Effects of Mo and Ta in Ni3Al based single crystal alloys on their stress–rupture properties at intermediate temperatures

Abstract

The effects of the addition of Mo and Ta elements to Ni3Al based single crystal (SC) alloys on their microstructures and stress–rupture properties were investigated. The results of this research indicate that in a Ni3Al based SC alloy with an added 3%Ta, the volume fraction, mean size and cubical degree of the γ′ phase increased and the segregation ratios of the refractory elements were not totally altered, compared to the effects seen after the addition of 3%Mo, as analysed using scanning electron microscopy (SEM) and energy dispersive spectroscopy. Moreover, the lattice mismatch of the γ/γ′ phase in the Ni3Al based SC alloy became smaller, as observed using X-ray diffraction. The stress–rupture lives of the Ni3Al based SC alloys were ∼330 and 260 h respectively. The results suggest that Mo promotes an increase in the stress–rupture property and that the effect of Ta is less than that of Mo. Analysis through transmission electron microscopy illustrated that, in the alloy with Mo, a multiple slip of <011> [111] dislocations was developed to form dense dislocation networks and a few of the dislocations sheared into γ′ by short <112> [111] stacking faults ribbons. In the alloy with Ta, the dislocations sheared into the γ′ precipitates by coupled dislocation pairs without dense dislocation networks. These differing results are mostly due to larger lattice mismatch and denser dislocation networks formed through the addition of Mo.