Enhancing structural stability of γ′-Co3(Al, Nb) phases: The key role of anti-site defects

Abstract

The design of the composition for two-phase superalloys of γ/γ′ has always been key to achieving a breakthrough in their mechanical performance. However, the underlying mechanism of the anti-site defects in γ′ precipitates that severely affect stability and performance have rarely been explored. The present work therefore specifically investigated the structural stability and mechanical properties of strictly stoichiometric and off-stoichiometric γ′-Co3(Al, Nb) phases by using the first-principles calculations. The results show that the change in the Al/Nb ratios in the compounds could induce the instability of the L12 structure, while an increase of Nb atoms leads to compounds with predominantly covalent bonds exhibiting stronger mechanical properties. After evaluating the Gibbs free energy of these γ′-Co3(Al, Nb) phases at finite temperature, it is found that Co29Al2Nb1 and Co29Al1Nb2 compounds with anti-site defects can be thermodynamically stable at greater than 710 and 905 K, respectively. Accordingly, Co-10Al-2Nb and Co-2.5Al-3Nb alloys were successfully prepared experimentally, further confirming that the γ′-Co3(Al, Nb) phase with anti-site defects can precipitate preferentially and remain stable during the 168 h annealing treatment.

Keywords

• Co-based superalloys
• Anti-site defects
• Structural stability
• Mechanical properties
• Gibbs free energy