高温硫化腐食環境中のNi基耐熱合金のクリープ破断特性におよぼす粒界性状の影響

抄録

The creep rupture tests of a nickel-base superalloy Inconel 751 were carried out at 800°C in static air for the specimens which were variously heat-treated and coated with synthetic ash mixture composed of 90% Na₂SO₄ plus 10% NaCl and the effect of the nature of grain boundaries on the creep rupture properties was investigated. In directly aged (DA) specimens which had zigzag boundaries with coarse titanium-rich carbides, a drop in the rupture strength due to hot corrosion was less pronounced than that in simply aged (SA) and triple heat treated (THT) specimens which had straight boundaries with chromium-rich carbides. The rupture ductility, however, lowered remarkably as a result of brittle fracture mode due to hot corrosion regardless of the nature of grain boundaries.
From the metallographic examination, it was clarified that the rupture life of Inconel 751 subjected to hot corrosion was chiefly determined by the behavior of aggressive intergranular penetration of sulfides, which depended strongly upon the grain boundary configuration and the boundary carbide constituents. It was also suggested that the zigzag boundaries and the existence of coarse titanium-rich boundary carbides would control the intergranular penetrations to obey a given kinetics by virtue of their beneficial effect on preventing both stress activation and chromium-depletion at grain boundaries, and therefore a decrease in the rupture life of DA specimen should be restrained appreciably as compared with SA and THT specimens.