CoNi-base superalloys with improved high temperature properties and 3D printability
Ni-base superalloys have been the material of choice for structural components in the hot sections of turbine engines since the 1940s. Superalloy research and development has proceeded with specific turbine applications such as blading, vanes, and disks in mind. For the blade alloys that must withstand the most thermomechanically severe environment downstream from the combustor, an excellent balance of high temperature mechanical properties (strength, ductility, creep, and fatigue), oxidation resistance, processability, coating compatibility, and cost must be achieved. Many of these exceptional properties are conferred by their microstructure that consists of a high volume fraction of sub-micron scale γ′ precipitates (Ni₃Al, L1₂) that are coherent with a refractory-rich solid-solution strengthened γ matrix (Ni, A1). These alloys are typically cast into single crystals that exhibit further improved high temperature creep performance due to the absence of grain boundaries. While decades of research have sought to discover materials that can perform at temperatures beyond what is capable with the technologically mature Ni-base superalloys, suitable replacements have not yet been found.
The discovery of a γ′ phase in the Co-Al-W ternary system by C.S. Lee in 1971, which was rediscovered and described in more detail in 2006 by Sato et al., has enabled the development of novel classes of γ′ strengthened Co-base superalloys for high temperature applications. Since the solidus temperatures of these alloys can be over 100 °C greater than their Ni-base counterparts, these alloys may provide a path to higher temperature capability than is currently available. Co-base superalloys form a similar γ-γ′ microstructure where the γ′ phase is based on Co₃(Al,W). Creep studies have shown that single crystal Co- and CoNi-base have creep performance greater or equal to 1ˢᵗ-generation Ni-base single crystals. However, early model alloys have also been demonstrated to have low γ′ solvus temperatures, poor oxidation resistance, and high mass density. There is also a dearth of data on their response to cyclic loading and fatigue failure. To achieve the balance of properties necessary for high temperature applications, the compositions of Co- and CoNi-base alloys need to be refined through alloying and the resultant mechanical and environmental properties assessed.
In this work, low cycle fatigue tests conducted on single crystal Co-base superalloys demonstrate that these alloys have low strength and poor resistance to oxidation-assisted surface cracking which severely reduces the fatigue life compared to legacy Ni-base alloys. These property shortcomings motivate the development of novel Co-base alloy compositions with improved high temperature properties. An alloy design study using high-throughput alloy synthesis and rapid characterization methods has identified a promising region of CoNi-base composition space with alloys that exhibit improved properties including elevated γ′ solvus temperature, favorable oxidation kinetics due to formation of a protective alumina scale, and reduced mass density. Low cycle fatigue tests on a novel alloy from this composition space, SB-CoNi-10+, demonstrate improved fatigue resistance that is similar to 1ˢᵗ-generation Ni-base blade alloys.
Single crystal castings of SB-CoNi-10+ revealed that this alloy exhibits a favorable solidification behavior with mild amounts of as-cast microsegregation. This suggests that these alloys may be amenable to the extreme thermal conditions present in additive manufacturing techniques such as selective laser melting and electron beam melting. High performance Ni-base superalloys are difficult to process with fusion-based processes since large amounts of cracks often form in the as-printed alloy that severely degrade the mechanical properties. The SB-CoNi-10 alloy has been fabricated with multiple additive manufacturing techniques and has been demonstrated to be resistant to the formation of cracks or significant porosity. Studies on the room temperature and elevated temperature tensile properties are presented along with investigations on the effect of various heat treatment schedules on the alloy microstructure. CoNi-base superalloys are suitable candidates for further development for additive manufacturing since they can produce a high γ′ volume fraction microstructure while still being 3D printable with minimal amounts of defects. These alloys could be utilized by engineers to leverage the design flexibility of additive manufacturing for novel high temperature components with complex geometries, innovative cooling channels, and reduced material waste.