EVOLUTIONS OF MICROSTRUCTURES AND MECHANICAL PROPERTIES OF TWO CAST Ni-BASED SUPERALLOYS DURING LONG-TERM THERMAL EXPOSURE

K452 and K446 alloys are two newly developed Ni-based cast superalloys, designed for microstructural component, applications of gas turbines in marine and industrial fields The two alloys perform well under laboratory conditions with good fatigue resistance, hot-corrosion resistance. and tensile- and stress-rupture properties, in addition to being completely oxidation resistant up to 900 degrees C However, due to the high contents of Cr, W and Mo, both K452 and K446 alloys tend to experience a severe microstructural degeneration when exposed at elevated temperatures In this paper, the microstructural stabilities and their influences oil the mechanical properties of the two alloys were comparatively examined during thermal exposure at 800-900 degrees C for 1 x 10(3)-1 x 10(4) h It is found that the gamma' phases in the two alloys keep coarsening and are both spherical during the whole exposure The coarsening rate of the gamma' phase in K446 is in general faster than that in K452, the probable reason of which is that elements diffuse faster in K446 than in K452 due to the different heat treatment regimes they suffered The thermal stabilities of primary MC carbides are closely related to the chemical compositions of both the carbides and the alloys The carbide in K452 is instable and tends to completely decompose via three different reactions, whereas the carbide in K446 is stable and degenerates only to a tiny degree mainly via one reaction Primary MC degeneration in K452 releases a significant amount of C into the supersaturated gamma matrix, facilitating the formation of M(23)C(6) throughout the alloy, whereas the, MC degeneration in K446 hardly releases C into the gamma matrix so that the mu phase precipitates everywhere due to the lack of C It seems that the kind of the phase precipitated from the supersaturated matrix during thermal exposure of service is dependent oil the stability of primary MC in K452 the blocky, closely spaced grain boundary M(23)C(6) particles engulfed in gamma' is the optimal structure of grain boundary, where a stress rupture life peak occurs, whereas in K446 the precipitation of a significant amount of mu phase degrades sharply the stress-rupture life.