Theoretical study of synergistic effect of P and Mg on the cohesive properties of Ni3Al grain boundaries

Abstract

The addition of specific alloying elements can effectively control the microstructure of alloys, so as to improve the cohesive properties of the GBs. In this work, the first-principles plane-wave pseudopotential method is used to investigate the segregation behavior of P and Mg doping at the Ni3Al GBs and reveal the physical mechanism. Different 100%Ni Σ5 (210) [010] GB systems with and without doping elements are relaxed and larger volume expansions are obtained. The calculated segregation energies show that P or Mg atoms tend to segregate to the GBs relative to the bulk. P atoms tend to stay at the interstitial sites surrounded by 8 Ni atoms in the pure Ni holes of the GBs while Mg atoms tend to substitute Ni atoms. The segregation of P or Mg to the GBs leads to an increase of the Griffith work at the GBs, indicating that the segregation can improve the bonding properties of the GBs. Especially, the synergistic segregation of Mg can improve the detrimental effect of P in the mixed hole on the cohesive properties of the GBs. It is also found that the site occupation of Mg can be reversed from the Ni site to the Al site by the synergistic alloying of P at the 50%Ni Σ5 GBs. P and Mg tend to bond with the host atom Ni regardless of GB structure. The investigation of electronic structure shows that GB strengthening of alloying elements is attributed to the increasing P/Mg-Ni interactions across the GBs.