Abnormal improvement of high-temperature stress rupture life in a new low cost Ni-based single crystal superalloy by adding minor nitrogen

Generally, elevated nitrogen (N) content promotes the precipitation of nitrides, which increase the size and quantity of porosity and thus degrade the mechanical performance of superalloys. However, in this study, N content was found to have a beneficial impact on the stress rupture property of single-crystal (SX) superalloys. The results indicated that as the N content increased from 5 ppm to 35 ppm, the porosity first decreased and then increased. The porosity of the sample with 14 ppm N was the lowest, 0.072% +/- 0.011%, which was mainly attributed to the TiN phases in interdendritic pools that compensated for volume shrinkage by lattice expansion. The addition of N reduced the volume fraction and antiphase boundary energy of the gamma ' phase because TiN nitride formation consumed Ti in large quantities. Additionally, the rupture life at 1100 degrees C/130 MPa was abnormally improved to 251.67 h +/- 19 h as the N content was increased to 14 ppm, while it was markedly decreased as the N content was continuously increased to 35 ppm. The improved stress fracture life of the alloy was primarily attributed to the formation of TiN with a small size, which retarded microporosity formation and effectively impeded dislocation movement in gamma channels. Moreover, during the stress rupture test, the size of the tertiary gamma ' phase first reduced to a minimum in the alloy containing 14 ppm N, which increased the resistance to dislocation movement. However, excessive addition of N drastically reduced the stress fracture life as the precipitation and solid solution strengthening in the gamma ' phase weakened. In addition, the agglomeration of nitride inclusions formed defects that aggravated crack nucleation and propagation. The overall N impact was studied, and the optimal N content was ascertained to provide further instructions for the commercial development of Re-free secondgeneration SX alloys.