The anisotropy of deformation and fracture in a directionally solidified Ni/Ni3Al-Ni3Cb lamellar eutectic alloy

The orientation dependence of deformation and fracture modes was investigated for a directionally-solidified Ni−Ni₃Al−Ni₃Cb lamellar eutectic alloy (Ni-20 wt pct Cb-2.5 wt pct al-6.0 wt pct Cr) using optical and transmission microscopy to examine tensile and compression specimens tested at temperatures below the softening point of the δ (Ni₃Cb) reinforcing phase (∼1050 K). In this temperature range there is a large difference between longitudinal and transverse tensile ductibility (>5 pct longitudinalvs<1 pct transverse). No single preferred fracture path (such as interfacial delamination) could be found to account for the low transverse tensile ductility. Analysis of the δ twinning geometry, however, indicated that the twinning strains for twins of the type {211}, which operate copiously in longitudinal tension, are negative in most transverse orientations, with Schmid factors being very low (<0.013) in the limited range of transverse orientations where {211} twin strains are positive. Examination of transverse tension test specimens broken at 1033 K confirm the absence of {211} twins, with only limited {011} twinning being found in selected grains, leading to the conclusion that the relatively low transverse tensile ductility of the eutectic results from the very limited number of deformation systems which operate in the δ reinforcing phase below the softening temperature.