Comparison of Creep Mechanisms Between Co-Al-W- and CoNi-Based Single Crystal Superalloys at Low Temperature and High Stresses

Abstract

TO further understand the creep performance of γ′-strengthened Co-based superalloys at low temperatures, the creep behaviors and mechanisms of a Co-Al-W-based and a CoNi-based single crystal superalloys (alloys 5Co and 7CoNi) were investigated at 1033 K/800 MPa and 1033 K/550 MPa based on their yield stresses, respectively. The creep behavior of alloy 5Co is characterized by forming a slow acceleration creep stage instead of a second deceleration creep stage in alloy 7CoNi after the rapid increase of creep rate in the acceleration creep stages of both alloys. The rapid shearing of multiple γ′ precipitates by the leading partial dislocations of individual stacking faults (SFs) in alloy 5Co are responsible for the rapid increase of creep rate, while the formations of anti-phase boundary (APB)-coupled dislocation pairs and SF-APB-SF structures in γ′ precipitates dominate the acceleration creep stage in alloy 7CoNi. The appearance of the slow acceleration creep stage in alloy 5Co can be ascribed to the generation of dislocations in the γ matrix and γ/γ′ interface. In contrast, the interactions of individual SFs, APBs and SF-APB-SF structures in alloy 7CoNi decrease the creep rate in the second deceleration creep stage. In both alloys, the W segregation at the SF improves the creep resistance by preventing the subsequent formation of APB in the same {111} plane, and contributes to the dissociation of the superdislocation(s) of APB into SF-APB-SF structures in the γ′ precipitates of alloy 7CoNi. This work will be beneficial for understanding the creep mechanism in different temperatures and further compositional optimization of γ′-strengthened Co-based superalloys.