Abstract
In this work, influence of ultrasonic nanocrystal surface modification (UNSM) on the degree of sensitization (DOS) in Inconel 718 has been studied and correlated with the resulting microstructure. The UNSM processed samples decreased their grain size from 11.9 µm to 7.75 µm, increasing the surface of grain boundaries, and thus enhancing the area where δ phase and niobium carbides precipitate. The effect of the UNSM process on the DOS of Inconel 718 was studied by the double loop electrochemical potentiokinetic reactivation (DL−EPR) test. The DL−EPR showed that for UNSM processed samples with no thermal treatment, the DOS increased up to 59.6%, while for UNSM treated samples that were post-annealed at 1000 °C for 10 min and water quenched the DOS decreased down to 40.9%. The increase of grain boundaries surface area and triple junctions after the UNSM process enables the formation of twice the amount of δ phase compared to the as-received Inconel 718 bulk sample. The area fraction of the grain boundary covered by δ phase was of 9.87% in the UNSM region while in the bulk the area fraction was 4.09%. In summary, it was found that after UNSM process, the annealing at 1000 °C for 10 min and water quenching promoted the transformation of γ″ to form δ phase on the grain boundaries, which reduces the intergranular corrosion susceptibility.
Highlights
Inconel 718 is a Ni-Fe-Cr superalloy widely used in the aerospace and nuclear industries due to its enhanced corrosion properties within extreme environment conditions [1,2,3]
The ultrasonic nanocrystal surface modification (UNSM) process causes the grains on the surface and at the nearest surroundings layers to be crushed and the grain boundaries increase, enlarging the available surface coverage for the chromium and niobium carbides to precipitate which will raise the degree of sensitization (DOS) and the intergranular corrosion susceptibility
Four different thermo-mechanically treated samples were studied: Sample I1 was thermally treated in the furnace for 2 h at 675 ◦ C and water quenched; sample I2 was thermally treated in the furnace at 1000 ◦ C for 10 min and water quenched, this process was repeated three times; sample I3 was mechanically processed with the UNSM treatment three times; and sample I4 was mechanically processed with the UNSM, annealed at 1000 ◦ C for 10 min and water quenched
Summary
Inconel 718 is a Ni-Fe-Cr superalloy widely used in the aerospace and nuclear industries due to its enhanced corrosion properties within extreme environment conditions [1,2,3]. The UNSM process causes the grains on the surface and at the nearest surroundings layers to be crushed and the grain boundaries increase, enlarging the available surface coverage for the chromium and niobium carbides to precipitate which will raise the degree of sensitization (DOS) and the intergranular corrosion susceptibility. This corrosion issue has not been considered in previous studies regarding the UNSM surface processing of Inconel and only few works mention it [17,20]. In addition to the electrochemical tests, a microstructural characterization was performed by optical, scanning electron microscopy (SEM) and X-ray diffraction (XRD) to support the DOS results
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