Nicklas Volz

Understanding the influence of γ′ and secondary-phase fractions on the mechanical properties of superalloys is very important to optimize these high-strength materials. So far, this has not been systematically investigated for the novel class of Co-based superalloys. In this study, a Co–Al–W–Ta model alloy series was designed with compositions of γ/γ′ on the tie-line and an increasing γ′ volume fraction of up to 70% after heat treatment at 900 °C, while a few alloys are unexpectedly out of γ/γ′ two-phase region with an additional secondary phase fraction of up to 15%. The high-temperature strength and creep properties were evaluated by compression tests up to 1050 °C and compressive creep experiments at 950 °C, respectively. At temperatures of up to 1050 °C, an increasing γ′ volume fraction consistently increased the yield strength, which was not dramatically changed by the presence of secondary phases. Significant work hardening was found in alloys with γ′ volume fractions of 65–70% during compression testing, but not in alloys with either a lower γ′ volume fraction (<50%) or a high fraction of secondary phases (~15%). Similar to the yield strength, the creep strength also increased continuously with the γ′ volume fraction, but was greatly reduced with an increasing fraction of secondary phases. The best creep performance at 950 °C and 200 MPa was found in the alloy with the highest γ′ volume fraction and no secondary phases. At higher creep stresses, rafting contributed significantly to the hardening and, again, the alloy with a high γ′ volume fraction and a small amount of secondary phases exhibited the highest strength.

The defect evolution associated with an anomalous work hardening behavior of a single crystalline quaternary Co-Al-W-Ta superalloy at 950 °C was investigated by transmission electron microscopy. As plastic deformation is initially confined to the γ matrix channels, a plateau arises in the stress-strain curve after yielding. At about 1% plastic strain, extensive shearing of the γ′ precipitates under superlattice stacking fault formation occurs leading to extreme work hardening rates up to 12 GPa and a total increase in stress of about 200 MPa. Additional investigations on the temperature and strain-rate dependence of the anomalous work hardening behavior reveal the significance of diffusion and segregation processes on the stress-strain curve and the work hardening behavior.

Co-base superalloys are considered as promising high temperature materials besides the well-established Ni-base superalloys. However, Ni appears to be an indispensable alloying element also in Co-base superalloys. To address the influence of the base elements on the deformation behaviour, high-temperature compressive creep experiments were performed on a single crystal alloy series that was designed to exhibit a varying Co/Ni ratio and a constant Al, W and Cr content. Creep tests were performed at 900 °C and 250 MPa and the resulting microstructures and defect configurations were characterised via electron microscopy. The minimum creep rates differ by more than one order of magnitude with changing Co/Ni ratio. An intermediate CoNi-base alloy exhibits the overall highest creep strength. Several strengthening contributions like solid solution strengthening of the γ phase, effective diffusion coefficients or stacking fault energies were quantified. Precipitate shearing mechanisms differ significantly when the base element content is varied. While the Ni-rich superalloys exhibit SISF and SESF shearing, the Co-rich alloys develop extended APBs when the γ′ phase is cut. This is mainly attributed to a difference in planar fault energies, caused by a changing segregation behaviour. As a result, it is assumed that the shearing resistivity and the occurring deformation mechanisms in the γ′ phase are crucial for the creep properties of the investigated alloy series.

Tailoring the partitioning behavior of solid solution strengtheners is a crucial design strategy for advanced Ni-based superalloys. The goal of this study was to maximize the enrichment of Mo, Re, Ru and W in the γ-matrix phase. To determine the composition dependency of the partitioning behavior, a set of polycrystalline superalloys with varying contents of these solid solution-strengthening elements and a W-containing single-crystalline alloy series with varying concentrations of the γ′-forming elements Ta and Ti was produced. Assessed properties include phase compositions by electron-probe micro-analysis, phase transformation temperatures by differential scanning calorimetry and creep behavior. Re exhibits the most pronounced enrichment in the γ-matrix, followed by Mo, Ru and W. Due to the preference of the Al-sites in the γ′-phase in the order Ta > Ti > W > Mo > Re, the solid solution strengthening elements Mo, Re and W are displaced from the γ′-phase by increasing Ti and Ta contents. The investigated solutes do not directly influence the partitioning behavior of Ru as it prefers Ni-sites in the γ′-phase. Compressive creep experiments reveal a correlation between the content of solid solution strengtheners in the γ-phase and creep performance.

