I am a PhD Researcher that focuses on characterising the microstructural behaviour of superalloys.
Don't creep me out: why is creep resistance essential for turbine engines?
In 10 Seconds? In the aerospace industry, safety is paramount due to the extreme environments turbine engines operate in. When it comes to designing turbine engines, high temperature alloys must have excellent resistance to creep. Creep is a process that involves the gradual plastic deformation of a material overtime – it is slow, temperature aided and time dependent.
Why is it so critical? Due to the exceedingly high temperatures found within turbine engines and being used for prolonged periods of time, the shape of critical components (such as blades, fan discs and compressor parts) can potentially change – and this, in turn can cause the engine to seize, which we definitely don’t want!
How is creep resistance being improved? Over the past couple of decades, the aerospace industry has been using more single crystal superalloys for turbine components due to their lack of grain boundaries. Grain boundaries are the interface between two grains within a poly-crystal material – if the grains are small, grain boundaries will consequently increase the mechanical strength of the material.
However, much research is currently being carried out to investigate the improvements that can be made by altering the chemical composition of single crystal superalloys – observe the pinned article “Effect of multiple alloying additions on microstructural features and creep performance at 950 °C and 400 MPa in Ru-containing single crystal superalloys”.
The science behind Single Crystal Superalloys
Single crystal superalloys enhance the resistance to creep because they are manufactured by one crystal/grain – unlike conventional superalloys, which are poly-crystals. Therefore, poly-crystal materials are congested with grain boundaries, which is one of the leading factors in reducing creep resistance, because this can lead to the occurrence of grain boundary sliding.
In conclusion, single crystal superalloys are finding more and more applications within the aerospace industry. However, currently their two main applications are stator and rotor blades within turbine engines - due to their excellent creep resistance and mechanical strength.
Abstract: Solid solution strengthening of the γ matrix is one key factor for improving the creep strength of single crystal nickel-base superalloys at high temperatures. Therefore a strong partitioning of solid solution hardening elements to the matrix is beneficial for high temperature creep strength. Different Rhenium-free alloys which are derived from CMSX-4 are investigated. The alloys have been characterized regarding microstructure, phase compositions as well as creep strength. It is found that increasing the Titanium (Ti) as well as the Tungsten (W) content causes a stronger partitioning of the solid solution strengtheners, in particular W, to the γ phase. As a result the creep resistance is significantly improved. Based on these ideas, a Rhenium-free alloy with an optimized chemistry regarding the partitioning behavior of W is developed and validated in the present study. It shows comparable creep strength to the Rhenium containing second generation alloy CMSX-4 in the high temperature / low stress creep regime and is less prone to the formation of deleterious topologically close packed (TCP) phases. This more effective usage of solid solution strengtheners can enhance the creep properties of nickel-base superalloys while reducing the content of strategic elements like Rhenium.
Pub.: 01 Sep '16, Pinned: 08 Jun '17
Abstract: The as-cast and heat-treated microstructures and high temperature creep properties have been investigated in four experimental Ni-based single crystal superalloys containing various levels of Hf addition (0~0.4 wt.%) and B addition (0~0.02 wt.%). The experimental results indicated that the creep rupture life showed an improvement with individual addition of Hf, but it was decreased with individual addition of B. The elemental partitioning ratio and interfacial dislocation spacing of γ/γ′ were obviously changed with individual Hf or B additions. Meanwhile, the formation of secondary phases, such as the blocky MC carbide, script-like shape M3B2 phases, was observed in the creep samples, which was also closely related to the high temperature creep behaviors. The high volume fraction of residual (γ+γ′) eutectics was mainly attributed to the significant decrease of creep rupture life for the present experimental alloy containing both Hf and B additions. This study is helpful to better understand Hf and B's role of strengthening mechanism and to optimize Hf and B additions in single crystal superalloys.
Pub.: 01 Jul '16, Pinned: 08 Jun '17
Abstract: The microstructure of a rare earth containing magnesium alloy, Mg-4.35Y-3RE-0.36Zr wt%, was engineered through applying accumulative back extrusion (ABE) process. Toward this end, the predetermined ABE cycles were applied at 400 °C up to five passes under a punch speed of 5 mm/min to study the ultrafine grained microstructure formation and its corresponding texture modification in the experimental material. A variety of bimodal grain size distributions were developed at all deformation conditions. In addition, the dissolution of eutectic phase stimulated the probability of dynamic precipitation of β phase during deformation. The latter caused a pinning effect on the grain boundary and gave rise to an inhomogeneous grain growth thereby intensified a bimodal grain size distribution (bimodality). In addition, the capability of experimental material to shear band formation during straining, even after one ABE pass, induced the level of bimodality. A remarkable grain refinement was achieved inside the shear bands due to the higher magnitude of shearing strain. Furthermore, the shear bands intersections provided suitable conditions for well defined ultrafine grain formation in between primary bands. The formation of noticeable number of these ultrafine grains within the shear bands could decrease the basal intensity thereby inducing a significant texture weakening effect. The obtained results indicated a significant improvement in both the strength (yield and ultimate) and elongation to fracture of the processed material. This was justified considering the effects of grain size, the level of bimodality and the texture weakening.
