The fracture behavior of a directionally solidified FeCrMnC superalloy

Abstract

The fracture behavior of a directionally solidified alloy, containing Fe-15Cr-15Mn-2Mo-1.5C-1Si-1Nb (where the composition is in weight per cent), has been investigated in both transverse and longitudinal orientations at room temperature in laboratory air. The directional solidification (DS) process resulted in an aligned polycrystal, with individual grains sharing a common (001) zone axis, and a network of interdendritic chromium-rich M 7C 3 blocky carbides and niobium-rich MC script carbides. A heat treatment, which simulates the brazing cycle used to assemble the heater heads found in prototype automotive Stirling engines, was applied to all specimens prior to the fracture experiments. Interestingly and unexpectedly, the specimens which were oriented with the crack propagation plane parallel to the DS direction, identified as the transverse orientation (T-L), exhibited a toughness value which was twice that of the orientation in which the crack propagated perpendicular to the DS direction, the longitudinal orientation (L-T). For the T-L orientation, the fracture path was observed to progress through the dendrite cores, resulting in a significant amount of plastic deformation and energy absorption during the fracture process. Extensive crack bifurcation and tortuosity constituted another mechanism by which the toughness was increased for the T-L orientation. For the L-T orientation, fracture proceeded in a brittle cleavage fashion, with the propagating crack passing predominantly through the less tough interdendritic carbide network and tearing through the f.c.c. matrix only as it traversed between adjacent carbide networks.