In order to further improve transverse rupture strength of MA 6000 at intermediate temperature, Hf addition was attempted at INCO. Its nominal composition in weight percent was Ni-15Cr-2Mo-4W-4.5Al-2.5Ti-2Ta-0.5Hf-0.15Zr-0.05C- $1.1Y_2O_3$.
This study has shown that the 100hrs rupture strength(411MPa) of the alloy in the long-transverse directions at 760℃ is increased by a factor of 1.24 compared with MA 6000, but rupture elongation is little affected by the Hf addition. The improvement in strength may be related to solid solution strengthening and increased volume fraction of γ'-precipitates by the Hf addition.
The fracture surfaces in the long-transverse specimen at 760℃ were a mixture of intergranular and transgranular fracture modes. The microcracks were initiated at grain boundaries perpendicular to the applied stress by formation and coalescence of voids. The grain boundary carbides in MA 6000+Hf(W, Mo-rich carbides) have a little effect on microcrack initiation since they are randomly distributed as fine and discrete shapes.
In extruded conditions, elongated Cr-rich inclusions were formed in the extruded direction. After secondary recrystallization, W, Mo-rich carbides are appeared as the ring type distribution. Also, in the vicinity of these W, Mo-rich carbides, fine Ti-rich carbides were present. These inclusions are considered to have detrimental effects on the high temperature machanical properties.
