Influence of minor boron on the microstructures of a second generation Nibased single crystal superalloy

Boron is added into single crystal superalloys as a micro-alloying element to strengthen low angle grain boundaries. However, systematic investigations on the effect of boron on microstructures of single crystal superalloys are limitedly reported. The effect of boron on as-cast and heat-treated microstructures was investigated in two experimental Ni-based single crystal superalloys containing 3?wt% Re. The current results indicated that the volume fraction of (γ?+?γ′) eutectic and M₃B₂ borides was evidently increased, while the number of micropores was evidently decreased with the addition of 0.02?wt% boron. The (γ?+?γ′) eutectic could not be dissolved completely due to the lower incipient melting temperature caused by the formation of M₃B₂ borides. Meanwhile, the M₃B₂ borides were found to be enriched with indispensable strengthening elements Cr, Mo, W and Re, and this may lower the strengthening effect and cause stress concentration during high temperature creep.