Ru-containing Single Crystal Superalloys Research Articles

Co content dependence on the microstructure characteristic and creep performance at elevated temperature in Ru-containing single crystal superalloys

The composition design and optimization of Ru-containing fourth generation single crystal superalloys, which are currently considered as the most advanced and practical materials for manufacturing high-pressure turbine blades of aero-engines, have been in increasing and urgent demands. In this work, significant adjustments to the pivotal alloying element Co were made and the synthetic effect of Co on the microstructure as well as high-temperature creep property in Ru-containing alloys were systematically evaluated. The results illustrated that the size of γ′ phase was inversely proportional to the Co content but the volume fraction of γ′ precipitates stayed the same in different alloys. The TCP phases were more likely to precipitate in alloys with lower or extremely high Co addition during long-term aging, while the morphology of these particles transformed from long needle-like to short rod-like or blocky as Co content increased. It was noted that the element diffusion as well as the resultant coarsening and rafting of γ/γ′ structures were favorably facilitated by the increasing Co addition. The alloy with 12 wt% Co exhibited the longest stress rupture life at 1140 °C/137 MPa, regardless of the moderate precipitation of blocky TCP particles during creep. Moreover, the TCP phases in different alloys were analyzed to be typical μ phase but the specific element distributions varied with Co addition. Moreover, plenty of 2nd γ′ precipitates were observed after creep rupture, while their nucleation and precipitation could be affected by the Co addition in alloys. Ultimately, the comprehensive Co-effect was discussed and the optimum Co content was determined, in order to provide further guidance in the design and development of Ru-containing single crystal superalloys.

Improvement of Creep Resistance at 950 °C and 400 MPa in Ru-Containing Single-Crystal Superalloys with a High Level of Co Addition

Microstructural features, including γ′ volume fraction and size, γ-γ′ lattice misfit, γ channel width, and dislocation substructure, are known to significantly influence the creep performance in Ni-base single-crystal superalloys. In this study, the microstructural characteristics of Ru-containing single-crystal superalloys with different levels of Co, Mo, and Ru additions were quantitatively investigated after ruptured and interrupted creep tests conducted at 1223 K (950 °C) and 400 MPa. The creep lifetime was slightly increased with the high level of Co addition and significantly increased with the coadditions of Mo and Ru. A minor effect of Co content on the γ channel width and γ′ volume fraction was found in experimental alloys. The alloy with high levels of Mo and Ru additions was determined to possess a more negative γ-γ′ lattice misfit, and a high density of stacking faults (SFs) was formed in the γ channels during creep. The combined effects of the SFs in the γ matrix serving as the barriers to dislocation movement, as well as the dense interfacial dislocation networks preventing dislocation to shear the γ′ phase, were considered as the main mechanism responsible for the improvement of creep resistance. Results from this study are helpful to understand the effect of microstructural features on creep performance and contribute to the knowledge of physical metallurgy in Ru-containing single-crystal superalloys.

Microstructural nature and stability of Co-rich TCP phases in Ru-containing single crystal superalloys

Destruction of microstructural stability caused by topologically close-packed (TCP) phase precipitation restricted the industrial application of advanced single crystal superalloys. However, systematic investigations on microstructural nature and evolution of Co-rich TCP phases in Ru-containing single crystal superalloys with high level of Co addition have been rarely reported. In this study, experimental results indicated that the main TCP phase was μ phase after thermal exposure at 950 °C for 1000 h, while R phase was the main phase after thermal exposure at 1100 °C for 1000 h in the alloys containing high level of Co and different levels of Cr and Mo. The experimental and thermodynamic calculation results showed that R phase was more stable at 1100 °C than that of μ phase in the experimental alloys. Meanwhile, μ phase started to transform to R phase at 1100 °C for 50 h, and the stress accelerated the transformation of μ phase and R phase during creep test at 1100 °C/140 MPa in an alloy containing high levels of Co, Cr and Mo additions. Both R phase and μ phase were enriched in Re, W, Mo, Cr and Co, while R phase contained more Cr and μ phase was more enriched in Mo. This study contributes to better understand the microstructural evolution of TCP phases at different temperatures and to optimize the alloy design and thermodynamic database of Ru-containing single crystal superalloys with high level of Co addition.

Microstructure stability of a ru-containing nickel based single crystal superalloy

The microstructure evolution and stress rupture properties at 1100°C/140MPa of a Ru-containing single crystal superalloy after long term aging at 1100°C for 200h-1000h were investigated. The results showed that the γ′ phase rafting and the γ matrix channel broadening appeared after long term aging. The rafting degree enhanced with increasing of aging time. A small amount of TCP phase began to appear after aging for 200h. The volume fraction of TCP phase after aging for 1000h was only 0.1%, which indicated the alloy had excellent microstructure stability. The stress rupture life of the alloy at 1100°C/140MPa represented the decrease tendency with increasing of aging time. The γ′ rafting and TCP phase precipitation were the main factor for this degradation.

