فصلنامه مواد پیشرفته در مهندسی
سال سی و نهم شماره 3 (پاییز 1399)

In this study, the effect of Ta/W ratio on the microstructure and stress rupture properties of Ni-based single crystal (SX) superalloy PWA1483 was investigated. For this purpose, single crystal (SX) superalloys with different Ta/W ratios (0.75, 1.0, 1.32 and 1.5 in wt.%) were fabricated. The alloys were directionally solidified by Bridgman method under the same solidification condition at withdrawal velocity of 3 mm/min and thermal gradient of about 7 K/mm followed by standard age hardening heat treatment. Microstructural characterization was performed using optical microscopy (OM) and scanning electron microscopy (SEM). The stress rupture properties were investigated at 982 °C and 248 MPa. The results showed that increasing the Ta/W ratio decreases the size and volume fraction of micro-pores together with the size of γchr('39') precipitates. Hence, the stress rupture life increased. The superalloy with Ta/W ratio of 1.5 showed the minimum size of micro-porosity (18.2 μm) and the maximum stress rupture life (~34 h). The superalloy with Ta/W ratio of 1 showed the most uniform microstructure and creep behavior. It seems that the presence of topologically closed packed (TCP) η-phases is the main reason for stress rupture life decrease in SX superalloy as micro-pores initiated from TCP phases or the TCP/matrix interfaces.

The aim of the study was to investigate the effect of temperature, time, pH, capping agent concentration (mercaptoacetic acid), Zn to Se and Se to reducing agent (NaBH4) mole ratios on morphology, phase developments and size of zinc selenide nanoparticles prepared by hydrothermal method. Characterization of zinc selenide nanoparticles was performed by Field Emission Electron Microscopy (FESEM), Energy Dispersive X-ray Spectrometry (EDS), X-ray Diffraction (XRD), Induced Coupled Plasma Spectrometry (ICP), Fourier Transform Infrared spectroscopy (FTIR) and Simultaneous Thermal Analysis (STA). The results of EDS showed that the ratio of atomic percentage of Zn to Se in the optimized zinc selenide nanoparticles is 1: 1 and elements are evenly distributed in the nanoparticles. Based on the results of FTIR and STA thermal analyses, MAA as the stabilizing agent binds to and stabilizes zinc selenide nanoparticles. Finally, fine nanoparticles of zinc selenide with narrow size distribution, spherical shape and cubic crystal structure were obtained at the minimum temperature (130 °C) and processing time (15 hours) with the least amount of reducing and capping agents compared to previous researches.

High-temperature erosion-oxidation behavior of nickel-based alloys containing 0-7 wt.% Mo in fluidized bed waste incineration conditions was studied. A stream of hot condensed air with a flow rate of 25 L/min caused hot silica sand (700 °C) mixed with 0.5 wt.% of chloride salts to hit the specimens for 250 h. By removing the erosive factor, the high-temperature oxidation behavior of the alloys in air and air-chlorine atmospheres was studied at 520 and 560 °C for 100 h. Mass gain measurement due to oxidation followed by thickness loss measurement in the erosion-oxidation tests showed that an increased Mo content led to improved oxidation resistance as a result of reduced scaling rate. However, under simultaneous oxidation and erosion conditions, the lower oxidation rate of the alloy with 7 wt.% Mo caused rapid removal of the protective scale and a reduction in erosion-oxidation resistance of the alloy. Under these conditions, the alloy with 3 wt.% Mo showed the smallest removal rate. Microscopic observations and XRD analysis confirmed formation of Cr2O3/NiCr2O4 scales on the surface.  Mo-free alloy with lowest oxidation resistance showed a higher erosion-oxidation resistance. However, the high oxidation rate of this alloy led to a severe Cr-depletion and internal oxidation in subsurface region.

In this study, the effect of welding heat input on microstructure and mechanical properties of dissimilar joints of API-X42 and API-B pipeline steels was investigated. Evaluation of the microstructures showed that increasing the welding heat input decreased acicular ferrite in weld metal microstructure, while amount of Widmanstatten ferrite, polygonal ferrite and grain boundary ferrite increased. Also, results of microhardness test showed that by increasing the heat input, hardness of weld metal and the heat affected zone decreased. Tensile test results showed that as the heat input increased, fracture transferred from base metal of API-B to the heat affected zone. Impact test results also showed that increasing the welding heat input could sharply drop the impact energy of the heat affected zone for both base metals due to extensive grain growth.

