Effect of C Content on the Microstructure and Stress Rupture Properties of Nickel-Based Superalloy K4750

The effect of C content on the microstructure and stress rupture properties of nickel-based superalloy K4750 was studied by SEM, EBSD, TEM, EPMA and stress rupture tests. With the increase in C content, the size and quantity of MC and M23C6 carbides in experimental alloys increased, and the morphology of MC carbides changed from small blocky or rod to large blocky or skeleton. The inhibition of MC carbides on grain boundary migration made the grain size of as-cast alloys inversely proportional to C content. As C content went from 0.07 to 0.10 wt.% and then to 0.12 wt.%, the stress rupture life of K4750 alloy at 750 °C/430 MPa increased from 160.7 to 222.0 h, and then decreased to 184.4 h, and the elongation increased from 5.1 to 11.5% and then decreased to 9.8%. The difference of stress rupture properties of K4750 alloys could be attributed to the influence of varied C content on carbides precipitation characteristics. The moderate increase in MC and M23C6 carbides could effectively pin grain boundaries and reduced the tendency of forming micro-voids and micro-cracks, which was beneficial to stress properties of K4750 alloy. On the contrary, when the size and quantity of carbides increased excessively, they could become potential crack sources in the process of stress rupture tests.