نشریه فرآیندهای نوین در مهندسی مواد
پیاپی 18 (پاییز 1390)

Experimental tests were carried out to explosively clad solution annealed Inconel 718 super alloy on quench-tempered AISI H13 hot tool steel. Experimental tests were carried out using various stand-off distances and explosive ratios. Various interface geometries were obtained from these experiments. Experiments were carried out with Ammatol explosive mixture. In this study, the integrity of the bond interface was examined by the shear test. The micro hardness profiles at the interface for all samples were performed. Metallographic inspections of the bond interfaces were also carried out. The shape of interface fell roughly into two classes, wavy or wavy with some vortex shedding. In some case, a gradual change in wavelength along the direction of welding was observed which was due to a change in impact angle following plate contacts. Hardness was increased with stand-off distance and explosive ratio. Shear strength was found to be functions of interface morphology and wave amplitude.

Sol-gel chemical processing route is a new and novel method for synthesizing ceramic nano powders. In this method, β-SiC nano powder can be synthesized via alkoxides and carbonaceous sources. High purity product, high chemical activity and ability of mixing in molecular scales are main advantages of the route. In this research, nano SiC powder was synthesized using TEOS and phenolic resin as raw materials. Also the effects of pH and gelation time on particle size of precursors were investigated. After sample preparation, heat treatment process was performed in a tube furnace and argon atmosphere at 700, 1400, 1500oC. Particle size distribution of the samples was investigated by TEM while SEM and XRD were used for microstructural and phase evolutions of heat treated samples, respectively. Finally, the optimum synthesis temperature and agglomerate size were reported.

In this paper, the effects of dolomite type and Al2O3 content on the phase composition in the preparation of the aluminous cement containing MA spinel are investigated. For this reason, different compositions of raw and calcined dolomites are used as raw materials along with calcined alumina in the preparation of the aluminous cement containing MA spinel. Then, the compositions are prepared at 1350 °C using the sintering method and their mineralogical compositions are investigated using the diffractometric technique. Also, their microstructures have evaluated by SEM. The results indicate that raw materials used have great effect on the type and amount of formed phases in cement composition. Independently of the dolomite type used, a mixed phase product consisting of spinel accompanied by CA and CA2 is obtained. The content of CA phase in the cement composition is decreased with increasing of Al2O3 and therefore, decreasing of CaO in the raw materials composition. On the other hand, the CA2 content is increased with the addition of Al2O3. In addition, the results show that the formation of C12A7 is favored by use of calcined dolomite.

Pitting corrosion mechanism of heat treated aluminum alloy 7075 in aqueous solution containing chloride ions was investigated, using impedance study. These tests were performed at three different potentials including pitting potential, 100 mV and 200 mV above the pitting potential. For T6 and W temper, nyquist plot showed two semi-capacitance loops at higher and medium frequencies and one inductive loop in lower frequencies. On the other hand, T7 temper showed one semi-capacitance loop at higher and medium frequency and one inductive loop in lower frequency. Results showed that, mass transfer is the prevalent mechanism of corrosion current control at higher frequencies for 200 mV above the pitting potential. After impedance tests, surfaces of samples was studied by optical and scanning electron microscopy. Also, role of intermetallic particles on pit initiation was investigated for each heat treatment cycles.

In the present study, the influence of annealing heat treatment and aging treatment after annealing process on the superelastic behavior of 40 and 60% cold worked Ni-rich NiTi shape memory alloy have been investigated. For evoluation of superelastic behavior, tensile tests were carried out at 37°C and a cross-head speed of 0.1mm/min using by loading and unloading regime up to 5.5% strain. Results showed that at the presence of the fine grains due to the recrystallization process, superelastic behavior improved; however, some strains remain in the strain-stress curves of alloy. With formation of recrystallized fine grains, due to annealing heat treatment, and Ni4Ti3 precipitates, due to aging treatment after annealing process, the best superelasticity behavior achieved. It is observed that the variation of grains size result in variation of precipitates distribution within microstructure. This, therefore, leads to variation of transformation temperatures. Consequently, it can be said that, with variation of grains sizes during annealing process and with aging treatment after that, transformation temperatures can be controlled.

Hydroxyapatite [HAp; Ca10(PO4)6(OH)2] biocomposites are ceramic materials that exhibit excellent bioactivity, but their inherent low mechanical behavior render them unusable for practical applications. Titania (TiO2) is added to improve inmechanical properties without diminution of biocompatibility. Two Step Sintering (TSS) has been applied, in the present work, to suppress the accelerated grain growth of Hydroxyapatite and Titania nanoparticles in the HA-TiO2 composites in the last sintering stage. Samples characterized by X-Ray diffraction and scanning electron microscopy. It was shown by application of this method, the final grain size of HA-15wt.%TiO2 maintainslower than 100nm while by application of conventional sintering it reaches higher than 100nm. The optimum regime consisted of heating the green bodies up to 1150˚C in the first step of sintering and then holding at 1050˚C for about 25h in the second step. This yielded the finest microstructure with no exaggeration in grain growth and density of specimens reaches higher than 95% theoretical density.

Al–5 vol.%SiC nanocomposite powder and pure aluminum nanostructure powder are produced by attrition milling and using pure aluminum powder with dimension of 20 to 50 µm and SiC with dimension of 20-50 nm. Then the produced powders are consolidated and strengthened by hot extrusion method. The wire cut machine was used to prepare flat tensile samples from extruded products. Tensile test were done from 25 to 300 oC. The results show that the strength of Al–5 vol. %SiC nanocomposite are higher than pure aluminum nanostructure in all temperatures. Fractography shows that the fracture of Al–5 vol. %SiC nanocomposite are brittle and pure aluminum nanostructure are ductile.

The aim of this study is to investigate the effect of increasing the thickness on the properties of titanium films created on AISI 316L stainless steel substrate by using the process of DC magnetron sputtering. To achieve this, deposition operation in various times 5, 10, 15 and 20 minutes were set to films with different thicknesses to create. The thickness of titanium films by using profilometery were measured. The morphology of films by field emission scanning electron microscope (FESEM) were investigated and titanium nanoparticles average size were determined. The topography of titanium films by using atomic force microscope (AFM) were investigated and the amount of average surface roughness of films were determined. In addition, corrosion resistance the thickest of created titanium film by potentiodynamic polarization test in SBF physiological solution at 37ºC temperature was evaluated and the values of corrosion potential and the corrosion current density were determined. The results of these tests showed that the titanium film with a thickness of 298 nm has the most dence and the most uniform structure and the highest amounts of titanium nanoparticles average size (65 nm) and the average surface roughness (3.767 nm). Also creating titanium film with a thickness of 298 nm on AISI 316L stainless steel increases the corrosion potential from -0.313 to -0.279 V and reduced corrosion current density from 451.1 to 163.4 nA/cm2 which resulted to improve corrosion resistance.