Enhancing phase stability, thermodynamic, mechanical, and thermophysical properties of Ni 3 Ti intermetallic alloy through Nb substitution—a first principles study

Abstract The Ni-based intermetallic compounds play a vital role on enhancing properties of superalloy. One such intermetallic compound is Ni 3 Nb which exists in various crystal structures. In the present study, the stability of binary intermediate compound Ni 3 Nb in D0 a and D0 22 structures is assessed via Density Functional Theory and their thermophysical properties are evaluated. Results show that even though the Ni 3 Nb-D0 a phase is the most stable phase at 0 K but the metastable Ni 3 Nb-D0 22 exhibits superior physical properties. In addition, phonon calculations show that entropy and heat capacity of Ni 3 Nb-D0 a are significantly greater than Ni 3 Nb-D0 a structure irrespective of temperature. Another binary Ni-based intermetallic phase that has drawn significant interest is Ni 3 Ti owing to its stability and ability to retain its strength at high temperatures. However, it suffers from low fracture toughness and poor formability. The physical properties of the Ni 3 Ti binary phase can be further altered by substituting Nb at Ti sites in the Ni 3 Ti parent phase for all possible space groups of a chosen structure. However, there is a lack of structural information on the Ni 3 (Ti, Nb) ternary intermediate phase. The structural identification of the stable ternary intermediate compound Ni 3 (Ti 0.67 Nb 0.33 ) is determined at 0 K by considering various possible structures, namely L1 2 , D0 22 , D0 19 , D0 a , D0 24 , and R-phase. Among various chosen structures, the density functional theory calculations show that the Ni 3 (Ti 0.67 Nb 0.33 )-R phase is the most stable structure with the least energy of formation. Physical properties of Ni 3 (Ti 0.67 Nb 0.33 ) are calculated using first principles calculations and compared with their respective binary phases, Ni 3 Nb-D0 a and Ni 3 Ti-D0 24 . The Pugh ratio of Ni 3 (Ti 0.67 Nb 0.33 )-R phase is ∼12% greater than the Ni 3 Ti-D0 24 which implies its greater ductility and formability than the binary phase. The hardness, Young’s modulus, thermal conductivity, and sound velocity of binary and ternary phases is of the order of Ni 3 Ti-D0 24 > Ni 3 (Ti 0.67 Nb 0.33 )-R > Ni 3 Nb-D0 22 > Ni 3 Nb-D0 a . The structural stability of Ni 3 (Ti 0.67 Nb 0.33 ) ternary R-phase at 300 K is further assessed via Ab initio Molecular Dynamics simulations.