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

Abstract

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 °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.