Effect of multiple alloying additions on microstructural features and creep performance at 950 °C and 400 MPa in Ru-containing single crystal superalloys

Microstructural features, including γ channel width, γ' size, γ' volume fraction, γ-γ' lattice misfit, TCP phase as well as dislocation substructures had influence on the creep performance in Ni-base single crystal superalloys. However, relatively limited work has been conducted to investigate the effect of microstructural features based on various alloying additions on creep properties in Ru-containing single crystal superalloys. In this study, the creep tests were conducted at 950°C and 400MPa in experimental alloys with different levels Co (7.0wt% and 15.0wt%), Cr (3.5wt% and 6.0wt%), Mo (1.0wt% and 2.5wt%) and Ru (2.5wt% and 4.0wt%) additions, and the microstructures during creep were characterized in detail. Co and Ru were found to decrease the γ channel width, respectively. The high level of Cr addition decreased the γ' volume fraction and promote the TCP phase formation during creep. The addition of Mo decreased the γ channel width and also acted as a TCP former. The γ-γ' lattice misfit was increased to more negative by the individual additions of Co, Ru, Cr, and Mo, respectively. The joint addition of microstructural stabilizers Co and Ru inhibited the TCP phase formation during creep. The precipitation of TCP phases served as one of the main factor to decrease the creep property in alloys with high level of Cr and Mo additions, respectively. The synergistic effect of Mo and Ru additions in the alloy with high level of Co content was found to increase the γ-γ' lattice misfit and the amount of stacking faults in γ matrix during creep process, which improved the creep resistance at 950°C and 400MPa. This study is helpful to understand the effect of alloying elements additions and microstructures on creep performance and to get better improvement of physical metallurgy knowledge and alloy design in Ru-containing single crystal superalloys.

Microstructural characteristics of σ phase and P phase in Ru-containing single crystal superalloys

Microstructural instability caused by topologically close-packed (TCP) phase precipitation restricts the useful compositional range of advanced Ni-base single crystal superalloys in industrial applications. Limited systematic investigations of TCP formers (Cr and Mo) additions on microstructural evolution of both the σ phase and the P phase in Ru-containing single crystal superalloys have been reported. In this study, the microstructural characteristics of σ phase and P phase were investigated in three Ru-containing superalloys with different levels of Cr and Mo additions at 950°C and 1100°C by using phase extraction, X-ray diffraction, scanning electron microscope and high resolution transmission electron microscopy. The experimental results indicated that the high level additions of Cr and Mo promoted the formation of σ phase and P phase, respectively. The amount of σ phase was much higher than that of P phase after long term exposure at 950°C and 1100°C. The sheet-like σ phase existed in the alloy with higher Cr addition after thermal exposure at 950°C and 1100°C for 1000h, while the needle-like P phase precipitated in high Mo content alloy after thermal exposure at 1100°C for 1000h and the intergrowth of σ phase and P phase was observed after thermal exposure at 950°C for 500h. Both the σ phase and P phase were enriched in Re, W, Cr and Mo, but the σ phase contained more Re and Cr while the P phase contained more Mo and Ni, and Ru was found in both phases. The nucleation of σ phase was much easier than P phase due to the more ledge steps in the interfacial structure between σ phase and matrix, as well as the higher partitioning ratios of Re, Cr and Mo. This study is helpful to understand the microstructural evolution of σ phase and P phase, and to optimize the alloy design in Ru-containing superalloys.

Dislocation motion during high-temperature low-stress creep in Ru-free and Ru-containing single-crystal superalloys

Creep deformation of the two experimental single crystal superalloys at high-temperature low-stress (1140°C/137MPa) has been analyzed through transmission electron microscopy. Emphasis is placed on elucidating the dependence of dislocation motion on microstructural evolution. The detailed analysis demonstrated that the stacking fault energy of the γ matrix significantly decreased with the addition of Ruthenium (Ru). The stacking faults presenting in the γ matrix after heat treatment has been rarely reported previously. During the primary creep stage, the dislocations can easily cross-slip on the different {111} planes in the horizontal matrix and leave 60° dislocation loops on the (001) γ/γ′ interfacial plane. Furthermore, calculations demonstrated that it is difficult for the slipping dislocations to bow into the vertical γ matrix channel. In the early stages of steady state creep, the interfacial dislocations reoriented slowly from the 〈110〉 slipping direction to the 〈100〉 well misfit stress relief direction. On the other hand, few dislocations shearing into the rafted γ′ phase have been observed. In fact, during the middle stages of the steady state creep, although perfect dislocation networks have formed, some dislocations shearing into the γ′ phase have also been observed. In addition, the a 〈010〉 type superdislocations (some with non-compact core) have also been observed in the two experimental alloys. At last, the Ru-containing alloy possesses more negative lattice misfit, denser γ/γ′ interfacial dislocation networks and higher microstructural stability, thus can maintain a minimum creep rate in the steady state stage and have a longer creep life.

