نشریه مهندسی متالورژی و مواد
سال بیست و هشتم شماره 2 (بهار و تابستان 1396)

In this research, TiAl/Al2O3 composite was synthesized from mechanically activated TiO2-Al powder mixtures using microwave heating. The powder mixtures were heated in a microwave oven after milling and pressing into cylindrical tablets. The effect of mechanical activation intensity was evaluated on the ignition time and the resultant reaction products. XRD and SEM analysis were used for evaluation of the synthesized samples. Moreover, the volume percent of the formed phases was calculated from XRD patterns using Maud software. The results confirmed that the composites mainly consist of TiAl and dispersed Al2O3 particles. The dominant phase is Al2O3 with 65-75 vol.% and the amount of TiAl phase varies from 18 to 30 vol.%. The results revealed that with increasing the intensity of activation, the ignition time first decreased and then increased. At optimum condition with an ignition time of 27 s, the composite structure is more uniform and less porous and it consists of 23.2% TiAl and 69.6% Al2O3.

Stray grain formation during single crystal growth with growth rates of 3, 5 and 7 mm/min has been assessed. The results revealed that increasing the growth rate resulted in the decrement of thermal gradient in the solidification front and caused stray grain formation in the solidification front of the columnar grain growth. Experimental and numerical results showed that a well-oriented single crystal structure is obtained at a growth rate of 3 mm/min in Ni-based superalloy. Stray grain formation occurred at growth rates greater than 3 mm/min, as a result of high growth rate and low thermal gradient.

In this research, the effect of cooling rate and copper content on the microstructure of Zn–27%Al alloy was investigated using chilled casting in a sand mould. Microstructural examination showed that with decreasing cooling rate, dendrite arm spacing and the percentage of interdendritic phases increased. Moreover, addition of copper up to 1 wt. % did not lead to significant change in microstructure, while addition of 2, and 4 wt% copper led to the formation of -CuZn4 phase in the interdendritic regions. In addition, with decreasing cooling rate, the morphology of - precipitates varied from semi-plate-like to spherical.

Since intercritical annealing is the main processing stage for the production of dual-phase steels (DPS), studies on the formation of austenite are of great importance. In this research, the effects of the cold rolling and intercritical annealing temperature on the formation of austenite in plain low carbon steel have been investigated. For this purpose, the steel sheets were cold rolled for different amounts of reductions and annealed at different intercritical temperatures followed by quenching into an ice brine solution. In order to investigate the microstructural evolution as well as the effect of controlling parameters on the nucleation and growth processes of austenite (martensite at room temperature) formation, the samples were quenched at various time intervals during intercritical annealing and their microstructures were analyzed using optical and scanning electron microscopes. The results showed that cold rolling and other processes occurring during the formation of austenite significantly influences the initial microstructure and the nucleation and growth of austenite phase. In un-deformed and 50% cold rolled specimens, austenite formed from the pre-existing pearlite colonies. However, in the 70 cold rolled specimens, cementite spheroidization within the deformed pearlite colonies caused austenite to be formed from a microstructure consisting of cementite particles embedded in a matrix of ferrite. In this situation, nucleation and growth of austenite phase took place mainly on the grain boundaries of ferrite matrix. Furthermore, it was found that the temperature of intercritical annealing treatment has a strong effect on the kinetics of austenite formation.

In this work, carbonate and oxalate creating factors were determined in the Bayer process. In the chemical removal of carbonate and sodium oxalate soluble in sodium aluminate liquor, barium aluminate with structures of Ba2Al2O5, Ba2Al4(OH)16 and BaAl2O4, calcium aluminate (Ca3Al2(OH)12), sodium phosphate (Na3PO4) and lime were separately used. Investigation of temperature, concentration of caustic soda, Al2O3 and sodium carbonate revealed that using barium aluminate with the structure of Ba2Al4(OH)16 with the removal efficiency of carbonate equal to 80.2% and sodium phosphate (Na3PO4) and lime with the removal efficiency equal to 66.9% in oxalate removal, have maximum efficiency. The results also showed that lime and bauxite lead to 66.9 and 99.9% of carbonate and oxalate pollutions in Bayer process liquor, respectively.

