Effects of B content on microstructure and high-temperature stress rupture properties of a high chromium polycrystalline nickel-based superalloy

Highlights: • Excessive boron addition aggravates the inhomogeneity of microstructures. • Boron addition weakens solution strengthening and precipitation strengthening. • A grain boundary strengthening mechanism of boron addition is proposed based on the crack nucleation and propagation. -- Abstract: Effects of boron (B) addition on microstructure and high-temperature stress rupture properties of a heat-treated high chromium polycrystalline Nickel-based superalloy were investigated by adjusting B content (0, 0.016 wt%, 0.048 wt%, 0.09 wt%). The results indicated that the grain boundary characteristics were altered by B addition, such as the morphologies of the MC carbides, M₂₃C₆ carbides and precipitation of γ′ particles along grain boundaries. Meanwhile, it was confirmed that B existed in the form of Cr-rich M₅B₃ borides in the boron-containing alloys. Moreover, boron addition aggravated the inhomogeneity of microstructures, and reduced the volume fraction of secondary γ′ particles. Additionally, the rupture life was remarkably improved as the B content was increased to 0.016 wt%, whilst the elongation tended to improve as the B content was increased to 0.048 wt%. The improvement of stress rupture properties was mainly contributed to the modification of the grain boundary characteristics which retarded crack nucleation and propagation at grain boundaries. However, with excessive addition of B, the stress rupture properties were reduced significantly, as the solid solution strengthening and precipitation strengthening tended to be weakened by B addition, in addition the incipient melting regions and lager size borides promoted the intragranular crack nucleation and propagation.