Multi-stage dwell fatigue crack growth behaviors in a nickel-based superalloy at elevated temperature

A series of continuous and interrupted cyclic tests in Inconel 718 with the dwell times ranging from 0 s to 10,800 s at 650 degrees C were carried out in order to study the multi-stage dwell fatigue crack growth (DFCG) behavior. Replica technique was used to measure the intermittent DFCG rates from short to long periods and to retrospect specific damage mechanisms based on microstructural characterizations. From the macroscopic viewpoint, the DFCG curves were classified into short, moderate and long dwell conditions according to the tendencies in DFCG rates. Correspondingly, the dwell fatigue life distribution was divided into three periods, which exhibit different sensitivities in the length of dwell time on dwell fatigue life. From the microscopic viewpoint, the high-scattered points reflected in the DFCG curves were elaborately discussed with the combinations of EBSD mapping and high-resolution TEM analysis. sigma 3 twin boundaries (sigma 3 TBs) were regarded as the barriers to resist the DFCG due to their special performances. In addition, with the increase of dwell times, the main crack morphologies changed from transgranular to intergranular mode. Simultaneously, local crack growth modes at high-scattered points changed from across to along the sigma 3 TBs with extreme low DFCG rates. Also, high-angle crack deflection was another factor that was responsible for the decrease of the DFCG rates.

Automated summary

This study investigated the multi-stage dwell fatigue crack growth (DFCG) behavior of Inconel 718 at high temperatures through a series of continuous and interrupted cyclic tests. The DFCG rates were measured and specific damage mechanisms were retrospect based on microstructural characterizations. Different sensitivities in dwell fatigue life were observed with varying dwell times, and sigma 3 twin boundaries (σ3 TBs) were identified as barriers to resist DFCG. Additionally, the main crack morphologies and local crack growth modes changed with the increase of dwell times, affecting the DFCG rates.