MTech(Res) Thesis Colloquium: Mr.Shiba Sankar Dash (07/08/26)
Thesis title:
Multiscale Modeling of Stress Effects on Phase Equilibrium in the Ni–H System
Faculty advisor(s):
Prof. Karthikeyan Subramanian
When?
07th August, 2026 (Friday), 04:30 PM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract
Hydrogen dissolved in nickel strongly influences performance of nickel-based alloys. In this work, a first-principles based study of phase equilibrium was carried out in the Ni–H interstitial solid solution under internal stresses. The main goals were to probe the conditions under which nickel hydrides would form, and if they do whether they would do so by nucleation and growth, or by spinodal decomposition. The results were then used to understand segregation of hydrogen, and formation of hydrides to microstructural sources of stress (such as dislocations, cracks, etc). Spin-polarized density functional theory calculations were performed for compositions spanning dilute hydrogen to full occupation of octahedral interstitial sites, for a range of strains. it was found that since hydrogen occupied octahedral sites, their effect on the thermodynamics of the system is only sensitive to the hydrostatic component of stress. Energy–volume data from firs-principles calculations described using the Birch–Murnaghan equation of state, enabled pressure-dependent thermodynamic properties to be obtained from −20 to +20 GPa. The calculations reveal that the presence of hydrogen caused significant lattice expansion while decreasing the shear modulus. . Pressure-dependent mixing enthalpies were represented using a sub-regular Redlich–Kister model, while configurational entropy was treated using an ideal interstitial lattice-gas model. Chemical free energies were used to determine binodal, chemical and coherent spinodal boundaries, showing that compression broadens the miscibility gap and raises the critical temperature, whereas tension suppresses phase separation. Under sufficiently high tensile stresses, the coherent spinodal disappears, i.e., there is a tendency for phase separation via nucleation and growth, instead of spinodal.