Theγ′volume fraction is a key parameter in precipitation-strengthened Co- and Ni-base superalloys and mainly determines the alloys’ properties. However, systematic studies with varyingγ′volume fractions are rare and the influence on thermal expansion has not been studied in detail. Therefore, a series of six Ta-containing Co-based alloys was designed with compositions on aγ–γ′tie-line, where theγ′volume fraction changes systematically. During solidification, Laves (C14-type) andµ(D8₅-type) phases formed in alloys with high levels of W and Ta. Single-phaseγor two-phaseγ/γ′microstructures were obtained in four experimental alloys after heat treatment as designed, whereas secondary precipitates, such asχ(D0₁₉-type), Laves, andμ, existed in alloys containing high levels ofγ′-forming elements. However, long-term heat treatments for 1000 hours revealed the formation of theχphase also in the formerχ-free alloys. The investigation of the thermal expansion behavior revealed a significant anomaly related to the dissolution ofγ′, which can be used to determine theγ′solvus temperature with high accuracy. Compared to thermodynamic calculations, differential scanning calorimetry (DSC) and thermal expansion analysis revealed a larger increase of theγ′solvus temperatures and a lesser decline of the solidus temperatures when the alloy composition approached the composition of the pureγ′phase.

The Ni content is a crucial factor for the development of γ′-strengthened Co-based superalloys and some studies have systematically addressed its influence on various properties in model superalloys. In this paper, we report for the first time the influence of the Co/Ni ratio in the more advanced nine-component superalloy ERBOCo-1: exchanging Co and Ni in this Co/Ni-based superalloy while keeping the other alloying elements constants has a big influence on a variety of material properties. The elemental segregation after casting is slightly more pronounced in the alloy with higher Ni-content. Microstructural characterization of this alloy termed ERBOCo-1X after heat-treatment reveals that the precipitates are cuboidal in the Co- and spherical in the Ni-rich alloy, indicating a decrease in the γ/γ′ lattice misfit. Analyzing the elemental partitioning behavior by atom probe tomography suggests that the partitioning behavior of W is responsible for that. Furthermore, it is found that even though Ni exhibits the highest overall concentration, the γ matrix phase is still Co-based, because Ni is strongly enriched in the γ′ precipitates. Creep tests at 900 °C reveal that even though the microstructure looks less favorable, the creep resistance of the Ni-rich alloy is slightly superior to the Co-rich variant.

Compositionally complex polycrystalline γ/γ′ CoNi-base superalloys, such as CoWAlloy2 (Co41-Ni32-Cr12-Al9-W5-Ti0.3-Ta0.2-Si0.4-Hf0.1-C-B-Zr) are interesting candidates for new high-temperature materials. To maximize their high-temperature strength, the γ/γ′ microstructure has to be optimized by adjusting the multi-step heat treatments. Various microstructures after different heat treatments were analyzed by scanning and transmission electron microscopy and especially in-situ small-angle neutron scattering during heat treatment experiments. The corresponding mechanical properties were determined by compression tests and hardness measurements. From this, an optimum γ′ precipitate size was determined that is adjusted mainly in the first precipitation heat treatment step. This is discussed on the basis of the theory of shearing of γ′ precipitates by weak and strong pair-couplings of dislocations. A second age hardening step leads to a further increase in the γ′ volume fraction above 70% and the formation of tertiary γ′ precipitates in the γ channels, resulting in an increased hardness and yield strength. A comparison between two different three-step heat treatments revealed an increase in strength of 75 MPa for the optimized heat treatment.

The creep strength of single-crystalline Co-based superalloys was found to be comparable to first-generation Ni-base superalloys. However, considerable shearing of the γ′ precipitates was observed in the early creep stages. To determine the strengthening contribution of the Ta-containing γ′-Co₃(Al,W) precipitates, the creep strength of several single-crystalline Co-Al-W-Ta superalloys was determined as a function of the γ′ volume fraction at 1223 K (950 °C) and stress levels between 25 and 600 MPa. Employing a Lagneborg–Bergman–Reppich (LBR) approach, it is found that the strengthening contribution of the γ′ precipitates increases significantly with increasing γ′ volume fraction. In a Co-base superalloy that exhibits a precipitate volume fraction of about 70 pct, the γ′-strengthening contribution calculated with the LBR approach ranges between the ones observed in first-generation Ni-base superalloy CMSX-6 and second-generation Ni-base superalloy CMSX-4.