نشریه مهندسی متالورژی و مواد
سال بیست و هفتم شماره 1 (پاییز و زمستان 1394)

Two batches has been prepared involve calcium carbonate- zirconia and dolomite- zirconia with mole ratio 1:1. After mixing in water and drying, samples were preesed in cylinder form and then fired at four temperatures for three hours. The phase evaluation was studied by XRD technique. Calcium zirconate were established at all samples. At 1300 ˚c monoclinic zirconia phase was observed that its intensity is more in dolomite samples. With increasing temperature the intensity of ZrO2 phase is decreased and stabilized zirconia with calcia (Ca0.15Zr0.85O1.85 ) is observed.

Nickel coating has a poor surface mechanical properties. In order to improve the mechanical properties, nickel were covered with ceramic particles. In this study, the Ni/TiN composite coatings were produced by Watts bath containing TiN nanoparticles. The effects of deposition current density, electrolyte agitation speed and the number of particles were investigated. The presence of particles was determined by EDS analysis and the coating morphology was examined by SEM. The corrosion samples were examined by linear polarization tests. It was observed that the maximum amount of particles is formed at the current density of 4 A/dm2 and the agitation speed of 450 rpm.

Dissimilar joint of low-alloy ferritic steel and austenitic stainless steel has been widely used in industries. In this study, the austenitic stainless steel A240-TP.316 and low-alloy ferritic steel A387-Gr.11 were welded together by gas tungsten arc welding (GTAW) method with constant and pulsed currents. Two types of filler metals, i.e. ER309L and ERNiCr-3, were used in this investigation. The microstructure of the dissimilar weldment was characterized by conventional metallography using optical and scanning electron microscopes and by energy disperse spectroscopy (EDS). The mechanical properties of test samples were evaluated by tension, charpy impact and microhardness experiments. It was found that the samples welded by pulsed current exhibit relatively higher impact energies mainly due to their lower heat input induction and creation of intense mixing in the welding pool. Consequently, the formation of undesirable microstructural features such as carbon depleted zone, transmission region and unmixed zones were reduced in the pulsed current welded specimens. Among all the filler metals used, the nickel based filler metal was found to be suitable since in this case, the migration of carbon into the welding pool was limited and therefore, the risk of transition region formation as compared with other filler metals decreased.

The use of methods employed for estimating the strength and toughness of composites reduces the trial-and-error in their design. The strength of interface between reinforcement and matrix and the effective surface of pulled-out particles have great influences on toughness and strength of these composites. In this study, a model was proposed to estimate the interface strength and matrix strength of the composite and their effects on the toughness and strength were investigated. In addition, the strength of interface and its influence on increasing the toughness and strength were examined for a composite sample containing M23C6 particles as the reinforcement component within the matrix of 1.2542 tool steel.

In this research, fabrication of an in-situ composite via aluminothermic combustion synthesis in Al–V2O5 system has been investigated. For this purpose, the mixed powders of Al and V2O5 with molar ratio of 28 to 3 were ball milled and then compressed. Differential thermal analysis was used to evaluate the temperatures of phase transitions. Using the peak temperature of the reactions, the samples were subjected to heat treatment. It was shown that the reduction reaction of V2O5 by Al is a stepwise process and V2O4 and V2O3 as the main intermediate phases and γ-Al2O3 and Al23V4 as the transmission phases were formed before the formation of Al3V and α- Al2O3. Microstructural studies showed that after sintering at the temperatures up to 1000 ºC, Al8V5 phase is formed at the interface between the two phases of Al3V and α-Al2O3. Hardness and compression test results also showed that the hardness and strength of the samples increase with increasing heating temperature due to the formation of greater volume fraction of intermetallic compounds and ceramic phases, however, the compressive strain decreased because of the formation of a greater amount of the brittle phase Al8V5.

In this research, mechanical alloying was used to synthesize ultrafine grained AA2024 and AA2024-B4C powders in an attrition mill under argon atmosphere up to 50 h with the rotational speed of 400 rpm. In order to determine the grain size of the matrix, X-Ray diffraction test and Williamson–Hall method was used. After mechanical alloying, hot pressing and hot extrusion were used for densification of powders. The microstructure and mechanical behavior of hot extruded samples were characterized by scanning electron microscopy (SEM), tension and compression and hardness tests. The microstructure of samples showed that the CuMgAl2 precipitate is present. Furthermore, the distribution of these precipitates in nanostructured samples was more uniform and their content was greater. The results of mechanical tests indicated that the strength and hardness of AA2024 alloy after mechanical alloying and addition of B4C particles increases for 64 and 49%, respectively. In other words, the AA2024-B4C nanocomposite showed the highest strength and hardness, however, its elongation was the lowest.

The critical temperatures of a thermomechanical process have a great importance for the final microstructure and mechanical properties of high strength low alloy steels. In this study, the average scheduling was used to determine the critical temperatures of API X70 steel. This steel is extensively used in Iran for large diameter, high-pressure gas transportation pipelines as well as the oil transmission networks. The critical temperatures in various conditions including different strains, strain rates (from 0.1 to 1 s-1) and rolling interpass times were determined and the effects of these parameters on no-recrystallization critical temperature (Tnr) was investigated. The results showed that Tnr decreased with an increase in the strain and strain rate. In addition, it was observed that Tnr increased with a decrease in the interpass time. A good agreement was found between the results of this research with those of existing empirical relations and those of similar steel. With regard to the lack of experimental data, the results obtained in the present study can be used for production of API X70 steel in Iran.

Titanium aluminide and its alloys have been utilized in automotive, aerospace and power industries because of their specific properties such as low density (~ 4 g/cm3), high melting temperature (~ 1460 °C) and excellent high-temperature strength. Ti-48Al intermetallic compound and its alloys with 1.5, 3 and 5 at.%Mn were produced with mechanical alloying of elemental powders for 50 hours. The process of mechanical alloying was carried out in a planetary ball mill machine with WC cups and balls under inert atmosphere of argon. To obtain bulk samples, the mechanically alloyed powders were cold pressed with the load of 40 ton and then heat treated at 1050 °C under argon atmosphere in an electrical furnace with quartz tubes. The X-ray diffraction analysis showed that the addition of manganese to Ti-48Al during mechanical alloying causes no formation of a new phase in titanium aluminide system. Scanning electron microscopy observations on the surface of sintered bulk samples revealed that boundaries between powder particles have been well formed at 1050 °C. The results of hardness tests showed that the addition of Mn up to 5 at.% increases the hardness value of Ti-48Al compound (~22.1 HRC) to ~ 40.7 HRC. Evaluation of oxidation behavior showed that oxidation resistance of Ti-48Al-5Mn alloy is slightly smaller (~0.015 g) than that of Ti-48Al. On the contrary to Ti-48Al-5Mn alloy, Ti-48Al compound had a steady-state oxidation behavior for oxidation periods longer than 14000 seconds. Scanning electron microscopy images and the results of energy dispersive spectroscopy showed that the lower oxidation resistance of Ti-48Al-5Mn compared to that of Ti-48Al is related to the formation of rutile at 900 °C on the oxidized surfaces of Ti-48Al-5Mn.