Microstructural effects on crack path selection in bending and fatigue in a Nb-19Si-5Cr-3.5Hf-24Ti-0.75Sn-1W alloy

Nb-Si alloy containing 19Si, 5Cr, 3.5Hf, 24Ti, 0.75Sn and 1W (in at.%) has been tested by three-point bending and fatigue at both room temperature and 400 deg. C. The microstructure and fracture surface associated with crack growth were investigated by scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD). The alloy contains Nb solid solution (Nb(ss)) and Nb- or Ti-rich silicides. Some pre-existing microcracks were found in the silicides. EBSD Analyses on the cast and HIPped Nb-Si alloy revealed a preferred orientation in the Nb(ss) similar to that of a unidirectionally solidified alloy. The fatigue crack path preferentially propagated either along the phase boundaries between the silicide and Nb(ss) or through the pre-existing silicide microcracks. Slip steps were observed inside the Nb(ss) during crack propagation, indicating deformation in the alloy occurred in the Nb(ss) while the silicides assisted the crack propagation.