Recently, surface coating process by mechanical alloying method was considered as a new and fundamental method. The method in this paper was introduced as "surface mechanical coating (SMC)". In this research, effect of speed milling parameters on mechanical surface coating process in the range of 200 to 500 rpm for both 20 and 60 hour milling time was considered. Since the milling speed leads to increased energy and higher chamber temperature, then microstructural and compositional changes in the powder and coating are inevitable. X-ray diffraction, electron probe micro analyzer and scanning electron microscope were used to study the structure and composition properties. It should be mentioned that in this research copper powder and nickel ball were used as initial materials. The results showed that the completed formation of Cu-Ni solid solution was acquired after 20 hours of milling at 400 rpm and 60 hours of milling at 300 rpm. It was found that the thickest coating layer of about 220 µm was obtained after 60 hours of milling at 400 rpm. The solid solution formation mechanism during SMC process was extracted according to microscopic examination.

The aim of the present study is to investigate the effect of Al-Fe intermetallic compounds on the strength of friction stir spot welded (FSSW) joints. The tool rotational speed and dwell time were the studied process parameters. The microstructure of the joint interface was characterized using optical and scanning electron microscopes. Tensile-shear testing results showed that the strength of the welds improved with the enhancement of dwell time for all of the applied rotational speeds. Furthermore, for a given dwell time, higher rotational speeds led to stronger joints. Microstructural observations revealed that the strength of the welds increased with the growth of the intermetallic layer thickness up to 5 µm.

An attempt was made to investigate the thermal and residual stress distribution in a novel three layer (La2Zr2O7/ 8YSZ/ NiCrAlY) during a real-like heating regime. The technique of reduction of solving time like mass scaling leads to a considerable reduction in running time while satisfying and not violating accuracy and converging criteria and constrains. Simulation results indicated that, most of damaging and harmful distortion and residual stress concentrate on ceramic top coats and this lead less harm and life time reduction in substrate.

Todays, aluminum matrix composites are widely used in many industrial applications due to their superior properties such as high strength to weight ratio. However, joining problems of these composite materials is a challenge which limits their applications. Consternation of alumina particles, micro-cracks and porosity were found in the joint areas that were filled from α-Al solid phase and fine CuAl2 precipitated particles. The filled areas increased as the joint temperature increase. It is due to a higher amount of the molten material produced during joining. Moreover, the amount of CuAl2 precipitated particles reduced as the joining temperature increased, resulting in reduction of the hardness of the α-Al solid phase in the joint area. The shear strength of the bonded joints was also evaluated using a tension test system. The maximum shear strength (82% of the base metal shear strength) was achieved at the bonding temperature of 590°C and process duration of 30 minutes. Analysis of the fractured surfaces showed plastic deformation, shear dimples and partially ductile failure. Some alumina particles were observed on the fractured surface. At low joining temperatures, the joint strength improved by increasing bonding time, whiles at high joining temperatures, the joint strength decreased due to reduction of CuAl2 particles in the α-Al solid phase.

The use of magnesia-doloma refractories is restricted due to their poor hydration resistance in variety industrial applications. In this investigation the effect of nano-Fe2O3 and nano-ZrO2 addition on the microstructure and hydration resistance of magnesia-doloma refractories was investigated. SEM and XRD analysis were used to investigate the microstructure and phases. Results show that the using nano-Fe2O3 causes the formation of low melting phases such as C2F, C3A and CAF surrounding CaO, MgO grains and grain boundaries at the working temperature which increased the hydration resistance, density, and decreased the porosity. Using nano-ZrO2 led to the formation of CaZrO3 phase in solid state at the working temperature which increased the hydration resistance. The results also show that the samples containing nano-ZrO2 have high hydration resistance compared to samples with nano-Fe2O3. The MC35F4Z2 sample, showed the highest hydration resistance than all nano-Fe2O3 samples and samples which contained 2, 4 and 6 wt. % nano-ZrO2.