Editing Gas turbine (section)

===Creep===
A major challenge facing turbine design, especially [[turbine blades]], is reducing the [[creep (deformation)|creep]] that is induced by the high temperatures and stresses that are experienced during operation. Higher operating temperatures are continuously sought in order to increase efficiency, but come at the cost of higher creep rates. Several methods have therefore been employed in an attempt to achieve optimal performance while limiting creep, with the most successful ones being high performance coatings and single crystal [[superalloy]]s.<ref>{{cite conference |first1=Brian |last1=Hazel |first2=Joe |last2=Rigney |first3=Mark |last3=Gorman |first4=Brett |last4=Boutwell |first5=Ram |last5=Darolia |title=Superalloys 2008 (Eleventh International Symposium) |chapter=Development of Improved Bond Coat for Enhanced Turbine Durability |doi=10.7449/2008/Superalloys_2008_753_760 |conference =Superalloys |publisher=The Minerals, Metals & Materials Society |location=US |year=2008|pages=753–760 |isbn=978-0-87339-728-5 |doi-access=free }}</ref> These technologies work by limiting deformation that occurs by mechanisms that can be broadly classified as dislocation glide, dislocation climb and diffusional flow.

Protective coatings provide [[thermal insulation]] of the blade and offer [[oxidation]] and [[corrosion]] resistance. Thermal barrier coatings (TBCs) are often stabilized [[zirconium dioxide]]-based ceramics and oxidation/corrosion resistant coatings (bond coats) typically consist of [[aluminide]]s or MCrAlY (where M is typically Fe and/or Cr) alloys. Using TBCs limits the temperature exposure of the superalloy substrate, thereby decreasing the diffusivity of the active species (typically vacancies) within the alloy and reducing dislocation and vacancy creep. It has been found that a coating of 1–200 μm can decrease blade temperatures by up to {{convert|200|C}}.<ref>{{Cite web|url=https://www.phase-trans.msm.cam.ac.uk/2003/Superalloys/coatings/index.html|title=Coatings for Turbine Blades|website=www.phase-trans.msm.cam.ac.uk}}</ref> 
Bond coats are directly applied onto the surface of the substrate using pack carburization and serve the dual purpose of providing improved adherence for the TBC and oxidation resistance for the substrate. The Al from the bond coats forms Al<sub>2</sub>O<sub>3</sub> on the TBC-bond coat interface which provides the oxidation resistance, but also results in the formation of an undesirable interdiffusion (ID) zone between itself and the substrate.<ref>A. W. James et al. "Gas turbines: operating conditions, components and material requirements"</ref> The oxidation resistance outweighs the drawbacks associated with the ID zone as it increases the lifetime of the blade and limits the efficiency losses caused by a buildup on the outside of the blades.<ref>Tamarin, Y. Protective Coatings for Turbine Blades. 2002. ASM International. pp 3–5</ref>

Nickel-based superalloys boast improved strength and creep resistance due to their composition and resultant [[microstructure]]. The gamma (γ) FCC nickel is alloyed with aluminum and titanium in order to precipitate a uniform dispersion of the coherent {{chem2|Ni3(Al,Ti)}} gamma-prime (γ') phases. The finely dispersed γ' precipitates impede dislocation motion and introduce a threshold stress, increasing the stress required for the onset of creep. Furthermore, γ' is an ordered L1<sub>2</sub> phase that makes it harder for dislocations to shear past it.<ref>A. Nowotnik "Nickel-Based Superalloys"</ref> Further [[Refractory]] elements such as [[rhenium]] and [[ruthenium]] can be added in solid solution to improve creep strength. The addition of these elements reduces the diffusion of the gamma prime phase, thus preserving the [[Fatigue (material)|fatigue]] resistance, strength, and creep resistance.<ref>Latief, F. H.; Kakehi, K. (2013) "Effects of Re content and crystallographic orientation on creep behavior of aluminized Ni-based single crystal superalloys". Materials & Design 49 : 485–492</ref> The development of single crystal superalloys has led to significant improvements in creep resistance as well. Due to the lack of grain boundaries, single crystals eliminate [[Coble creep]] and consequently deform by fewer modes – decreasing the creep rate.<ref>Caron P., Khan T. "Evolution of Ni-based superalloys for single crystal gas turbine blade applications"</ref> Although single crystals have lower creep at high temperatures, they have significantly lower yield stresses at room temperature where strength is determined by the Hall-Petch relationship. Care needs to be taken in order to optimize the design parameters to limit high temperature creep while not decreasing low temperature yield strength.