Microstructure-informed modelling of the fracture toughness of alloys representing macro-segregated zones in heavy forgings

Abstract

The macro-segregation of carbon and alloying elements in heavy forged nuclear components can lead to significant variations in mechanical properties, and especially in fracture toughness. In the current work, the prediction of fracture toughness was addressed for three model alloys chemically representative of the compositions encountered in macro-segregated zones. Characterisations of the microstructure and of the fracture toughness properties, measured by compact tension (CT) specimens, were performed. Tensile properties were measured around the reference temperature for each material for the calibration of elasto-plastic parameters. Carbide size distributions were determined thanks to scanning electron microscopy (SEM) image analysis. The microstructure-informed brittle fracture (MIBF) local approach model was applied here to predict the scatter of brittle fracture toughness. This model involves two sources of scatter: the stress distribution inside a representative volume of the bainitic microstructure and the size distribution of carbides, which are assumed to be the brittle facture initiators. The simulation results demonstrated the capabilities of the MIBF model for predicting the fracture toughness scatter and the shift towards higher temperature of the brittle-to-ductile transition with the increase of carbon and alloying elements. The only parameter of the MIBF model to be calibrated is the effective surface energy γf which was found very close to estimated values for ferrite. The observed variation of γf in these three model alloys also suggests a possible effect of microstructural evolution on fracture toughness, in addition to the constitutive law and carbide distributions.