The phase decomposition with related element interaction and redistribution during long term aging of 740H superalloy

Microstructure evolution of alloy 740H for advanced ultra-supercritical (AUSC) power plants are studied during long term aging with a temperature scope of 600-720 degrees C up to 10,000 h and 800 degrees C up to 2000 h. Field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM) and micro-chemical phase analysis are carried out for detailed investigation of phase evolution. Though phase morphology does not change dramatically during 720 degrees C aging, apparent element redistribution are detected after 200 h aging. Different from common knowledge, decomposition of MC carbide is detected during aging. Element redistribution relationship between gamma' phase, MC carbide and M23C6 carbide during 10,000 h aging is established based on quantitative analysis and morphology observation. Nb, Ti and C elements are released into matrix during decomposition of MC carbide. Nb and Ti elements diffuse into gamma' phase during growth of gamma' particles and Cr element is rejected out of the phase. The abundant C element released by MC carbide and Cr element rejected from gamma' phase are beneficial for growth of M23C6 carbide and weight fraction of M23C6 carbide doubles after 720 degrees C/10,000 h aging. This time-consuming and element diffusion related process can be modified into the reaction: MC+gamma -> (MC)(t)+gamma'+(Cr)(enrichment) -> M23C6+gamma'. The evolutions of different phases are linked by redistribution of elements during long thermal exposure. (C) 2018 Elsevier B.V. All rights reserved.