The aim of this research is to study and evaluate the effects of cobalt and nickel additions on heat-treated and thermal-exposed microstructures in cast nickel-base superalloy grades based on GTD-111 with cobalt and nickel modifications. In the study, 1 wt.-% aluminium, as well as various cobalt and nickel contents, was added to all the modified alloy. After standard heat treatment, it was found that increasing the 1 wt.-% aluminium content affected the morphology of gamma prime particles, causing them to become more cuboidal in shape and larger in size. On the other hand, the size of gamma prime in all modified alloys decreased slightly with increasing cobalt contents. However, cobalt and nickel additions did not strongly affect the shape of the gamma prime particles. After long-term exposure at temperatures of 1173 K and 1273 K for 1440 ks, these gamma prime particles were much larger in size than those of heat-treated ones. The higher temperature caused extreme increases in the average size of thermal-exposed gamma prime particles. Also, the gamma prime particles became nearly spherical in shape and had lower coarsening rate with higher cobalt and lower nickel contents. Moreover, the addition of cobalt reduced the coalescence of gamma prime particles. Samples with cobalt content higher than 13.1 wt.-% were able to maintain the cubic shape of the particles even after long-term exposure at a temperature of 1273 K for 1440 ks. No topologically close-packed phase was found in any analyzed specimens.
Morphological evolution of gamma prime precipitates was governed by an elemental partitioning ratio between the gamma and gamma prime phases. In the present study, Ni-based MGA 1400 alloy was modified by adding 4 wt% of Re and 6.4 wt% of Co by vacuum arc melting. The effects of Re and Co additions on lattice parameters of gamma and gamma prime phases were studied by TEM and XRD techniques. Calculated lattice misfit values obtained from XRD and TEM techniques were used to identify the shape and morphology of gamma prime precipitated particles after long-term exposure at 1173 K for 1800 ks? After long-term exposure, gamma prime particles became much coarser. The addition of Re retarded the coarsening rate of the gamma prime precipitates, resulting in smaller gamma prime particles at the same annealing times. Even though, both Re and Co atoms are strongly partitioned in the gamma matrix, the effect of Re and Co additions on lattice misfits was contradictory. The addition of Re produced rounded corner cuboidal morphology of gamma prime precipitates due to higher magnitude with negative lattice misfit (−0.3857%), while Co addition provided a more rounded shape of gamma prime particles due to the lattice misfit value approach to zero (+0.0936%).
This research work studied and evaluated the effects of reheat treatment conditions, which consisted of solution treatment at a temperature of 1448 K for 14.4 ks, followed by air cooling and precipitate aging at a temperature of 1118 K for 86.4 ks, on the microstructural rejuvenation or refurbishment of various modified alloys based on the cast nickel base superalloy, GTD-111 with aluminum, nickel and/or cobalt additions after long term heating at temperatures of 1173 K and 1273 K for 1440 ks. From the results obtained, it was found that the reheat treatment conditions applied are more suitable for microstructures after long term heating at a temperature of 1173 K. However, such reheat treatment conditions could not fully return reheat treated microstructures to microstructures similar to those of previous research work. It seems that the selected solutioning temperatures and/or times were not sufficient to completely dissolve all coarse gamma prime particles after long term heating for all samples with alloying additions. Typical size and area fractions of the gamma prime particles of the reheat treated microstructures are very similar to those of the original alloyed ones but with lower values, especially those related to the size of the gamma prime particles.
This research work studied the effect of aluminium, cobalt and nickel additions in superalloys grade GTD-111 and IN-738 on morphology of gamma prime particles and their lattice misfits after subjected to heat treatment and long-term cyclic exposure by using SEM and XRD. It was found that the particles became more cuboidal in shape after heat treatment in modified GTD-111 and IN-738 alloys. When adding 1 wt% aluminium and 6 wt% nickel, larger negative values of lattice misfit were found at −0.57 % and −0.28 %, respectively. However, 6 wt% cobalt addition reduced the negative value of the lattice misfit to −0.20 % in modified GTD-111 and small positive lattice misfit (+0.08 %) was found in modified IN-738. Also, modified GTD-111 alloy shows more spherical shape of the gamma prime precipitates than those in modified IN-738. After long-term cyclic exposure at 1173 K for 1440 ks, it was found that the particles become more spherical shape comparing to those of heat treated ones. The shape of particles in both modified alloys with 1 wt% aluminium and 6 wt% nickel additions maintained more cuboidal compared to others. Furthermore, the effect of 6 wt% cobalt addition is more pronounced for modified IN-738 than that of modified GTD-111.
In this study, a nickel-tungsten/amorphous boron composite coating (Ni-W/B) was successfully deposited on carbon steel using the electrodeposition method. Electrodeposition was performed by dispersing varying quantities of amorphous boron (0, 0.5, 1, and 3 g/L) particles in a Ni-W electrolytic bath. The microstructure and morphology of composite coatings were characterized by scanning electron microscope (SEM). The electrochemical properties of the Ni-W alloy and Ni-W/B composite coatings were studied using electrochemical impedance spectroscopy (EIS), polarization curves, and immersion testing. It was found that the addition of amorphous boron particles to Ni-W coatings can significantly smooth the modified composite coatings and improve the corrosion resistance, probably by changing the corrosion from pitting to uniform corrosion with increasing boron content. The boron concentration of 0.5 g/L in the bath provided the highest corrosion resistance among all the modified coatings.
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