An order-disorder phase transition in alloy 783 bolts after long-term service

Abstract

Ni-Co-Fe based alloy 783 is commonly employed as bolt material in power plants. Its initial state possesses a γ matrix strengthened through B2-type β phase and L12-type γ' phase. Over prolonged service, the alloy displays an observed increase in strength along with certain embrittlement. After 100 and 123 months of service at 600 °C, a pine-like phase enriched with Co and Fe occurs in the matrix, which is confirmed to be a BCC phase with a lattice parameter of 2.87 Å. These BCC precipitates also manifest within the coarse β particles and are coherent with the B2 phase, showing a cuboidal shape. Thermodynamic calculations suggest the transformation from the ordered B2 phase to the disordered BCC phase at temperatures below 670 °C. The identification of the order-disorder phase transition deepens the understanding of phase transition after long-term service in alloy 783 and the related mechanical properties evolution in addition to previous works.