Abstract
In this work, the influence of thermal (TT), mechanical, and thermomechanical (TMT) treatments using the ultrasonic nanocrystal surface modification (UNSM) on the corrosion protection properties of Inconel 718 was studied, correlating the changes in the electrochemical properties with the promoted microstructure. The UNSM treatment had a grain refinement effect on the top surface, reducing the grain size from 11.5 to 7.4 µm for the first 10 µm in depth. The high grain boundary density, due to the grain refinement, enabled a faster growth of the passive film. The impedance showed a decrease in the charge transfer resistance by three orders of magnitude, from 106 to 103 Ω cm2 for as-received to 1000 °C, as the TT temperature crossed the solvus of the γ′/γ″ and approached the solvus of the δ-phase. The UNSM treatment lowered the pitting corrosion susceptibility, increasing the charge transfer resistance and decreasing the effective capacitance of the double layer, leading to the thickest passive film with 6.8 nm.
Highlights
Inconel 718 is a Ni-based superalloy widely used for aerospace and nuclear applications due to its outstanding strength, creep, fatigue, and wear resistance at elevated temperatures, close to 700 ◦ C [1,2,3,4]
The γ00 -phase is an unstable phase that grants most of the hardness to Inconel 718; given a thermal stimulus it transforms into the δ-phase, a more stable form of the Ni3 Nb
The microstructure seen in both images resembles those found in the literature, mainly γ-phase matrix with some deformation twins scattered found in the literature, mainly γ-phase matrix with some deformation twins scattered around the microstructure [20,25]
Summary
Inconel 718, besides keeping its mechanical and corrosive properties under highly aggressive environments, possesses good weldability [5,6,7]. The strength and durability of its properties come from the single-phase austenite (γ) matrix microstructure, thanks to the high Ni equivalent content promoting a stable γ-phase, based on the Schaeffler diagram [8]. Inconel 718 presents other secondary phases and precipitates, which influence the mechanical and electrochemical properties, among the most common of which are: gamma prime (γ0 ), a Ni3 (Al,Ti) phase with face centered cubic (fcc) crystal structure; gamma double-prime (γ00 ), a Nb3 (Al,Ti) with body centered tetragonal (bct) unit cell; and delta (δ), a Ni3 Nb phase with an orthorhombic crystal structure [9,10,11]. The γ00 -phase is an unstable phase that grants most of the hardness to Inconel 718; given a thermal stimulus it transforms into the δ-phase, a more stable form of the Ni3 Nb
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