Early precipitation of Ni2(Cr,Mo) phase

The early precipitation behavior of long-range-ordered (LRO) Ni2(Cr,Mo) strengthening phase in a Ni-21Cr-17Mo (wt%) superalloy that was subjected to a patent two-step heat treatment (705 °C/16 h/furnace cooling - 605 °C/32 h/air cooling) was revealed by means of transmission electron microscopy (TEM) and electron diffraction pattern (EDP) analysis. The coherent interfacial structure between the long-range-ordered Ni2(Cr,Mo) precipitate and the Ni-based matrix leads to the excellent strengthening effect of the alloy. High-spatial-resolution energy-disperse X-ray spectrum (EDX) analysis reveals that the Ni2(Cr,Mo) precipitates were slightly rich in Mo. The minor microstructure change after the key first-step heat treatment was especially focused on. It was found that weak streak fringes were present along 〈311〉Ni-base⁎ directions in the EDP along the [112] zone axis and the ordered nano-regions parallel to {311}Ni-base planes were distributed homogeneously in the matrix. Combined with the detailed microstructural characteristic, it is concluded that Mo atoms might enrich at every two {311}Ni-base planes during the first step of aging at high temperature (705 °C), which could increase the transition temperature of Ni2Cr resulting in its formation when aging at the second aging step of 605 °C. Meanwhile, Ni2(Cr,Mo) phase precipitates by the substitution of Mo atoms for Cr atoms every three {311}Ni-base planes in Ni2Cr phase with the continuous long-range diffusion of Mo atoms.