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The isothermal oxidation behavior of directionally solidified Ni-base superalloy DZ951 at 900, 1000 and 1100°C were investigated. Oxidized alloy was characterized by SEM, EDAX, XRD. A subparabolic time dependence (n = 0.18 at 900°C and n = 0.08 at 1000°C) of the oxide growth rate was determined at relatively low temperature, while alloy nearly followed parabolic law at 1100°C (n = 0.42). The severe composition segregation, which resulted from the solidification process, led to the formation of different scale on the dendritic and interdendritic regions at 900°C, however, this difference disappeared after oxidation at 1000 and 1100°C for long time. Furthermore, faceted and needle-shaped AlN precipitates developed in the alloy subsurface region after oxidation at 1100°C for 200h, while no internal corrosion products were found in 900 and 1000°C tests.

Advancements in superalloys permit hot gas path components to operate for many thousands of hours under severe centrifugal, thermal and vibratory stresses. The blade of a gas turbine must withstand the most severe combination of temperature, stress, and environment. After a certain period of operation, the blade is damaged by the turbine operation mode. To recover the similar initial mechanical properties, the blade of nickel-based superalloy component undergoes a replacement repair process. A fluoride ion cleaning (FIC) process is used to assist in the successful welding repair of nickel-based superalloy components. This work is to study the FIC influence on a gas turbine blade that was serviced 25,748 equivalent operating hour (EOH) after first overhaul. The blades, including the fractured regions were obtained from a plant. The conditions of blade samples were observed by optical microscope and SEM, and the chemical composition of the flaws in the blade was investigated by EDS. γ′ phase morphology is also an important indicator of the mechanical properties of blades at the high temperatures. The blade condition was examined from morphology and size of the γ′ phase, because the γ′ phase coarsening rate depends largely on the exposure time and temperature.

The bare superalloy DZ 125 alloy, the aluminide coated specimens and electron beam physical vapor deposited (EB-PVD) thermal barrier coating (TBC) consisting of yttria stabilizied zirconia (YSZ) topcoat and NiCoCrAlY bond coat specimens were exposed to atomized seawater and kerosene at 900°C and the cyclic hot-corrosion behaviors of the specimens were investigated. Disastrous spallation of the bare superalloy occurred within 50 h hot-corrosion. In contrast to this, after 100 h hot-corrosion, the average mass change for the aluminized and TBC coated specimens is 0.7 mg/cm² and 0.63 mg/cm², respectively, exhibiting excellent hot-corrosion resistance.

This study investigated various performance measures — material removal rate (MRR), surface roughness (SR), and dimensional deviation (DD) — in wire cut electrical discharge machining of the Incoloy-800 superalloy under the effect of purposefully varied process inputs. Superalloys comprise a bright category of alloys that possess excellent mechanical strength at elevated temperatures, superior surface stability, magnificent creep resistance, and corrosion and oxidation resistance. Analysis of variance was performed to reveal influential and crucial inputs for machining measures; results were interpreted and modeled statistically. After conducting the experimental runs, the novel attempt has further been made to conduct the multi-response optimization with the desirability method and the teaching-learning-based optimization (TLBO) algorithm to develop the best process setting for optimizing all considered measures jointly. Furthermore, machined sample’s microstructure analysis was conducted with the scanning electron microscopy (SEM) analysis. The optimized parametric setting was obtained as follows: pulse on time: 2 $\mu$s, pulse off time: 4 $\mu$s, peak current: 2 A, and wire feed rate: 5 m/min. Obtained experimental values for MRR, SR, and DD were 0.936 mm/min, 3.797 $\mu$m, and 0.084 mm, respectively, with the combined desirability value of 0.601. Optimization values of MRR, SR, and DD through the TLBO method varied from predicted values by 3.01%, 6.86%, and 6.32%, respectively.

The drilling of film cooling holes on gas turbine blades made out of ceramic-coated superalloy improves the efficiency of the gas turbine and prolongs the life of the turbine blade. The purpose of this study was to investigate the effects of different stand-off distances (SODs) on abrasive water jet (AWJ)-pierced holes, in which the machining time, entry and exit hole diameters, overcut, hole taper, and surface morphology were studied. In this study, the water jet pressure (WJP) of 275MPa, piercing angle of $90^{\circ }$, dwell time of 0.2s, and abrasive flow rates (AFRs) of 350g/min and 400g/min were considered for the AWJ piercing operations. The entry and exit hole diameters and overcut linearly increased with an increase of SOD with different abrasive flow rates. And hole taper was observed at the coating and substrate sections in which it decreased with an increase of SOD up to 2mm, and a further increase of SOD increased the hole taper. Besides, the drilled holes were found to have an absence of delamination, cracks, and thermal defects. It was also noted that there is a transformation from a brittle to a ductile mode of erosion that may occur in the high-erosion kinetic energy impact region in the YSZ material section. Based on the experimental results, it is confirmed that SOD of 2mm became an influencing factor in AWJ for piercing quality holes in the YSZ-coated superalloy.

New superalloys are potential materials in aircraft and power plant industries because of their properties like high-temperature strength, creep life and resistance to corrosion and oxidation at elevated temperatures. Because of the superior properties of superalloys, machining them using the conventional processes is a difficult task that is associated with high cost and poor accuracy. In this study, an attempt has been made to machine NIMONIC 75 superalloy by the electro discharge machining (EDM) process, using the Taguchi-based Gray Relational Analysis method for multi-objective optimization of material removal rate (MRR), tool electrode wear rate (TEWR) and surface finish (SF). The experiments conducted were based on $L_{18}$ (2${}^1\times 3^5$) orthogonal array. Six input parameters namely tool material, peak current, gap voltage, pulse on-time, pulse off-time and tool lift time were considered in this study. The validation results proved that the parametric setting of tool material as copper, peak current as 12A, gap voltage as 50V, pulse on-time as 200$\mu$s, pulse off-time as 15$\mu$s and tool lift time as 2s, yields optimized values of the performance characteristics. SEM images indicate the presence of numerous surface irregularities, whereas the XRD test shows the formation of various carbides on the EDMed surface.