A new observation of strain-induced grain boundary serration and its underlying mechanism in a Ni–20Cr binary model alloy

Highlights: • Grain boundary serration was first found to occur in Ni–Cr binary alloy with no precipitate. • The mechanism of segregation-induced serration was proposed. • The serrated grain boundary was enriched with Cr atoms as high as 87 at.%. • The critical strain energy for the serration was determined to be lower than 0.97 kJ/mol. - Abstract: The formation of serrated grain boundaries and its mechanism correlated with straining exerted on grain boundaries were investigated in a Ni–20Cr binary model alloy. This model alloy is a full solid-solution which is deliberately free of carbon and aluminum to exclude precipitation of second phase. A special heat treatment involving slow cooling was used to promote grain boundary serration, which is known to enhance creep properties by decreasing interfacial free energy. No sign of serration could be observed for the Ni–20Cr model alloy, as expected. However, we found that a compressive 5% strain hold applied during slow cooling successfully induced grain boundary serration. The compressive 5% strain hold generated dislocations near the grain boundary, which promoted Cr enrichment near and at the grain boundary. To dismiss the effect of dislocations, straining was removed and the solutionizing temperature was increased in order to promote Cr enrichment near and at grain boundaries by means of vacancy-assisted diffusion of Cr atoms from the bulk to grain boundaries during slow cooling. Significant grain boundary serration was again observed. Atom probe tomography analyses showed that the serrated grain boundary with a wide width of ~ 10 nm was enriched with Cr atoms, as much as 87 at.%, while the γ matrix only contained 34 at.%. First-principles calculations showed that the strain energy necessary for the onset of grain boundary serration is lower than 0.97 kJ/mol.