A Study of Interdiffusion in the Fe-C/Ti System Under Equilibrium and Nonequilibrium Conditions

In the present study, diffusion behavior under equilibrium and nonequilibrium conditions in a Fe-C/Ti system is studied in the temperature range of 773 K to 1073 K (500 °C to 800 °C). A defect-free weld joint between mild steel (MS) (Fe-0.14 pct C) and Ti Grade 2 obtained by friction welding is diffusion annealed for various durations to study the interdiffusion behavior under equilibrium conditions, while an explosive clad joint is used to study interdiffusion under nonequilibrium conditions. From the elemental concentration profiles obtained across the MS-Ti interface using electron-probe microanalysis and imaging of the interface, the formation of distinct diffusion zones as a function of temperature and time is established. Concentration and temperature dependence of the interdiffusion coefficients (D(c)) and activation energies are determined. Under equilibrium conditions, the change in molar volume with concentration shows a close match with the ideal Vegard’s law, whereas a negative deviation is observed for nonequilibrium conditions. This deviation can be attributed to the formation of secondary phases, which, in turn, alters the D(c) values of diffusing species. Calculations showed that the D ₀ and activation energy for interdiffusion under equilibrium is on the order of 10⁻¹¹ m²/s and 147 kJ/mol, whereas it is far lower in the nonequilibrium case (10⁻¹⁰ m²/s and 117 kJ/mol) in the compositional range of 40 to 50 wt pct Fe, which also manifests as accelerated growth kinetics of the different diffusion zones.