Impact of boron as an alloying addition on the microstructure, thermo-physical properties and creep resistance of a tungsten-free Co-base γ/γ′ superalloy

Abstract

While boron is added to many structural alloys primarily as a grain-boundary strengthener, it has myriad other implications on the microstructure and thermophysical properties of these alloys. In the present study, we report these effects by varying boron addition in the range of 0.02 at.% to 0.8 at.% on the microstructure, thermo-physical properties and the creep resistance of tungsten free, γ/γ′ base Co–30Ni–10Al–5Mo–2Ta–2Ti (at.%) polycrystalline superalloy. Without boron addition, the plastic strain during creep at 850 °C is primarily accommodated through grain-boundary decohesion, often enhanced by severe oxygen attack along the grain boundaries. The addition of boron alleviates the poor grain-boundary cohesivity while inhibiting internal oxidation. An increase in the solvus temperature with boron addition was found, leading to an increase in the high-temperature capability of this alloy (∼40 °C). However, a concurrent decrease in the alloy's melting point has been observed. Coupled with improved grain-boundary cohesivity and larger grain size, improvement in creep resistance by almost an order of magnitude could be achieved. While at low amounts of boron (0.02–0.08 at.%) borides do not form, indicating solid solubility of boron within the bulk alloy, at high boron contents (0.2–0.8 at.%), Co and Mo rich boride forms primarily along the grain boundaries. These borides are surrounded by a distribution of fine secondary γ′ precipitates around them. In contrast, borides in Co–Al–W alloys promote precipitate free zones (PDZ), causing deterioration of mechanical properties. The beneficial effect of boron along grain-boundaries, both in elemental form and in the form of borides, has been highlighted in the present study which paves way for optimizing boron addition in other alloys of this class.