مجله مواد نوین
سال دوازدهم شماره 2 (پیاپی 44، تابستان 1400)

In this research, the effect of current intensity on the corrosion resistance of Ti-6Al-4V alloy welded by Tungsten-inert Gas (TIG) method was investigated. For this purpose, 3 × 50 × 40 mm specimens were prepared and after preparing the surfaces, in a way but joint two-way single pass in vacuum chamber with inert argon with a flow rate of 70, 80 and 90 A were welded. Microstructural studies of base metal (BM), fusion zone (FZ) and heat affected zone (HAZ) were performed using optical microscopy and scanning electron microscopy (SEM). Then, in order to investigate the corrosion resistance of Ti-6Al-4V welded cross sections, the roots of the welded specimens were examined by potentiostatic polarization in seawater and 3.5% NaCl. The results showed that by increasing the welding current intensity from 70 to 80A, the grain size in the weld area as well as the corrosion rate of the weld area in Ti-6Al-4V alloy increased. By varying the flow intensity from 80 to 90A, the corrosion rate and grain size of the weld zone did not change significantly. The corrosion rate of Ti-6Al-4V alloy in 3.5% NaCl solution compared to seawater revealed that the corrosion rate of alloy in 3.5% NaCl solution is higher than its corrosion rate in seawater.

Flow line model as an efficient model in ECAP process analysis is based on functions that evaluate the material plastic flow inside the die. Among the various flow functions, the general flow function has the highest ability in analyzing of ECAP process. All parameters of this function have so far been considered as fixed parameters in calculating velocity and velocity gradient fields. Previous studies have shown a relatively linear relationship between the power in the function and the initial position of the flow lines. Accordingly, in the present work, the mentioned fields have been calculated based on the variable power and in order to investigate its effect, the experimental results of the ECAP process with an angle of 90 degree of aluminum alloy AA2124 have been used. The experimental flow lines were analyzed to find the linear relationship between the function’s power and the initial position of the flow lines. Simulations of the texture evolution were carried out subsequently using the general flow function in both constant and variable power conditions. Comparison of the simulated textures with the experimental results showed a much better performance of the variable power mode; in such a way that compared to the constant power condition, the deviation in the position of the simulated texture components in the variable power mode is significantly lower. However, the change in the power mode of the function did not show a significant effect on the intensity of the simulated texture.

In the present study, the Al72Cr17Fe11 quasicrystal nano-powder was synthesized through mechanical alloying. Phase investigation revealed the formation of decagonal quasicrystal structure on a nanometer scale. After that, the aluminum matrix composites reinforced by 3 wt.% of produced quasicrystal powder were fabricated by using the accumulative roll bonding process. Microstructural and mechanical properties were evaluated using scanning electron microscopy and microhardness and tensile tests, respectively. Microstructural studies showed that there was an improvement in the distribution of the quasicrystalline reinforcement phase in the aluminum metal matrix composite as well as an improvement in the bonding between the layers by increasing the cycles of the accumulative roll bonding process. Investigation of microhardness changes showed an increase in hardness value by adding a reinforcing phase in comparison to pure aluminum in the range of 74.5 to 105.8 Vickers after 8 cycles. Stress-strain curves of composite specimens showed that the tensile strength of composite specimens and their elongation increase continuously with increasing cycles of the accumulative roll bonding process. Tensile strength increased between 165 to 250 MPa with increasing rolling cycles from 2 to 8. The maximum toughness (30.2 (energy/volume (MPa)) was obtained after 8 cycles.

In this research, NiAl-TiC-TiB2 composites with different contents of TiC-TiB2 were produced via the self-propagating high-temperature synthesis (SHS) process using the exothermic reactions in compressed mixtures of Ni, Al, Ti, and B4C. The synthesis of composites was performed using induction assisted heating, and the formation of phases and the morphology of reinforcing particles were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, respectively. Results indicate that the TiC-TiB2 ceramic phases were successfully synthesized and dispersed throughout a NiAl matrix. By increasing the TiC-TiB2 content, the porosity in the structure gets a rising trend due to the enhancement of reaction kinetics and intensity of the reaction, and reaches from 24 Vol.% in the sample without reinforcement phases to 39% in the sample with 15% of ceramic reinforcements. Evaluation of mechanical and tribological properties of the fabricated ceramic samples shows that - although the addition of TiC-TiB2 increases the volume fraction of voids which cause destructive effects - a desirable distribution of ceramic particles within the NiAl matrix leads to improved properties for the composite. The distribution of TiC-TiB2 particles throughout the matrix results in a remarkable decrease in sample weight loss in the pin-on-disk wear test. Also, the microhardness mean values increase from 497 HV for the unreinforced NiAl sample to values up to 1015 HV for the composite sample containing 15% of reinforcing particles.

