The temperature dependence of strengthening mechanisms in Ni-based superalloys: A newly re-defined cuboidal model and its implications for strength design

Abstract

The development of advanced Ni-based superalloys for gas turbine applications is strongly reliant on alloy strength design and optimisation through microstructure control. Herein, a new model for precipitation strengthening in Ni-based superalloys with large amounts of cuboidal γ’ particles has been further developed from a previous work with re-defined physical parameters, allowing for a smooth integration of weak and strong pair-coupling models. The predicted results were directly validated using commercial alloy (Alloy720Li)-based single-crystal tie-line model alloys with specially designed γ’ particle sizes and volume fractions, which made it possible to extract the temperature dependence of all factors responsible for the overall strength, such as the γ/γ’ mixture, particle strengthening, and solid-solution strengthening. The physical parameters controlling the strength increment in particle strengthening were thoroughly examined as well. The proposed pair-coupling model with the assumption of cuboidal γ’ particles accurately predicts alloy strength over a wide temperature range (up to 760 °C) for volume fractions > 45%, whereas the classical model that assumes the presence of spherical γ’ particles is valid only for volume fractions < 20%. More importantly, our finding suggests that the cuboidal γ’, with its stronger dependence of strengthening on volume fractions than on size and higher strength increment with increasing APB energy, can allow easier strength optimization of turbine discs over spherical γ’.