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

摘要

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 Cr_(23)C_6 carbides from semi-continuous rods to discrete granular particles, and leads to the decrease of the amount of the Cr_(23)C_6 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 γ′ phase, but slightly decreases the γ′ fraction. The average grain size continuously decreases with the increase of N content and the proportion of low I coincident site lattice (CSL) boundary significantly increases at the 0.0077% N content. The 750 °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 γ′ bypassing via Orowan looping regardless of the N content, while deformation microtwinning and stacking faults shearing the γ′ precipitates are confirmed to be the assistant deformation mechanisms at the 0.0077% N content.