Co Effect on As-cast and Heat-Treated Microstructures in Ru-Containing Single-Crystal Superalloys

The effect of Co on the as-cast and heat-treated microstructures was investigated in two experimental Ni-based single-crystal superalloys containing low levels of Re and Ru. The experimental results indicated that increasing the Co content from 7.9 to 15.8 wt pct decreased the volume fraction of (γ + γ′) eutectic and the solidification segregation ratio of W. High levels of Co additions were also found to decrease the solvus temperatures of the γ′ phase and (γ + γ′) eutectic as well as the solidus temperature. During the long-term thermal exposure at 1373 K (1100 °C), no TCP phases precipitated in either alloy. However, the coarsening and coalescence of γ′ precipitates in the alloy containing 15.8 wt pct Co was slower than that in the other alloy with 7.9 wt pct Co. In the current study, high levels of Co additions decreased the equilibrium volume fraction of γ′ phase, leading to a change in the partitioning ratios of TCP-forming elements Cr, Mo, Re, and W between the γ and γ′ phases. This change resulted in a lower degree of elemental supersaturation in the γ matrix and improved the phase stability of the γ/γ′ microstructure. These experimental results were then compared with those obtained from multi-component thermodynamic calculations, and good agreement was observed.

Effects of Ru on Solidification Characteristic and Microstructures of Ni-Based Single Crystal Superalloy

The Ru-free and Ru-containing single crystal superalloys were cast in the directionally solidified furnace, while other alloying element contents were basically kept unchanged. The effects of Ru on the solidification characteristic and microstructures of single crystal superalloy were investigated with differential scanning calorimetry, electron probe micro analyzer, energy-dispersive X-ray spectroscope, scanning electron microscope and transmission electron microscope. The results show that the liquidus temperature of the single crystal superalloy decreases with Ru addition. The primary dendrite arm spacing and volume fraction of γ/γ′ eutectic both decrease with Ru addition. The sizes of γ′ phase of dendritic core and interdendritic region have no obvious change with Ru addition. Ru tends to segregate slightly in the dendritic core. The extent of elements segregation decreases with the Ru addition. Ru tends to partition preferentially into the γ matrix. The addition of Ru decreases the partition ratio (the ratio of the γ phases composition over the γ′ phases composition) of elements Re, W and Mo.

Topologically close-packed phase promotion in a Ru-containing single crystal superalloy

It is widely accepted that Ru suppresses the formation of topologically close-packed (TCP) phases in advanced Ni-based single crystal superalloys. However, in the current study, the addition of a large amount of Ru promoted the precipitation of TCP phases in a Ni-based single crystal superalloy containing a high level of Cr during thermal exposure at both 1000 and 1100 °C. It is suggested that the addition of Ru increased the degree of Re and Cr supersaturation in the γ phase and resulted in the TCP phase formation.

Development of γ phase stacking faults during high temperature creep of Ru-containing single crystal superalloys

An unusual deformation mode involving the formation of intrinsic stacking faults in the γ matrix of experimental Ru-containing γ– γ′ superalloys with high Co and Re contents during high temperature creep at 950 °C/290 MPa has been observed. The morphology, distribution and dependence of these stacking faults on alloy chemistry has been investigated along with their formation mechanism. Additions of Re and Co substantially decrease the stacking fault energy of the γ matrix. The observed stacking faults in the γ matrix form by the dissociation of a/2〈1 1 0〉 matrix dislocations with Burgers vectors perpendicular to the loading direction in the early stages of creep. The dependence of creep properties on elemental additions that influence stacking fault energy is discussed.

Cyclic oxidation of Ru-containing single crystal superalloys at 1100 °C

The cyclic oxidation behavior of Ru-containing single crystal superalloys at 1100 °C has been investigated in alloys with Ru contents ranging from 3.5 to 9 at.% (5.7–14.1 wt.%) and Cr additions ranging from 0 to 8 at.% (0–6.7 wt.%). High levels of Cr additions (8 at.%) significantly improved oxidation resistance, while increased levels of Ru adversely affected the oxidation performance. A multi-layered scale formed on the alloy surface, and it generally consisted of an external layer of NiO, an intermediate layer of spinel and an Al 2O 3 inner layer. The presence of a continuous α-Al 2O 3 inner layer clearly improved oxidation performance. Ru-rich precipitates were observed in the spinel layer of alloys which displayed poor oxidation resistance. High levels of Cr additions not only reduced the propensity for formation of Ru-rich phases in the oxide, but also promoted the formation of a continuous alumina layer adjacent to the superalloy, without stabilization of any detrimental phases in the superalloy.