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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.


Enhancing the strength and ductility in accumulative back extruded WE43 magnesium alloy through achieving bimodal grain size distribution and texture weakening

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

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

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

Evolution of superdislocation structures during tertiary creep of a nickel-based single-crystal superalloy at high temperature and low stress

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 [001] 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