Strain-induced microstructural evolution and its implication on high-temperature hot corrosion (HTHC) phenomena in Alloy 617

Abstract

The influence of various extents of strain on the microstructure and the high-temperature hot corrosion (HTHC) behavior in Alloy 617 is examined in this study. Electron backscatter diffraction characterization revealed an increase in the spread in the local misorientation zones from the vicinity of boundaries to the grain interiors with the extent of strain. Moreover, the increase in the degree of deformation also increased the connectivity of the random high angle grain boundaries (HAGBs), as corroborated by the fractal analysis, since most of the twin boundaries changed their character from ‘special’ to ‘random’ on the application of strain. The HTHC behavior of the as-received and the strained specimens is evaluated by exposing them to a salt mixture (Na 2 SO 4 + NaCl+V 2 O 5 ) at 1273 K for 24 h. The hot corrosion damage is assessed based on oxide scale characteristics (morphology, thickness, and composition), internal percolation depth, Micro-CT (porosity), and Mott-Schottky (passive film defects) analyses. In-depth characterization of the corroded specimens unveils an improved resistance to HTHC in the highly strained specimens due to the enhanced Cr diffusion through the defects and connected random HAGBs which facilitated the formation of a continuous, and compact Cr-rich oxide film. Importantly, the occurrence of recrystallization and the evolution of the Cr-rich carbides in the strained specimens in the course of the HTHC test increased its likelihood of resisting the aggressive environment for a prolonged duration. • Local misorientations, TTBs, and random HAGBs connectivity increased on straining. • Oxide layer formed on strained specimens is continuous, dense and protective. • Higher HTHC resistance in strained specimens is attributed to greater Cr diffusion. • The enhanced Cr diffusion is due to increased defects and random HAGBs connectivity. • Straining even led to superior resistance against damage during prolonged HTHC test.