فهرست مطالب امیر سیف الدینی

One of the most crucial factors influencing the strength and quality of steels is the presence of non-metallic inclusions. Studies demonstrated that non-metallic inclusions in industrial parts diminish their fatigue strength. On the other hand, their hardness is another influential parameter affecting the fatigue strength of steels. Therefore, this research investigated the impact of non-metallic inclusions on the fatigue strength of AISI 304 steel with three hardness of 170, 250, and 330 Vickers. This research revealed that as the hardness decreases, the fatigue strength of the part also decreases. Additionally, an examination of the failure surfaces of the parts using scanning electron microscopy (SEM) indicates that non-metallic inclusions are the root cause of fatigue failure in the samples. Finally, employing Murakami relations, parameters such as stress intensity factor and critical inclusion size were determined for steel with hardness levels of 170, 250, and 330 Vickers. The critical inclusion size for steel with hardness levels of 170, 250, and 330 Vickers was calculated as 32.7, 24.8, and 15 micrometers, respectively.

In this work, the effect of Ni addition on the glass forming ability (GFA) of FeMoPCB composite alloy was investigated and the optimal amount of Ni to improve GFA was determined. So, the thermal stability of this alloy with the various percent of nickel was investigated by not only experimental methods including; differential scanning calorimetry, X-ray diffraction, optical microscope and transmission electron microscopy but also computational methods such as α، β، γ and etc. The results of XRD test, TEM showed that in the presence of 10 at.% nickel , the glass-forming ability of this alloy increased, while in the presence of lower or higher percent nickel, different crystalline phases were detected. The presence of crystalline phases was also investigated by microhardness test. Finally, it was found that the results of the computational methods regardless of the experimental results don’t have enough accurate .

In this research, fabrication of an in-situ composite via spark plasma sintering method in an Al-V2O5 system was investigated. For this purpose, the powders of Al and V2O5 with three combination Al-41.94wt%V2O5, Al- 22.93wt%V2O5 and Al-15.78wt%V2O5 were mixed for 2 hours and then compressed and sintered at 550˚C by spark plasma sintering method. To study the temperatures of occurrence of phase changes was used from differential thermal analysis. spark plasma sintering samples with respect to peak reaction temperatures in differential thermal analysis were subjected to heat treatment for 10 minutes at a temperature range from 650 to 1050 ˚C and then cooled inside the furnace to ambient temperature. According to the X ray diffraction results, it was found that the phases of VO, α-Al2O3, Al3V, Al6V, Al11V were formed in the samples. Samples sintered at 650˚C have a highest density. With increasing temperature and percentage of aluminum in the initial powder mixture, the density of the samples decreased due to increase of porosity and cavity. Measured densities were in the range of 2.5 -3 g/cm3. The hardness of the samples was increased by an increase in temperature and decreased by an increase in the percentage of aluminum in the mixture of the initial powder. Hardness of the tested specimens was in the range of 52 to 123 HV.

In this research, the effect of heating rate on oxidation kinetics of magnesium powder particles under non-isothermal conditions was studied. For this purpose, differential thermal analysis (DTA) and thermogravimetry analysis (TGA) was done on magnesium powder particles at three heating rates of 5, 10 and 20 K min-1 up to 1000 °C under air atmosphere. Also, in order to better understand the oxidation process of magnesium powder, three temperatures were selected according to the DTA curve at a heating rate of 20 K min-1. Then, samples of magnesium powder were heated up to these three temperatures with heating rate of 20 K min-1 and were subjected to X-ray diffraction (XRD) and scanning electron microscopy (SEM) for phase and microstructural analysis. Then, kinetic studies were performed using some isoconversional methods such as Starink and Friedman as well as direct and indirect fitting methods. The activation energy (E) and pre-exponential factor (lnA) for oxidation of magnesium powder were in the range of 327-956 kJ mol-1 and 45-135 min-1, respectively. The reaction models for heating rates of 5, 10 and 20 K min-1 were obtained to be A3/2, R2 and D1, respectively.

In the present study, effect of Ni alloying element on the characteristics of deposited weld metal of E7018-G electrode was evaluated. Therefore, electrodes contained different amounts of Ni (0-1.7wt.%) were designed, manufactured and welded via SMAW process. Microstructural studies revealed dichotomy effect of Ni on the deposited weld metal microstructure, i.e. increasing the Ni content up to 1.2wt.% improved the formation of acicular ferrite in the weld metal microstructure and caused significant grain refinement at the reheated zone of weld metal. While, higher Ni content (>1.2wt.%) resulted in some raising in the widmannstatten ferrite content in the weld metal. Strength multiplied by impact energy parameter (UTS×CVN) was used for mechanical properties assessment. Mechanical properties evaluation revealed the highest UTS×CVN parameter achieved in the weld metal contained 1.2wt.% Ni. Hardness of the weld metal increased with increasing Ni content which is related to the formation of micro constituents in the microstructure of weld metal and increasing their content with increasing Ni content.

Aluminum and its alloys have an ever growing demand in many industries such as aerospace, automotive due to their high strength to weight ratio and corrosion resistance. In this research, in order to improve the wettability and distribution of nano-sized Al2O3particles within the matrix with low agglomeration and to achieve the good mechanical properties, injection of the milled nano- Al2O3/Al composite powder within the molten and mechanical stirring was used. Different mass fractions of nano alumina particles up to 0/3, 0/5, 0/7 and 0/9 wt% were injected into the melt under stirring. The microstructure and mechanical properties of the fabricated nanocomposites were studied. The density measurements showed that the porosity in the composites increased with increasing the mass fraction of Al2O3 Hardness, yield and ultimate tensile strengths of the composites increased with increasing Al2O3 particles up to 0/5 wt% The increase in mechanical properties can be explained by more uniform distribution of Al2O3 in the matrix, and grain refinement. The further addition of Al2O3 particles (>0/5%) particle agglomeration and porosity were the two important factors for decreasing strength and ductility.

In this study, Al/(SiCӘ) hybrid composite was produced on the surface of Al-1050 substrate via friction stir processing to achieve the high hardness of SiC particle and lubricating property of BNh simultaneously. Microstructural studies revealed that grain refinement was occurred in the stir zone (22% reduction of grain size in comparison to base metal). Closer microstructural studies by using scanning electron microscope equipped to Map-EDS analyzer illustrate the relatively uniform distribution of SiC and BNh particles in the Al-1050 matrix. Microstructural studies indicated that increasing FSP passes increased the grain sizes of TMAZ and HAZ more than 20% of base metal grain size. Microhardness evaluation showed that increasing the number of FSP passes resulted in increasing hardness of stir zone (100± 5HV in 8 pass sample in comparison with 60± 5 in 1 pass sample) that was ~70% more than hardness of base metal.

Corrosion resistance of (1) Fe77Mo5P9C7.5B1.5 glass matrix composite containing α-Fe dendrites and (2) AISI Type 301 stainless steel was assessed in deaerated 1 M HCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. Results showed that the excessive difference in composition between α-Fe dendrites and glass matrix induced a galvanic effect that causes preferential dissolution of α dendrites. However, no transpassive behavior was observed by anodic polarization up to 1000 mV for composite specimen. In contrast, the stainless steel revealed lower corrosion rate and passive current density. SEM micrographs were also used to confirm these findings.