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

Abstract

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℃.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 on the mechanical properties of the two alloys were comparatively examined during thermal exposure at 800-900℃for 1×10~3-1×10~4 h.It is found that theγ′phases in the two alloys keep coarsening and are both spherical during the whole exposure.The coarsening rate of theγ′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 supersaturatedγmatrix,facilitating the formation of M_(23)C_6 throughout the alloy,whereas the MC degeneration in K446 hardly releases C into theγmatrix so that theμphase precipitates everywhere due to the lack of C.It seems that the kind of the phase precipitated from the supersaturated matrix during thermal exposure or service is dependent on the stability of primary MC.In K452 the blocky,closely spaced grain boundary M_(23)C_6 particles engulfed inγ′is the optimal structure of grain boundary,where a stress-rupture life peak occurs,whereas in K446 the precipitation of a significant amount ofμphase degrades sharply the stress-rupture life.