Ordering Transformation and Age Hardening in a Ni-Cr-W Superalloy

The microstructural changes occurring in a Ni-Cr-W superalloy during prolonged exposure to proper temperature have been investigated using transmission electron microscopy. It is demonstrated that nanometer-sized C11_b (Pt₂Mo-type) and DO₂₂ superlattices can precipitate in the Ni-Cr-W alloy by means of a simple aging treatment at temperatures varying in the range of 773 K to 973 K (500 °C to 700 °C). The mechanism of transformation to long-range order has been revealed to accord with continuous mode based on transmission electron microscopy results and variation trend in Vickers microhardness. No signs of overaging and coarsening of C11_b and DO₂₂ phases with further aging have been found, which indicates that both of them have a high-thermal stability. The orientation relationships and interfaces between C11_b/DO₂₂ precipitates and Ni-based matrix have been investigated by high-resolution transmission electron microscopy, and the results reveal that the interfaces between C11_b/DO₂₂ precipitates and surrounding matrix are coherent at the atomic scale. Because of the high-density nanometer-sized C11_b/DO₂₂ precipitates, the microhardness of the alloy is improved remarkably.