kamran dehghani

In this study, the microstructural evolutions, formed phases, micro- and macro-hardness changes and shear properties were evaluated in the as-cast, homogenized, cold-rolled and annealed AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA). These studies were carried out using X-ray fluorescence (XRF), scanning electron microscopy (SEM), Rockwell C and Vickers hardness testing as well as shear punch testing (SPT). The microstructure after cold rolling followed by annealing was consisted of an elongated phases/grains in the direction of cold rolling and in almost equiaxed phases/grains. The homogenized specimen has the minimum amounts of hardness and yield/ultimate shear strength. Due to the presence of both elongated and equiaxed structures in the cold-rolled and annealed sample, the hardness and yield/ultimate shear strength of this specimen is between that of as-cast and homogenized cases.

In this research, the effect of major welding parameters on microstructural evolutions and mechanical properties of double-sided friction stir welded thick 5083 aluminum alloy joints was investigated. For the traverse speed of 80 mm/min, the increase in the rotational speed does not have any effect on the average grain size. In this case, it can be said that the annealing grain growth and recrystallization effect counteract and neutralize each other. However, for the traverse speed of 40 mm/min, with increasing the rotational speed, the grain size increases due to the dominance of annealing grain growth effect. Grain size in the stir zone was decreased about ten time as compared with base metal; however, no considerable increase was observed in the hardness of the stir zone and the hardness profile remained almost uniform alongside the joint area. The best joint efficiency was equal to 99.4 % with the pin length of 6 mm and transverse and rotational speeds of 80 mm/min and 1000 rpm, respectively. Also, the elongation was improved about 9 % compared to the base metal.

Although tantalum nitride coatings have recently been attracted researchers’ attentions due to their high hardness and corrosion resistance, the fracture toughness and deformation plasticity of thin tantalum nitride film has not been well investigated yet. In this research, for the first time, the fracture toughness, strain rate sensitivity and plasticity of sputter deposited tantalum nitride thin films have been evaluated using nano indentation technique and SEM micrographs. It was shown that the fracture toughness was increases from 0.6 to 7.8 MPa√m with increasing the nitrogen in sputtering chamber. This increase was attributed to the structural evolution from a hexagonal γ-Ta2N phase to the hexagonal ε-TaN and face centered cubic δ-TaN phases, determined by X-Ray Diffraction analysis. The plasticity of the TaN films evaluations indicated that the ratio of plastic work to total work was increased from 50% to 57% and 80%with phase variation from γ-Ta2N to ε-TaN and δ-TaN, respectively.

In this study, the microstructural properties and hardness of Friction Stir Welded (FSWed) of high strength low alloy API X70 steel at the presence of titanium dioxide (TiO2) particles were investigated. In this regard, a homogeneous mixture of TiO2 and X70 steel powders were inserted into the weld groove before applying FSW. The FSW method was applied on HSLA X70 with and without addition of titanium oxide (TiO2) particles. The optical microscopy and Vickers microhardness measurements were employed to evaluate the microstructure and hardness of the different weldments zones. The results showed that the hardness of various zones in the weldment are strongly depended on the microstructure which is affected by heat input and stiring action. In addition, the TiO2 particles were homogenously dispersed in the stir zone of TiO2-doped weldment and subsequently has changed the microstructure and hardness. In particular, a transition from an acicular ferrite (AF) dominant microstructure with a hardness value of 300 HV to a polygonal ferrite (PF) dominant microstructure with a hardness value of 180 HV was observed by moving from top surface region to near root region.