Discovering novel γ-γ′ Pt-Al superalloys via lattice stability in Pt3Al induced by local atomic environment distortion

Abstract

The strength of Pt-Al superalloys can be effectively improved using a coherent structure composed of a disordered γ phase and an ordered γ′ phase. However, γ′-Pt₃Al has a cubic phase only at high temperatures and a tetragonal phase at low temperatures, thus breaking the coherent interface and deteriorating its mechanical properties. Here, high throughput first-principles calculations and rational experiments were conducted to discover novel γ-γ′ Pt-Al superalloys. The alloying strategy stabilizes the γ′ phase through the local charge distortion (LCD) or the local lattice distortion (LLD). Alloying elements, such as, Ti, Hf, and Ta, occupy the Al site in combination with Pt, making the spherical shape of change density in Pt₃Al showing obvious directionality, i.e., LCD. Therefore, the stability of the γ′ phase is proposed at a lower alloying element concentration (3.125 at.%). However, alloying elements near Pt, e.g., Ni, Co, and Ru, tend to occupy the Pt site and induce LLD to stabilize the γ′ phase at higher alloying element concentration (12.5 at.%). The shape of the change density is similar to that of Pt₃Al, and no obvious LCD is found when alloying elements occupy the Pt site. LCD-induced phase stability is observed exclusively at the Al site, whereas LLD-induced phase stability is solely present at the Pt site. The Pt-12Al-6X (X = Hf, Ti, Ta, Cr, and Ni in at.%) alloys were prepared, among which Pt-12Al-6Hf possessed a stable γ′ phase, a lower lattice misfit between γ and γ′, and a higher precipitation strengthening, resulting in its hardness (442.1 H_V) surpassing that of Ni-based superalloys IN718 (413.6 H_V). The γ′ phase in Pt-12Al-6Hf was characterized by high-resolution transmission electron microscopy, suggesting that the Pt site of the γ′ phase was occupied by Pt, while the Al site was shared by Pt, Al, and Hf. This is consistent with the calculated results.