Effect of Hot Corrosion on the Creep Rupture Properties of a Nickel-Base Superalloy

Abstract

The creep rupture tests of a nickel-base superalloy Inconel 751 were carried out at 800°C in static air for the specimens with or without coating of synthetic ash mixture composed of 90% Na₂SO₄ plus 10% NaCl and the effect of hot corrosion on the creep rupture properties was investigated.
In ash-coated specimens the rupture strength was extremely low and showed large difference from each other. The rupture ductility also lowered remarkably as a result of brittle fracture mode due to hot corrosion.
The creep fracture of specimens without coating of ash mixture occurred as a result of the growth and the coalescence of a large number of grain boundary cracks, as usually observed. In ash-coated specimens, on the other hand, only a few aggressive intergranular penetration of sulfides, which had already initiated during apparent a steady-state creep stage, opened in an accelerated creep stage, propagated rapidly as a main crack and consequently a premature fracture resulted in. These results indicate that hot corrosion could essentially alter the creep fracture mechanism.
From the observation of the density and the length of aggressive intergranular penetrations existing in ruptured or unruptured specimens, it was also suggested that in hot corrosive environment the rupture life of Inconel 751 was mostly determined by the initiating process of only a few aggressive intergranular penetration.