Microstructure evolution during the precipitation and growth of fully coherent DO₂₂ superlattice in an Ni-Cr-W alloy

The ordering transformation occurring in a model Ni-Cr-W superalloy during prolonged exposure to proper temperature has been investigated systematically. It is demonstrated that nanometer-sized precipitates with a DO₂₂ structure can precipitate in the Ni-Cr-W alloy by means of simple aging treatment at 650–700 °C. The mechanism of transformation to DO₂₂ superlattice has been determined to be continuous ordering based on the results of high resolution transmission electron microscopy investigation and variation trend in Vickers microhardness. Different variants of DO₂₂ phase can coexist in the matrix with no signs of overaging as aging time increases, indicating it has a high thermal stability. The precipitates of DO₂₂ superlattice has been found to be of ellipsoidal shape which results in the greatest reduction of strain energy. The interfaces between DO₂₂ precipitates and matrix have been revealed to be coherent at the atomic scale, resulting in considerable coherency strain attributing to the lattice misfit between DO₂₂ particle and matrix. Because of the high-density nanometer-sized DO₂₂ phase, the microhardness of the alloy has been improved remarkably after aging treatment.