Effect of duplex grain size distributions on long term stress-rupture life at 600 degrees C/450 MPa of Nimonic 80A

Long term stress-rupture life at 600 degrees C/450 MPa of Nimonic 80A with various duplex grain size distributions has been investigated. Stress-rupture life of duplex grain size samples significantly increases from about 4512 h in the bimodal condition to 6863 h in the banding condition and to 10,765 h in the wide-range condition. Plate-like Cr23C6 prefers to precipitate at transition region of small dynamic recrystallization (DRX) grains and large original grains in the bimodal condition, which can easily lead to grain boundary (GB) cracking. Cracking is also found at small equiaxed recrystallized GB where is enriched with strip-like Cr23C6 in sample with abnormal grains in the banding condition. Blocky Cr23C6 exhibits an orientation relationship with gamma at GB in sample in the wide-range condition. Blocky TiC with average size about 7.5 mu m precipitates at GBs firstly and then also in the grain interior, while the fine TiC with size about 300 nm is found in grain interior and the morphology changes from lageniform or oval shape to spherical one with prolonged stress-rupture life. The gamma' which maintains a coherent orientation relationship with gamma is very stable. The duplex grain size distribution in the wide-range condition significantly improves stress-rupture life. (C) 2014 Elsevier Ltd. All rights reserved.