DESIGN AND PROCESSING OF NICO-BASED SUPERALLOYS FOR THE STUDY OF SOLUTE SEGREGATION AT PLANAR DEFECTS DURING HIGH TEMPERATURE DEFORMATION
Ni-based superalloys have been widely used for high temperature applications such as turbine blades for jet propulsion and power plants due to their excellent creep, fatigue, and corrosion resistance. But as the demand for higher temperature capability and strength increases, there remains a need to better understand high temperature deformation mechanisms and improve and strengthen superalloys at these elevated temperatures. Recently, a correlation has been observed between solute segregation at planar defects (stacking faults, antiphase boundaries, etc) and enhanced high temperature creep properties – known colloquially as phase transformation strengthening. Experimentally, regardless of alloy composition, strong Co segregation at planar defects along with Cr has been observed. In addition, it has been suggested by density functional theory work that Co would promote Cr concentration at stacking faults by forming strong Cr-Co bonds. Based on these findings, it was hypothesized the presence of Co provides a significant thermodynamic driving force for segregation to planar defects.
In order to further investigate the correlation between solute segregation and deformation mechanisms the fabrication of a planar front single crystal Ni-based superalloy and its microstructure, alloy composition, and microhardness properties of the as-zone melted and solution heat treated states were investigated and compared to the directionally-solidified state to study the effect of microsegregation on these alloy characteristics. Next, new Co-containing, Cr-free alloys are designed to optimize g-g’ volume fraction, size, and morphology to mimic microstructures observed in single crystal superalloys. The general alloy design strategy and approach are outlined, and the composition, microstructure, phase transformation temperatures, and mechanical properties of new Cr-free and Co-containing alloys are reported. A new set of Cr-free alloys have thus been designed, with modifications of Nb, Ta, and Ti additions ranging from 3 to 7 at.% to investigate the role of these elements on the phase transformation strengthening mechanism at elevated temperatures.