spallations. The mechanism of the corrosion has been explained on the basis of scale characterizations in SEM, EDS and XRD for different grain sizes.

Zeolite framework stabilized erbium(III) and thulium(III) complex nanoparticles was prepared for the first time by using a two step procedure: the ion exchange of Ln 3+ (Ln: Er and Tm) with the extra framework Na + ions in zeolite-Y, the template synthesis of 2,6-diformyl-4-methylphenol with different diamines. Analysis of the data indicated that the Ln(III) complexes has been encapsulated within the nanocavity of zeolite Y and exhibit different property from those of the free complexes, which can arise from distortions caused by steric effects due to the presence of sodium cations, or from interactions with the zeolite matrix.

The Raman frequencies of the lattice modes (147 cm −1 and 207 cm −1 ) are analyzed for the α - β transition in quartz according to a power-law formula with the critical exponent by using the experimental data. The temperature dependence of the Raman frequency is associated with the order parameter (polarization P) for this transition in the quartz crystal. The damping constant of the lattice modes studied here is calculated using the Raman frequencies at various temperatures for the α - β transition in quartz ( T c  = 846 K) using the soft mode – hard mode and the energy fluctuation models. Our calculations for the damping constant (bandwidths) give an evidence that the lattice mode of the 147 cm −1 exhibits a soft mode behavior for the α - β transition in quartz.

Hydroxyapatite (HA) ceramics are used extensively in different medical applications, such as biomaterial for repair or replacement of bone tissues since it resembles mineral component of bone and teeth. However, HA exhibits low fracture toughness due to its lack of strength and brittleness, thereby providing an obstacle to its application in implants that must withstand to high loads. In this study, HA was synthesized from human teeth by using a single calcinations method. Hydroxyapatite powders were mixed with different amounts of zirconia. The composites were sintered at different temperatures and characterized in terms of mechanical properties and the optimum sintering temperature was determined for good mechanical properties.

By means of long-term aging treatment, creep properties measurement and microstructure observation, the influences of long-term aging on the microstructure and creep behaviors of FGH95 Superalloy are investigated. Results show that the fine γ ′ particles dispersedly precipitate in the alloy after fully heat treated, and coarser γ ′ phase discontinuously distribute along the grain boundaries. After the alloy is long-term aged at 723 K for 500 h, the fine γ ′ phase with better cube degree and carbide particles distribute dispersedly in the alloy to enhance the creep resistance of the alloy for possessing a longer creep lifetime at the condition of 923 K and 1034 MPa. The deformed feature of the alloy is that configurations of slipping dislocations and the stacking fault appear in the alloy. As the creep goes on, the cracks initiate and propagate along the boundaries due to dislocations tangling in the grain boundaries for bringing out the stress concentration.

13.5 year service exposed (SE) catalyst primary reformer tube material made of H39WM micro-paralloy grade used in feritilizer plant was assessed for remaining life. The investigation includes mechanical properties evaluation; microstructural analysis and accelerated stress rupture tests. Failed tube portions showed coarsening primary carbides of Chromium and Niobium along the grain boundaries. Degradation of Niobium carbide (NbC) into Ni-Nb-Si phase and partial conversion this phase back to NbC was observed. Secondary carbides within grains were almost absent. Degradation in tensile strength for a range of temperature from 1123 to 1223 K was also observed but they were within the specified limits. Premature failures within 3–5 years service exposure are more common in reformer tubes. The failure was attributed to localized overheating leading to creep damage. The cast tube material may undergo microstructural changes during service exposure which is the main cause of degradation in strength and hardness changes. Accelerated stress rupture tests were performed in the range of 1173–1248 K on samples machined from 13.5 years at 1191 K and 15.4 MPa exposed reformer steel tubing, did not reveal any degradation of rupture behavior compared to that of the virgin alloy. An additional life of at least 10.6 years is predicted at the operating stress-temperature conditions.

In this work first cadmium sulfide-organic (CdS-or) nanostructures were synthesized via a simple chemical reaction at room temperature and then for preparation of the polymeric nanocomposite, CdS-or nanostructures were added to the poly methyl methacrylate (PMMA). The effect of the nanostructures on the thermal properties of the PMMA matrix was studied using thermogravimetric analysis (TGA). The thermal decomposition of the nanocomposite was shifted towards higher temperature in the presence of the CdS-or nanostructures. Nanostructures and nanocomposite materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), photoluminescence (PL) and ultraviolet-visible (UV-vis) spectroscopy.

The oxidation kinetics of four Fe-Cr-Mn-Ni based steels, utilized in automobile industry, have been studied as a function of temperature (973–1273 K) and oxygen partial pressure (5–10 5  Pa). It has been shown that the rate of corrosion of these steels under isothermal conditions is determined by diffusion of reagents through the scale, which phase composition and morphology depend first of all on chromium and to some extend also on nickel and manganese contents. The highest oxidation resistance at high temperatures is observed in the case of the X33CrNiMn23-8 steel, containing highest chromium concentration, equal 23.4 wt.% and nickel equal 7.8 wt.%. The remaining three steels with virtually the same chromium content (≈20 wt.%), but lower than that in the first one, show comparable oxidation resistance. Small differences in the oxidation rates of these three steels may be related to different nickel and manganese contents. It has been found also that the rate of corrosion of all steels under investigation does not depend under steady state conditions on oxygen partial pressure.