مجله مهندسی متالورژی
پیاپی 66 (تابستان 1396)

Formation and evolution of order-disorder microstructure of γ/γʹ phases plays an important role in high temperature strength of cast nickel-based superalloys. Accordingly, the characterization of formation mechanism, as well as morphology and interface of γ and γʹ phases, are very important to control the mechanical properties of nickel-based superalloys. Formation of γ' precipitates inside the γ phase matrix often occurs by classical nucleation and growth mechanism. Recently, some researches have reported the formation of γ' precipitates by spinodal decomposition mechanism in some Ni-based superalloys. In the present study, the spinodal decomposition phenomena in a Ni-Al binary system (as a simple model alloy for Ni-based superalloys) has been studied. For this purpose, the Ni-rich region of Ni-Al alloy system was assessed thermodynamically using CALPHAD method. The Gibbs free energy of γ and γʹ phases for this alloy system was calculated employing two sublattice model. Then, the possibility of spinodal decomposition was investigated from the thermodynamic point of view by the study of the Gibbs energy curves in different ranges of temperature. Due to the thermodynamic modeling results, a proper heat treatment procedure was designed for Ni-18Al (at.%) alloy and characterized by means of 3D atom probe tomography (APT) analysis. According to the modeling and experimental results, it was found that γ' precipitates were formed by spinodal decomposition under higher supersaturation conditions.

In this study, the influence of cold work on precipitation behavior in Ni Span C902 super alloy is considered to investigate. The samples were homogenized at 1000 ᵒC, and then samples were subjected to different amount of cold work of 30, 40 and 60%. Precipitation hardening treatments were performed at 450, 550, 650 and 750 ᵒC for 1 to 6 hours. Microstructural observations were done by optical microscope and precipitation hardening behaviors were investigated by Vickers hardening test. The results indicated that precipitation hardening at the temperatures of 450 and 550 ᵒC could not provide proper conditions for formation of strengthening precipitations (γ´); while hardness increased significantly by increasing the aging temperature. The maximum hardness has been achieved for the homogenized and the 60% cold worked samples at 750 and 650 ᵒC, respectively. The results of empirical observations of the experiments in the present study are in well consistent in comparing to Johnson-Mehl-Avrami equations.

Mixer- settlers are widely used for solvent extraction process in industry. The aims of this project are simulation of fluid flow and optimizing the operational conditions of settler in solvent extraction process. Designing and meshing of settler geometry is done by Gambit software. Then, in order to simulate the fluid flow, the meshed designed imported to Ansys Fluent software. Simulation results were verified before simulation. Effect of Inlet volumetric on phase separation investigated. The effect of geometry of picket fences was investigated by the presence of cubic, cylindrical picket fences, picket fences with 5 corner cross section and half cylindrical picket fences. A few extra plates of picket fences were located near the entrance of settler. Phase separation in presence of two and three rows of picket fences investigated. Results indicate that by reducing the inlet volumetric rate, increasing the number of rows, putting extra plates of picket fences in front of entrance and decreasing the closed to open surface ratio to 2, separation improves.

In this research Cu-12wt%Al-4wt%Ni shape memory alloy powder with and without addition of boron as alloying element was produced by using mechanical alloying method. Bulk samples were produced from the powder by press-sinter and subsequently rolling-heat treatment processes. The density and the amount of porosity of the samples were measured, subsequently. The effect of milling time on phase changes in produced powder, size of crystallites and lattice strain was also investigated. Results showed that after 20h of milling with rotating speed of 250 rpm and ball to powder mass ratio of 20:1, peaks of pure elements disappear and solid solution with FCC structure and lattice parameter close to that of copper forms. Morphology of milled powder and microstructure of bulk samples were studied by optical microscope and scanning electro-microscope. It was observed that adding boron lead to decrease in the size of particles after milling. Results showed that the density of samples after press-sinter processes decreases. However, the final density increased after rolling-heat treatment processes. It was also shown that adding boron to the powder mixture results in more homogenous distribution of porosities after rolling-heat treatment processes.

The isothermal high temperature corrosion behavior of an Fe3Al coating coated on plain carbon steel through tungsten inert gas (TIG) surface alloying with current of 40A was studied in the atmosphere of air. The specimens were tested at 700 and 900 °C for 4–100 h. SEM, EDS and XRD analyses were utilized to study the oxidation behavior of the iron aluminide intermetallic coating. The weight gain data determined a parabolic rate law. The results also revealed that the initial scale formed on the coating was predominately θ-Al2O3 which was substituted by α-Al2O3 after about 64 hours. The SEM observation and XRD results illustrated that at shorted times, when the θ phase was stable, spalling was more usual while with the increase in the amount of the α phase the spalling decreased significantly. The EDS results confirmed the presence of aluminum oxide at shorted times and iron oxide at longer times.

Friction stir processing as a relatively new and effective solid-state process is used for surface modification of metals and alloys in order to modify their microstructure and properties. In this study, the effect of friction stir processing on the microstructure and mechanical properties of an Al-0.9Ni-Fe alloy was investigated. The process was performed under the rotational speeds of 600-2000 rpm and the traverse speed of 25 mm/min. According to the results, the severe surface plastic deformation refined the intermetallic compounds present in the microstructure, improved their distribution within the matrix, and substantially reduced the amount of casting defects. As a result of these microstructural variations, the tensile strength and percent elongation of the base alloy are increased by almost 40 and 205%, respectively. The fracture toughness is also improved by 205%. It was also found that the hardness of processed alloy is about 24% higher as compared to the base alloy.

In the countries with natural gas, repair of gas transmission pips is a problem. In this study, the epoxy composite with single wall carbon nanotubes reinforcements as patch was used and the von Mises stress distribution was determined. For this purpose, the tubular steel with thickness and length 6 mm 1.5 m was simulated in ANSYS software at both ends of the pipe patched-epoxy composite single-walled carbon nanotubes with reinforcing particles ratios 0.01, 0.03, 0.05, 0.07, 0.1, 0.13 and 0.15 were applied. Then, according to the true values of internal pressure and external pressure, the stress of gas pipe distribution on the patch was determined. Von Mises stress results showed that, carbon nanotubes 0.01 times the maximum repair patch has 0.19 MPa von Mises stress is the maximum that occurred in the casing without patches that 1.48 Mpa is, much less. It was also found to increase the percentage of carbon nanotube to 0.15, von Mises stress to the maximum reaches 0.43 MPa. As a result, it can be stated that the nanocomposite patch greater ease in pipes, can be improved tensile properties are repaired areas. As well as increasing the percentage of nanotubes in composites, von Mises stress increases the maximum in the elderly.