Creep deformation and damage behavior of an alumina-forming austenitic alloy strengthened by intermetallic compounds

Based on Fe-30Ni-15Cr, a new alumina-forming austenitic (AFA) alloy, free of carbon, was designed for potential use in ultra-supercritical generator set. The creep behavior of alloy has been investigated by analyzing the creep constitutive equation, creep damage tolerance factor (lambda), microstructure images, and oxide layer structure under different stresses at 750 degrees C. The creep microstructure and oxide morphology were obtained by electron microscopy. The minimum creep rate ((center dot)epsilon m) is positively correlated with the stress, and when the stress increases from 120 MPa to 250 MPa, (center dot)epsilon m increases from 6.26E-3 1/h to 6.34E-4 1/h, almost ten times of the latter. The creep constitutive equation at 750 degrees C was determined as: (center dot)epsilon m = 9.29 x 10-11 sigma 3.3. B2-NiAl and L12-Ni3Al were precipitated during creep. The relationship between the misfit of nanoscale L12-Ni3Al phase and gamma matrix are perfectly coherent. The L12-Ni3Al phase grows more slowly and remains at the nanoscale size after creep at 750 degrees C for 869h, which contribute to increase creep strength and reduce the creep rate of the material. However, the coarsened sigma-FeCr phase reduced the creep productivity of the alloy. The microstructural degradation caused by precipitate coarsening was found to be responsible for creep failure, as indicated by the creep damage tolerance factor (lambda) and creep fracture morphology. The elemental distribution of the oxide layer indicates the absence of a dense alumina protective scale at 750 degrees C. The extensive presence of oxidation cracks in the matrix suggests that the effect of oxidation cannot be ignored in the analysis of creep life. The fact that sigma-FeCr is detrimental to creep damage indicates that the alloy composition needs further optimization.