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A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 1 9 2 7 γ ’ phase is a pronounced loss in creep strength at temperatures above 900°C. However, precipitate strengthened nickel-base superalloys are far from ideal once their long-term precipitate derived strength is taken into consider- ation. It is these γ′ precipitates that un- dergo coarsening, a process where large precipitates grow at the expense of smaller ones to formnewmorphologies at elevated temperatures. The problem of accelerated solute diffusivity, along with the driving force for reducing the overall interfacial area of γ′ precipitates at high operating temperatures, often triggers a continuous surface-energy driven precipitate coarsening process. This scenario leads to less than optimal mechanical properties. Materials scientists at Idaho Na- tional Laboratory (INL) are working on developing γ - γ′ nickel alloys that fea- ture substantially greater microstruc- tural stability with regard to secondary phase composition and distribution than commercially available superal- loys. The result of this alloy research is a consistent set of mechanical proper- ties at elevated temperatures. The sci- entific breakthrough recently published in Science Advances [2] by a team led by INL staff scientist Subhashish Meher proposed an alternate method to re- duce or halt the coarsening of γ′ pre- cipitates by introducing the perplexing concept of hierarchical microstructure. This novel nanoscale hierarchy can make superalloys even more super, ex- tending their useful life by thousands of hours. Previous research has shown that performance can be improved if the material structure of the superalloy repeats in some way from very small sizes to very large, like a box within a box within a box. This is called a hierarchical micro- structure. In a superalloy, it consists of a metallic matrix with precipitates, re- gions where the composition of the mixture differs from the rest of the met- al. Embedded within the precipitates are still finer-scale particles that are the same composition as the matrix outside the precipitates. The chemical supersaturation induced phase sepa- ration of gamma prime precipitates results in formation of nanoscale dis- ordered gamma precipitates inside of them (Fig. 1). The key is to heat and cool the superalloy in a specific way to achieve this microstructure. This cre- ates a microstructure within the ma- terial that can withstand high heat more than six times longer than an un- treated counterpart. OBSERVATION AND MODELING The researchers used transmission electronmicroscopy to observe the evo- lution of the γ′ precipitates and found that nanoscale γ precipitates act to slow the rate of coarsening of the larg- er γ′ precipitates at 800°C but are dis- solved after a certain time, after which coarsening re- sumes as for a convention- al γ - γ′ system. Although the coarsening resistance de- rived from the examined superalloys is in the range of 500 hours, various chang- es in the overall alloy com- position can be made to retain the hierarchical mi- crostructure even longer. The microstructure lifetime could be extended up to a few thousand hours, which would add several years of service life to components made of these alloys. “The hierarchical microstructure artificially increases the radius of γ′ precipitates by embedding nanoscale γ precipitates within them,” explains Meher. “The enhanced γ′ size will hin- der the dislocation motions at high temperatures, imparting enhanced me- chanical strength.” Meher also used a local electrode atom probe for 3D vi- sualization with nanoscale accuracy to derive both structural and chemical in- formation associated with this peculiar microstructure (Fig. 2). He concludes that elements such as rhenium, cobalt, and ruthenium present in this alloy are responsible for creating the hierarch- ical microstructure. Fig. 1 — A basic schematic describes the different variants that deviate from typical γ - γ′ nickel-base superalloys where γ′ precipitates are embedded in a continuous γ matrix. An invertedmicrostructure contains γ precipitates in a γ′ matrix. On the other hand, a hierarchical microstructure is composed of a γ matrix with γ′ precipitates that contain embedded, spherical γ precipitates. Fig. 2 — Summary of a hierarchical nickel-base superalloy, published by Meher et al. in Science Advances. The analysis revolves around explaining this peculiar microstructure by different experimental techniques as well as computational modeling.