The demand for efficient jet-engines/gas-turbines and stringent environmental norms requires the development of alloys that can sustain temperatures beyond the current operating temperatures of these jet-engines/gas-turbines. The currently used Ni-based superalloys are limited by their relatively solvus temperatures. The base metal Ni (Tm=1455°C) limits the highest operating temperature of these engines. Refractory alloys with good strength and ductility are being actively developed as an alternative to superalloys. Most of these refractory alloys are body-centered cubic (BCC).
The deformation behavior of BCC alloys is sensitive to temperature, and alloying elements,). D-parameter is the ratio of surface energy and unstable stacking fault energy (USFE), is an established method to assess the ductility of BCC alloys. A general theme in the existing refractory alloy ductilizing studies has been to add low USFE elements to reduce the overall unstable USFE of the alloy and increase the D-parameter value. There is a lack of comprehensive study on the effect of alloying elements on the D-parameter and their thermodynamic stability. Lack of rigorous error estimation in the existing procedure of calculating USFE and surface energy of concentrated alloys using DFT make calculated values unreliable.
The study aims to establish a DFT-based framework to accurately calculate unstable SFE and surface energy of concentrated alloys. Propose strategy to select alloying elements that give maximum ductilizing effect and form stable (does not phase separate or form intermetallic) solid solution. We propose to study the alloying behavior in refractory binary alloys first and then narrow it down to ternaries and to higher-order alloys. The refractory elements will be selected from Group-IV (It, Zr, Hf), Group-V(V, Nb, Ta), Group-VI(Mo, W), and Group-VII(Re) elements.

Keywords: stacking fault energy; deformability; ductility; superalloys