Effect of Carbon on the Microstructures and Stress Rupture Properties of a Polycrystalline Ni-Based Superalloy

The effect of carbon content on the microstructures and stress rupture properties of a newly developed polycrystalline Ni-based superalloy with high Cr content has been studied. It was observed that both grain size and the number of carbides increased with an increase in carbon content. After heat treatment, granular M23C6 carbides were dispersed around MC carbides along grain boundaries and inside grains. The quantity of granular M23C6 carbides increased while their sizes decreased. These findings can be verified with the results of thermodynamic calculation and differential scanning calorimetry analysis. The stress rupture times (975 degree celsius/225 MPa) increased from 13.3 to 25.5 h with the carbon content increased from 0.1 to 0.2 wt.%. The improvement can be attributed to two primary factors. Firstly, grain boundary is typically weak region during deformation process and the grain size increased as carbon content increased in the alloy. Secondly, carbides act as hindrances to impede dislocation movement, leading to dislocation entanglement. As carbon content rose, the quantity of carbides in interdendritic regions and grain boundaries increased, providing a certain degree of strengthening effect and resulting in a longer stress rupture time.