مجله مواد نوین
پیاپی 1 (پاییز 1389)

Titania porous bioceramic coating covered by hydroxyapatite causes the substrate to be more corrosion resistant and biocompatible. In this study, Ca and P contained titania was coated on Ti-6Al-4V alloy substrate by applying plasma electrolytic oxidation (PEO) with changing parameters such as time and voltages. After obtaining optimized sample, then it was treated hydrothermally at different time intervals until hydroxyapatite crystals were nucleated on titania porosities. Morphology and cross section, chemical composition, topography and hardness of the coating were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), Atomic force microscope (AFM) and nano indentation test, respectively. Also, the corrosion behaviors of the coating were studied by polarization and electrochemical impedance spectroscopy. According to the obtained results, hydrothermal treatment on optimized sample causes the formation of hydroxyapatite crystals in Titania porosities. These crystals are in nanometric dimensions and lumped shape. In addition, the coating on the substrate caused an increase in corrosion resistance of the sample in simulated body environment considerably.

Centrifugal casting as a high capable technique has been frequently used for production of high performance castings. In two decades its capabilities has been progressed towards the production of cast components with functionally graded microstructures. A controlled attribution of the centrifugal force in this technique can cause a desirable grading of the microstructures hence properties in different sections of the castings. In this research, functionally graded Al-wt.%MgSi hypoeutectic alloy with graded distribution of α-Al phase was cast as a cylindrical specimen using the centrifugal casting. During the solidification, the initially formed α-Al grains due to the higher density in compared with the remained melt, were segregated in the centrifugal force direction. The microstructural observations reveal two distinguished layers in radial direction of the cast cylinder: the outer layer with a higher volume fraction of α-Al grains and the inner one with the eutectic structure. To modify the cast structure hence mechanical properties, the cast sample was subjected under solution and ageing heat treatments. The microstructural observations of the heat treated sample shows no noticeable change in the cast graded microstructure; however the eutectic structure was distorted from laminar to a fine dot-like structure. This minor microstructural evolution is accompanied with an increase in hardness of different sections of the sample.

In this investigation different series of Fe-Cr-C hardfacing alloys with the constant ratio of were fabricated by GTAW on AISI 00 mild steel substrates. OES method was used for determining chemical composition of surface layers while OM, SEM, XRD were used for studying the microstructure and phases characteristics, hardness test (Vickers macro hardness method) was also used to determine hardness of surface layers. The metallographic examination and XRD results indicated that all specimens microstructure consisted of austenite and chromium-iron carbides, (Cr,Fe)7C, moreover by increasing of carbon and chromium content in hardface layers, the volume fraction of (Cr,Fe)7C increased while it caused to decreasing the austenite volume fraction and increasing hardness of the top surface layers. In addition wear Test result Shows that with increasing the chromium and carbon content in hypoeutectic alloys, the wear resistance increase, study of SEM of worn surface determined that wear mechanism of hypo eutectic alloys was micro ploughing, and in eutectic and hyper eutectic were microcutting and micro cracking.

In this study Ni50Al50 and Ni5Cr5Al50 were undergone mechanical alloying and heat treatment. The structural changes of powder particles during processes were studied by X-ray diffraction (XRD). It appeared that there are different phase transformations after both treatments: Ni50Al50 formed a nanocrystalline, orderly NiAl intermetallic compound; on the other hand (NiCr)Al intermetallic which itself was produced from Ni50Cr5Al5. The morphology and cross sectional area of the powder particles were characterized by scanning electron microscopy (SEM). The conclusion was that during initial stages of milling the layers of Cr, Ni, and Al are produced which subsequently are transformed into the final product upon further treatments.

(Cu-Cr)-AlO nanocomposite with different contents of AlO has been successfully produced by using mechanochemical routs. This approach involves mechanical milling of Cu, Al, CuO and CrO in a high energy ball mill to produce (Cu-Cr)-AlO composite. Different amount of initial Cu contents has been used to control the adiabatic temperature, AlO particles size and AlO content in the final products. The results of micro structure analysis with scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies showed that in the early stage of ball milling particles agglomeration occurred and subsequently a solid solution of Cr in Cu, Cu (Cr), formed. In next step reaction between solute Al and CuO resulted in formation of fine AlO particles and fracture of particles caused particles refinement. The results of X-ray diffraction showed that AlO particles were very fine.

In the present study, nanocrystalline Nickel-Cobalt coating was electrodeposited by pulse current from a Watts bath. Electrodeposition was carried out in the current densities of, 4, 6 A/dm at different frequencies and on-time periods in the baths containing of 0.5,, g/l saccharin. Morphology, analysis and crystallography of the coating were studied by means of SEM, EDS and XRD methods, respectively. The grain size of the coating was also calculated from XRD patterns using Scherrer equation. The results of polarization tests carried out in. 5% NaCl solution indicated that by increasing the cobalt content in the coating, although the corrosion potential slightly shifted to more negative values, but the corrosion rate decreased, which can be related to more activity of cobalt with respect to nickel. A decrease in grain size of the coating increased the corrosion rate and shifted the corrosion potential to more positive values.

In the present work, the electrophoretic deposition (EPD) process was utillized to fabricate NiO-YSZ composite layer for SOFC anodes on stainless steel substrate. Various suspensions were examined and it was found that co-deposition of the composite layer could be performed more sucsessfully in isopropanol interms of the suspension stability and deposition quality. The EPD parameters were tailored by a trial-and-error-approach to obtain crack-free layers. The effect of applied dc voltage from 50 to 00 V and deposition time from 0 sec to 0 min on the deposition weight, thickness and composition was also studied. The results determined that the presence of nickel oxide nanoparticles in the suspension increases the NiO content of the deposit while improving the microscopic feature of deposition.

In this research microsegregation in the dendritic directional solidification has been investigated using phase field simulation and some of analytical models. Comparison of the numerical and analytical results showed that in the investigation conditions, the analytical equations overestimated the phase field simulation results. It is found under both back diffusion and no back diffusion conditions. In a range of the solid fraction numerical and analytical results were comparable and difference between them was increased by increasing of the solid fraction.