Coarsening kinetics of cuboidal γ′ precipitates in single crystal nickel base superalloy CMSX-4

Coarsening kinetics of cuboidal gamma' precipitates was studied in a single crystal nickel base superalloy CMSX-4 at five temperatures ranging from 850 to 1000 degrees C and five ageing times from 100 to 2000 h. The gamma' precipitates preserve their cuboidal shape morphology during ageing at temperatures from 850 to 950 degrees 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 gamma' precipitates follows cube rate law and is controlled by volume diffusion of alloying elements in the gamma matrix according to Lifshitz-Slyozov-Wagner (LSW) theory. The activation energy for coarsening is calculated to be 272.4 kJ mol(-1). Critical input parameters such as gamma/gamma' 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.