The impact of carbon on single crystal nickel-base superalloys: Carbide behavior and alloy performance

Advanced single crystal nickel-base superalloys are prone to the formation of casting grain defects, which hinders their practical implementation in large gas turbine components. Additions of carbon (C) have recently been identified as a means of reducing grain defects, but the full impact of C on single crystal superalloy behavior is not entirely understood. A study was conducted to determine the effects of C and other minor elemental additions on the behavior of CMSX-4, a commercially relevant 2nd generation single crystal superalloy. Baseline CMSX-4 and three alloy modifications (CMSX-4 + 0.05 wt. % C, CMSX-4 + 0.05 wt. % C and 68 ppm boron (B), and CMSX-4 + 0.05 wt. % C and 23 ppm nitrogen (N)) were heat treated before being tested in high temperature creep and high cycle fatigue (HCF). Select samples were subjected to long term thermal exposure (1000 °C/1000 hrs) to assess microstructural stability. The C modifications resulted in significant differences in microstructure and alloy performance as compared to the baseline. These variations were generally attributed to the behavior of carbide phases in the alloy modifications.;The C modification and the C+B modification, which both exhibited script carbide networks, were 25% more effective than the C+N modification (small blocky carbides) and 10% more effective than the baseline at preventing grain defects in cast bars. All C-modified alloys exhibited reduced as-cast gamma/gamma' eutectic and increased casting porosity as compared to baseline CMSX-4. The higher levels of porosity (volume fractions 0.002 - 0.005 greater than the baseline) were attributed to carbides blocking molten fluid flow during the final stages of solidification. Although the minor additions resulted in reduced solidus temperature by up to 16 °C, all alloys were successfully heat treated without incipient melting by modifying commercial heat treatment schedules. In the B-containing alloy, heat treatment resulted in the transformation of script MC (M -- metal, C -- carbon) carbide networks into clusters of small, spherical MC carbides without a significant change in composition. Formation of topologically close packed phases during thermal exposure was suppressed in the B-containing alloy due the decomposition of primary MC carbides and the preferential formation of secondary M23C6 carbides.;All of the modified alloys exhibited shorter creep rupture lifetimes than the baseline at all creep conditions (950 °C/300 MPa, 850 °C/550 MPa, 750 °C/800 MPa). The most significant decrease in lifetime occurred at the 750 °C condition due to large primary creep strains of up to nearly 10% in the C-containing alloys. In HCF testing at 850 °C, the presence of carbides and increased porosity led to reduced lifetimes in the modified alloys. HIP (hot isostatic pressing) processing significantly improved fatigue performance, accounting for average lifetime increases ranging from 77% to 4490% over Un-HIPed material. HIP also isolated the effect of carbide morphology on fatigue behavior by changing active crack initiation sites from pores to carbides.