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Abstract
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.
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