فصلنامه علم مواد
سال دوم شماره 2 (بهار 1389)

Flame spray is a widely used coating process due to its low cost and wide variety of usable materials. But weak adhesion between splats as well as between coating and substrate and also high amounts of coating porosity are main drawbacks of this technique. To overcome these problems, usually heat treatment process is applied on these coatings. The NiCrBSi coatings which have high wear and corrosion resistance are used at severe tribological conditions. In the present research, Ni-Based NiCrBSicoatings were applied on CK45 steel substrates by flame spray and the effect of spray and heat treatment parameters on their microstructure were studied. The results showed that applied coatings on the sand blasted substrates under the condition of 150 g/min powder feed rate and 15 cm spray distance, have laminar splat microstructure with the lowest amounts of porosity. Also the effect of heat treatment’s temperature and time on the microstructure and generated phases in the coatings was investigated.Phase analysis showed that Ni, CrB, (Cr,Fe)7C3, Ni3B and Ni5Si2 phases are generated in the coatings microstructure which were heat treated at 1075°C. In addition to theses phases, Cr2Ni3B6 phase was also detected in the coatings which were heat treated at 1100°C.

Pulse plating processes are widely used in a variety of industrial applications to improve corrosion resistance and mechanical properties of metals and alloys surfaces. In this study chromium plate is deposited on the surface of a st37 steel plate from a standard chromium plating bath using direct and pulse current electroplating method. Among different pulse parameters, duty cycle was optimized to obtain better properties. Surface morphology and coating thickness were investigated using scanning electron and optical microscopes. Wear properties and friction coefficient were investigated using a pin-on-plate reciprocating wear machine. The results showed that the crystal growth and grain size significantly depend on the duty cycle and on/off time ratio. Decreasing duty cycle, cause to uniform and fine grain size, increased resistant to wear and changing morphology of the coating from needle like to cubic structure.

Optical ceramics forms an important section of technology in optical industry. Magnesium fluoride is one of these optical ceramics, which has various applications such as infra-red domes. In comparison with other optical materials MgF2 has high durability and suitable mechanical and thermal properties. These windows must be transparent in the suitablerange and transmit high percent of incident IR beam towards the detector. In addition to optical properties these windows must have suitable physical, mechanical, thermal, and chemical properties. In this study MgF2 polycrystalline samples were produced using hot pressing process and effect of process parameters and microstructure on optical properties and densification were studied. Results showd that main problem for achievinghigh transmittance is removing porosity and control of grain growth.Although surface reflection greatly decreased transmittance, this effect can be removed by adequate optical polishing

Nanocrystalline powders of hydroxyapatite (HA) were prepared from Ca(NO3)2·4H2O and P2O5 using a simple sol–gel approach. The resultant gel precursors obtained based on the concentration of the starting solutions were either transparent or translucent. Fourier transform-infrared spectroscopy (FT-IR) results combined with the X-ray diffraction (XRD)indicated the presence of amorphous HA in the as-dried gel precursor. Xray patterns collected on the powder after heat-treatment at 900 °C for 12 h in air exhibited single phase of HA. Scanning electron microscopy (SEM) showed that nanocrystalline (50 nm) HA particles have obtained in the powder after heat treatment at 900◦C.

In the present study, microstructural characteristics of tungsten inert gas (TIG) welded AISI409 ferritic stainless steel were studied. The effect of the welding parameters on grain size local misorientation and low angle grain boundaries was investigated. It was found that the base metal has partial recrystallization state. After the joining process due to heating cycle, complete recrystallization followed by severe grain growth in theHAZ. A decrease in the number of low angle grain boundaries in HAZ was observed. Nevertheless the welding plastic strain is an increasing factor for local misorientation and low angle grain boundaries. This investigation shows that the final state of strain is the result of the competition between welding plastic strains and stress relieving from recrystallization.

Aluminum alloys are largely used because of their inherent good properties. Nowadays, 5xxx series alloys are developed and one of their new categories is Al-4.5Mg. High strength in this alloy is due to work hardening. It has ultimate tensile strength of 330 MPa. The most important challenge during fusion welding of this alloy is HAZ softening. The aim ofthe present investigation is to study the effect of pulse current on mechanical and metallurgical properties of AA5083 aluminum alloy welds through pulsed tungsten inert gas (TIG) welding process. Microstructures of all the welds were studied and correlated with the mechanical properties. It was seemed that pulse current, has an optimum amount to reach the highest strength. All the welded specimens were failed within theweld region. Pulsed welds showed fine grain structure due to thermal disturbances and decrease in heat input. In general, hardness is lower in HAZ region compared to the weld metal and base metal regions, irrespective of welding technique, which is characterized by the coarse dendrite grains and lack of the strengthener phase. Hardness was highercompared to the continuous welds and this could be due to refinement of grain structure. The best peak current should be selected between 220 & 240 A.

High-Chromium iron alloy due to hardship and high abrasion resistance of cement and mining industry has many applications. However, now increasing impact strength and toughness of this alloy, the issue has become important in research. This study attempts to influence the use of ceramic foam filters made of SA on alloy microstructure and mechanical properties of white cast iron containing 17% chromium is studied. For thisfirst model was designed to allow most of the standard examples of mechanical tests such as hardness, impact resistance, and resistance to fracture of the surface wear and metallographic, with minimum need for turning, be prepared. Finally, the above tests on standard samples prepared from the two alloys were performed and the results compared and evaluated. These results indicate a small increase in impact strength and resistance to abrasion and also reduce the difficulty of this alloy were.Also observed that application of ceramic foam filters, clear impact on phase morphology and microstructure of this alloy, but failure would be softer and more homogeneous cracks are in the following examples.