Carbon is cheap and abundant in nature which can significantly reduce the cost of solar cell fabrication. In recent years, carbon nanostructures have gained special attention for application in perovskite solar cells. In this research, graphene oxide quantum dots (GOQDs) have been used in a planar perovskite solar cell. For this purpose, GOQDs with sizes smaller than 10 nm were synthesized by the hydrothermal method. The GOQDs were spin coated on ITO to make a planar n-i-p perovskite solar cell with the structure ITO/GOQD/MAPbI3/Spiro-OMETAD/Ag. The absorption spectrum of the GOQDs shows no overlap with absorption band of the MAPbI3 perovskite layer. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis show that a uniform film of crystalline MAPbI3 perovskite has been formed on the GOQD layer. The best device performance achieved in this research for the planar perovskite solar cell is as follows: Jsc=21.9 mA/cm2, Voc=1.02 V, FF=0.67 and PCE=15%.

The effect of metallic agents such as aluminium, magnesium and mixture of magnesium-zinc was studied on the carbothermic reduction of titanium dioxide in this research under mechanical milling. The mechanical milling runs were done in a planetary ball mill in different times. The stoichiometric mixtures of TiO2-Mg-C, TiO2-Mg-Zn-C and TiO2-Al-C were prepared based on stoichiometric ratios for each reaction. The acid washing process using 2 N HCl solution was done to remove the soluble phases after finishing mechanical milling of each sample. The isothermal heating was done at temperature of 1000 ºC for one hour for solid residues of acid washing under flowing of argon atmosphere. The analysis of products and phases was done using XRD and the microstructure of samples studied by SEM. The results showed the reaction between titanium dioxide and carbon would be exothermic after the adding metallic agent. The reaction progresses via MSR mode for the mixture of TiO2-Al-C in milling conditions and the products were titanium carbide and alumina in the 5 h milled sample. The reduction reaction however progressed via intermediate titanium dioxide in the milled samples for mixtures of TiO2-Mg-C and TiO2-Mg-Zn-C. The signs of titanium carbide and intermediate titanium oxide such as Ti2O3 were observed in the milled mixtures of TiO2-Mg-C and TiO2-Mg-Zn-C after isothermal heating under argon atmosphere. The signs of Ti2O3 phase reveal that the reduction reaction of titanium dioxide does not complete after 10 h milling when metallic agents such as magnesium and/or magnesium-zinc mixture are used.

The microstructural evolutions and mechanical properties of the IN617 superalloy aged at 750°C for 105000 hours were investigated. Microstructural examinations were carried out by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Microstructural evaluations shows that continuous carbides have been formed in all grain boundaries and large carbides have been formed within the grains and twin boundaries, due to long-term aging at high temperature. The volume fraction of carbides increases from 0.5% in the as-received plate to 6.5% in the aged sample. In addition to the three main compounds of M6C, M23C6 and Ti(C,N), the γ' phase and a small amount of harmful δ phase have been identified in the aged specimen. The stability of mechanical properties at high temperature is attributed to the presence of γ' phase after 105000 hours of aging. Based on γ' particles sizes (4-10 nm) as well as their morphology, this phase is in the dissolution stage after 105000 hours. Furthermore, M6C is in the transformation stage to M23C6 carbide and γ' phase, and a small amount of Ti(C,N) has been transformed to carbide. The presence of M23C6 carbides with dimensions less than 200 nm inside the grains indicates that the nucleation process of carbides continues. More than 90% of the carbides have spherical, quasi-spherical and irregular morphologies, and the rest are plate-like and rod-shaped. The morphology and size of the carbides formed at grain boundaries and intragranular regionsindicate that they are in the process of dissolution and agglomeration. The strength and hardness of the aged alloy is better than the as-received plate; However, due to the formation of large carbides in the grain boundaries, the impact energy of the aged alloy has been reduced by more than 75% as compared to the as-received plate. The results shows that IN617 is a suitable alloy for the long-term services at temperature range of 700-800°C .