Abstract
Coarsening kinetics of cuboidal γ’ precipitates was studied in a single crystal nickel base superalloy CMSX-4 at five temperatures ranging from 850 to 1000 °C and five ageing times from 100 to 2000 h. The γ’ precipitates preserve their cuboidal shape morphology during ageing at temperatures from 850 to 950 °C up to 2000 h. At a higher ageing temperature of 1000 °C, the cuboidal shape is preserved up to 500 h and longer ageing time leads to directional coarsening of the precipitates and formation of spontaneously rafted microstructure. The coarsening kinetics of cuboidal γ’ precipitates follows cube rate law and is controlled by volume diffusion of alloying elements in the γ matrix according to Lifshitz-Slyozov-Wagner (LSW) theory. The activation energy for coarsening is calculated to be 272.4 kJ/mol. Critical input parameters such as γ/γ’ interfacial energy and the effective diffusion coefficient controlling coarsening kinetics of the &gamma’ precipitates are estimated and verified by comparing the calculated and experimental coarsening rate coefficients with those published for numerous multicomponent as well as binary nickel base alloys.

Key words
nickel alloy, single crystal superalloy, ageing, coarsening kinetics, microstructure degradation

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