The existence and strengthening mechanism of phosphorus in Ni17Cr15Fe-based alloys

Abstract In this paper, the existing form of phosphorus (P) and its strengthening mechanism in the simplified alloys Ni-17%Cr-15%Fe were investigated. Two different post heat treatments were applied on the as solutionized alloys in order to investigate the precipitates and their effects on the mechanical properties. For the water quenched alloys, no precipitated compounds are observed either at grain boundary or within grain, the rupture strength was enhanced, and the tensile fracture mode transformed from mixture of intergranular and transgranular fracture to single transgranular fracture, which indicated the segregation of P atoms along grain boundary strengthened grain boundary. For the water quenched alloys with additional further aging at 800 °C for 8 h and subsequent water quenching, P facilitated the precipitation of intragranular M23C6 carbide and thus the intragranular strength was effectively improved. The results of quantitative chemical element analysis and qualitative TEM/EDS analysis indicated that P enriched in intragranular M23C6 carbide. At the same time, the amount of intergranular M23C6 carbide increased with an increase in P content and its morphology transformed from short rod to granular particle gradually, resulting in the grain boundary precipitation strengthening. The findings of existing state and strengthening mechanism of phosphorus in Ni-17%Cr-15%Fe alloys are of critical importance, which will provide useful information for further interpreting the beneficial effect of P in Ni-based superalloys.