The effects of boron addition on the microstructure stability and mechanical properties of a Ni-Cr based superalloy

This study is focused on the effect of boron additions on the microstructure stability and mechanical properties of a Ni-Cr based superalloy designed for application of 700 °C advanced ultra-supercritical units component. The results show that boron in the content range of 25-75 ppm has no obvious influence on as-cast and heat-treated microstructure characteristics compared with that of the alloy without boron. During long-term thermal exposure, the variation of B content has no obvious effect on γ′′ coarsening, δ phase precipitation and MC carbide degeneration, but inhibits the agglomeration of M23C6 carbide along grain boundary. According to the mechanical property analysis, the boron additions contribute to mainly tensile ductility especially the elongation of whether the heat treated alloy or the long-term thermal exposed alloy. The strength especially yield strength of the B doped alloy almost keeps the same level as that of the B-free alloy. Furthermore, the SEM fractography shows a change from intergranular fracture to a mixed fracture and the dimples on fracture surface obviously increase with the addition of boron. The mechanism discussion reveals that the segregation of boron at grain boundary resulted in the improvement of grain boundary cohesion and the optimization of carbides distribution at grain boundary. The strengthening mechanism of δ phase is confirmed as its obstacle effect to dislocation movement.