Effect of small amounts of nitrogen and silicon on microstructure and properties of MAR-M002 nickel-base superalloy

Abstract

The microstructure and the tensile and stress-rupture properties of the nickel-base superalloy M A R-M002 have been studied using material of normal commercial composition and three melts with additions of 0·0024 and 0·0050%N and of 0·16%Si. Increasing the nitrogen content resulted in a change in carbide morphology, from ‘Chinese script’ to a blocky form, and to increased microporosity. Interference-film microscopy revealed characteristic centres in some of the blocky carbides that had high Ti contents consistent with carbide nucleation on Ti(C, N) particles formed in the melt. The increased nitrogen content was associated with a significant decrease in the rupture life at 760°C and 695 MN m⁻², and a change in the fracture morphology. In the Si-doped alloy, which contained little microporosity, there were increased amounts of a phase rich in Ni–Hf (probably Ni₅Hf) that also contained Si, but there was only a slight decrease in stress-rupture properties.