Influence of Replacing Molybdenum with Tungsten on the Creep Fracture Property of Waspaloy Nickel-Based Alloy

Abstract

Alloys meeting the requirements of “700 °C and above” advanced ultra-super-critical technology, with higher thermal efficiency, have been developed in recent years. Here, a new wrought Ni-based superalloy with excellent high-temperature creep strength based on Waspaloy has been developed and is proposed as a candidate material for application in 700 °C class advanced ultra-super-critical steam turbine blades. In this new alloy, the Molybdenum (Mo) in Waspaloy is partially replaced by Tungsten (W). Creep tests have shown that this new Ni-based alloy has a 70 MPa higher creep-rupture strength than that of Waspaloy at 700 °C by extrapolating the experimental data. Detailed creep-rupture mechanisms have been analyzed by means of scanning electron microscopy, transmission electron microscopy, and chemical phase analysis with a view to devising potential approaches for performance improvements. The results showed that the partial replacement of Mo by W had negligible effect on the composition of carbides precipitated in the alloy. Instead, the amount of the γ′ phase was significantly increased, and mismatch between the γ and γ′ phases was reduced. In this way, the stability of the γ′ phase was increased, its coarsening rate was reduced, and its critical shear stress was increased. As a result, the high-temperature creep-fracture strength of the new alloy was increased.