Effects of nitrogen content on microstructures and tensile properties of a new Ni-Fe based wrought superalloy

The influence of the nitrogen (N) content (0.0013-0.034 wt%) on the microstructures and 750 ?C tensile properties of a newly developed Ni-Fe based wrought superalloy (named as HT700T alloy) for ultra-supercritical power plant applications beyond 700 ?C was investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The results show that the increase in the N content promotes the morphological transformation of the intergranular Cr23C6 carbides from semi-continuous rods to discrete granular particles, and leads to the decrease of the amount of the Cr23C6 carbides. Meanwhile, the increase in the N content promotes the formation of the MX phase and the MX phase more preferentially forms at the grain boundary. The N content has no obvious effects on the morphology of the gamma ' phase, but slightly decreases the gamma ' fraction. The average grain size continuously decreases with the increase of N content and the proportion of low Sigma coincident site lattice (CSL) boundary significantly increases at the 0.0077% N content. The 750 degrees C tensile properties show that the increase of N content has little effect on the yield and tensile strength of the alloy, while the ductility increases remarkably. A mixture fracture mode of intergranular and transgranular occurs in all samples regardless of the N content. The dominant deformation mechanism is gamma ' bypassing via Orowan looping regardless of the N content, while deformation microtwinning and stacking faults shearing the gamma ' precipitates are confirmed to be the assistant deformation mechanisms at the 0.0077% N content.

Automated summary

The study investigated the influence of nitrogen content on a newly developed Ni-Fe based superalloy for ultra-supercritical power plant applications beyond 700℃. The results showed that an increase in nitrogen content promoted changes in carbide morphology and grain boundary phase formation, with little effect on yield and tensile strength but significantly improving ductility. The alloy exhibited a mixture fracture mode and a dominant deformation mechanism of gamma ' bypassing via Orowan looping.