Some Effects of Carbon in the Production of Single Crystal Superalloy Castings

As nickel-base single crystals are being implemented in gas turbine engines operating at temperatures in excess of 1600K, maintaining structural properties at these elevated temperatures has become an increasingly important problem. Typically, elevated levels refractory alloying elements are added to these single crystal alloys to enhance the degree of solid solution strengthening within the microstructure. The presence of Re, W, Mo, Ta in Ni-base superalloys strongly influences parameters, such as the stacking fault energies, lattice misfit, and shear modulus of the crystalline lattices, that govern the creep response of the material. However, this is a challenge when dealing with high refractory alloys that are susceptible to the precipitation of deleterious topologically-close-packed (TCP) phases. Recent investigations have demonstrated Ru additions to be beneficial with respect to hindering the formation of these TCP phases and consequently improving the creep properties of these advanced single crystal alloys. Substantial effort has been dedicated towards understanding the fundamental effects and the mechanisms by which Ru additions alter the structure of the γ and γ' phases. Much of this knowledge has been effectively applied to the development of a new class of single crystal Ni-base superalloys with improved strength and a temperature capability significantly higher than those of existing "second" or "third" generation alloys. Nevertheless, optimization of the alloy chemistries remains notoriously difficult as various component design requirements related to the solidification characteristics, overall blade density, coating compatibility and cost must also be carefully considered when developing these materials. This investigation addresses some of the issues associated with the development of Ru-bearing, high refractory content Ni-base superalloys and describes how interrelationships between the properties, processing, microstructure and chemistry of these alloys can be balanced to yield a practical "fourth" generation single crystal superalloy.

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“…Another alternative method to prevent the formation of eutectic scale on the surface of castings is to utilize minor additions of carbon [30] .…”

Section: Discussionmentioning

confidence: 99%

Linking the Properties, Processing and Chemistry of Advanced Single Crystal Ni-Base Superalloys

Tin

Zhang

Hobbs

et al. 2008

Superalloys 2008 (Eleventh International Symposium)

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“…However, more recent results clearly indicate that carbon additions result in the formation of carbides in the interdendritic region that block fluid flow [4,5] which reduces the thermal solutal convection. The presence of the interdendritic carbide dendrites also reduces the scale formation during casting of CMSX-4 [6].…”

Section: Introductionmentioning

confidence: 98%

“…Studies have also shown boron to be effective at reducing carbide size, improving mechanical properties and retarding grain boundary cracking in polycrystalline Ni-base alloy [9][10][11][12]. In the case of single crystals, the addition of carbon and boron has proven beneficial in reducing solidification defects, improving the tolerance for minor solidification defects and improving mechanical properties [6,[14][15][16]. However, the effect of boron, carbon and nitrogen additions on the primary MC-type carbide morphology and composition, and the segregation during solidification has not been reported.…”

Section: Introductionmentioning

confidence: 99%

Effect of minor alloying additions on the solidification of single-crystal Ni-base superalloys

Cutler

Wasson

Fuchs

2009

Journal of Crystal Growth

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“…Studies have also indicated that carbon additions can also prevent the formation of c9 scale formation on directionally solidified superalloy components. 40 The combined effects associated with carbon are maximised when carbide precipitation occurs just below the liquidus temperature of the alloy. [19][20][21]36 Carbon additions are particularly useful for reducing the occurrence of freckles during low gradient solidification conditions, indicative of the processing parameters associated with physically large turbine blades.…”

Section: Discussionmentioning

confidence: 99%

Intelligent alloy design: Engineering single crystals superalloys amenable for manufacture

Tin

2009

Materials Science and Technology

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The solidification characteristics of high refractory content nickel based superalloy single crystals have been studied over a wide range of composition. To assess the influence of composition on distribution coefficients, quantitative segregation mapping experiments have been conducted on as cast single crystals. Additionally, differential thermal analysis was utilised to measure the composition dependence of liquidus, solidus and γ′ solvus temperatures. Over the range of composition characteristic of the most advanced commercial single crystal alloys, synergistic interactions existing between select constituent elements were found to mitigate the severity of Re and W segregation during dendritic solidification. Minor additions of carbon, in the range of 0·05–0·15 wt-%, were also found to reduce the degree of Re and W segregation during solidification. In both instances, controlling the extent of Re and W segregation during solidification was shown to strongly influence the tendency of the alloy to form macroscopic grain defects, such as freckles, during unidirectional solidification. Moreover, precipitation of primary carbides within the mushy zone during solidification was also found to impact the solidification characteristics of the alloy. The implications of these alloying approaches for developing novel, high refractory content, single crystal superalloys compositions that are amenable for manufacturing are discussed.

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Some Effects of Carbon in the Production of Single Crystal Superalloy Castings

Cited by 7 publications

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Linking the Properties, Processing and Chemistry of Advanced Single Crystal Ni-Base Superalloys

Linking the Properties, Processing and Chemistry of Advanced Single Crystal Ni-Base Superalloys

Effect of minor alloying additions on the solidification of single-crystal Ni-base superalloys

Intelligent alloy design: Engineering single crystals superalloys amenable for manufacture

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