The present work aims to modify surface properties of pure Ti by development of Ti-Al-N intermetallic composite coatings. In this regard, tungsten inert gas (TIG) cladding process was carried out using Al 1100 as filler rod with Ar and Ar+N2 as shielding gases. Phase and structure of the samples were investigated by X-ray diffraction (XRD) technique, optical microscopy (OM) and scanning electron microscopy (SEM). Hardness values and corrosion behavior of the obtained coatings were also compared using Vickers microhardness tester and potentiostat, respectively. The results showed that composite structure containing Al3Ti, Ti3Al2N2 and Ti3Al intermetallic compounds could be formed on the surface of pure Ti. Amounts of brittle phases and welding defects at the titanium-coating interface were least by welding under pure Ar shielding. Despite the increasing amount of structural defects such as porosity and non-uniformity under Ar+N2 shielding, the prepared coatings had higher hardness (more than 100 HV) and corrosion resistance (more than twice) compared with those obtained under Ar shielding.

The aim of this study was to investigate the suitable temperature range for hot deformation of three medium carbon Ni-Cr-Mo low alloy steels by hot tensile and hot torsion tests. Hot tensile tests were carried out in the te,prature range of 850-1150°C at a constant strain rate of 0.1 s-1 until fracture. Then, the tensile flow behavior, hot ductility and microstructural evolution of the steels were studied. Hot torsion tests were performed in the temperature range of 1200-780°C at strain of 0.1 with strain rate of 1s-1. The effect of titanium and niobium on the mean flow stress and the non-recrystallization temperature were investigated. The tensile test results showed that dynamic recrystallization was the dominant mechanism at temperatures above 950°C in the base steel and temperatures above 1050°C in the microalloyed steels. The results of hot torsion tests showed that the non-recrystallization temperatures of the base, Ti containing and Nb containing steels were 1070°C, 1069°C and 1116°C, respectively. Finally, the suitable hot deformation temperature range to achieve optimum mechanical properties in the base and Ti containing steels obtained as 950-1070°C and that of Nb containing steel obtained as 950-1100°C.

In this paper, the effect of adding aluminum trihydrate (ATH) on electrical (including dielectric constant, dielectric loss and strength, volume and surface resistivity) and hydrophobic properties of two-part room temperature volcanized (RTV) silicone rubber resin coatings were investigated. For this purpose, the RTV-ATH nanocomposite was made by physical mixing and its electrical and hydrophobic properties were compared with those of pure RTV. The results showed an increase in the dielectric constant (from 3.11 to 4.13), the dielectric loss (by ~ 0.06) and the dielectric strength (by ~ 4 KV/mm) of the RTV-ATH composite compared to the pure RTV. Moreover, ATH increased volume resistivity and reduced the surface resistivity of composite coatings. On the other hand, by spraying of coatings, no improvement in the hydrophobicity class of composite coating compared to pure RTV was observed and both samples were standardized in the HC2 standard class. Structural analysis of samples was performed by X-ray diffraction, optical microscopy and scanning electron microscopy and the presence of the main phases in them was confirmed.

In this study, the influence of temperature and strain rate on plastic flow of a Zr-based bulk metallic glass (BMG) during the three-point bending test was studied to find a correlation between strain rate sensitivity (m) and flow behavior. The flexural stress-deflection curves revealed two distinct types of dynamics, serrated and non-serrated flow, related to temperature and strain rate. The serrated flow which appeared at temperatures higher than a critical value or strain rates lower than a critical value, was simultaneously due to activation of shear transformation zones (STZs) and time-dependent structural relaxations. Further results indicated negative and positive values of m at temperatures above and below 0.4 Tg, respectively. The main reason for negative strain rate sensitivity was insufficient time of structural relaxation at high strain rates which lead to generation of free volume inside shear bands making the BMG softer. Comparison of STZ activation energy with activation energy for the onset of serration indicated almost equal values and there was a close relationship between serrated flow and STZ operation.