Pub.: 27 Apr '17, Pinned: 05 Jul '17
Abstract: Effects of Re on the formation of surface eutectics have been investigated by using Ni-base single crystal superalloys with different Re additions. It was found that Re promotes the segregation of Al and Ta to the eutectic melt, leading to an increase of the surface and internal eutectics. In addition, the addition of Re also increased the freezing range, the local solidification time, and the permeability of the dendritic network within the mushy zone. These factors ultimately promoted the outflow of the interdendritic residual liquid with the action of solidification shrinkage, and led to the formation of more surface eutectics. In contrast, the addition of Re had no obvious influence on the surface eutectic microstructures.
Pub.: 06 Apr '17, Pinned: 08 Jun '17
Abstract: Authors: B. Wang ; J. Zhang ; T. W. Huang ; H. J. Su ; Z. R. Li ; W. C. Yang ; L. Liu ; H. Z. Fu Article URL: http://www.tandfonline.com/doi/full/10.1080/02670836.2016.1216029?ai=1xji1&mi=kpptjb&af=R Citation: Materials Science and Technology Publication Date: 2016-08-05T10:20:11Z Journal: Materials Science and Technology
Pub.: 05 Aug '16, Pinned: 08 Jun '17
Abstract: 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 400 MPa in experimental alloys with different levels Co (7.0 wt% and 15.0 wt%), Cr (3.5 wt% and 6.0 wt%), Mo (1.0 wt% and 2.5 wt%) and Ru (2.5 wt% and 4.0 wt%) 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 400 MPa. 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.
Pub.: 23 Mar '17, Pinned: 08 Jun '17
Abstract: Publication date: Available online 13 December 2016 Source:Progress in Natural Science: Materials International Author(s): Shiyu Ma, Jianxin Zhang The alloying effects of W, Cr and Re in the  (010) edge dislocation cores (EDC) of Ni-based single crystal superalloys are investigated using first-principles based on the density functional theory (DFT). The binding energy, Mulliken orbital population, density of states, charge density and radial distribution functions are discussed, respectively. It is clearly demonstrated that the addition of refractory elements improves the stability of the EDC systems. In addition, they can form tougher bonds with their nearest neighbour (NN) Ni atoms, which enhance the mechanical properties of the Ni-based single crystal superalloys. Through comparative analysis, Cr-doped system has lower binding energy, and Cr atom has evident effect to improve the systemic stability. However, Re atom has the stronger alloying effect in Ni-based single crystal superalloys, much more effectively hindering dislocation motion than W and Cr atoms.
Pub.: 16 Dec '16, Pinned: 08 Jun '17
Abstract: Publication date: March 2017 Source:Acta Materialia, Volume 126 Author(s): Yuling Tang, Ming Huang, Jichun Xiong, Jiarong Li, Jing Zhu A second-generation nickel-based single crystal superalloys DD6 were creep tested in the  direction (within 15°) at 1100 °C/140 MPa. The specimen tested until rupture was investigated using scanning and transmission electron microscopy to determine the evolution of the dislocation behaviors during tertiary creep. It was found that the tertiary creep deformation was highly localized and inhomogeneous along the gauge length, and the types of superdislocations in γ′ rafts varied with the distances from the rupture surface, including individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. It can be concluded that γ′ rafts shearing events occur in the following sequence with the evolution of tertiary creep: individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. The origin of these transformations of superdislocation types and its influence on tertiary creep rate are discussed. It is proposed that at the microscopic level, a more reasonable explanation for the strain softening mechanism during the tertiary creep of nickel-based superalloys at high temperatures and low stresses is the emergence of new superdislocation types with higher mobility rather than the density rise of a single type of superdislocation produced during the later secondary creep stage. Graphical abstract
Pub.: 14 Jan '17, Pinned: 08 Jun '17
Abstract: Single-crystal Ru-containing nickel-base superalloys with spherical γ′ precipitates have been observed in alloys with substantial amounts of Re and W and high levels of Ru. The γ′ precipitates did not experience stress-induced shape changes (rafting) during creep deformation at 950 °C and 290 MPa, indicative of a γ-γ′ lattice misfit very near zero. Furthermore, interfacial dislocation networks were not formed during creep deformation in the low misfit alloys. The alloys containing spherical precipitates had lower creep strengths than the alloys containing cuboidal precipitates at 950 °C and 290 MPa. Element partitioning between the phases was investigated in order to determine the origin of the unusual microstructural features. Transmission electron microscopy (TEM)-based energy-dispersive spectroscopy (EDS) analysis of the γ and γ′ phases indicates that Ru affects the partitioning of Re, which partitions much less strongly to the matrix than previously observed in Re-containing superalloys, consistent with a lattice misfit very near zero. With high levels of Ru, the addition of Cr also has a strong influence on partitioning. These investigations demonstrate that Ru and Cr control the lattice misfit, precipitate shape, and creep behavior, through the associated changes in the γ-γ′ phase equilibrium.
Pub.: 01 Oct '06, Pinned: 08 Jun '17