  The reduction of iron ore with carbon-carriers is one of the largest sources
of greenhouse gas emissions in the industry, motivating global activities to
replace the coke-based blast furnace reduction by hydrogen-based direct
reduction (HyDR). Iron oxide reduction with hydrogen has been widely
investigated both experimentally and theoretically. The process includes
multiple types of chemical reactions, solid state and defect-mediated diffusion
(by oxygen and hydrogen species), several phase transformations, as well as
massive volume shrinkage and mechanical stress buildup. In this work, a
chemo-mechanically coupled phase-field (PF) model has been developed to explore
the interplay between phase transformation, chemical reaction, species
diffusion, large elasto-plastic deformation and microstructure evolution.
Energetic constitutive relations of the model are based on the system free
energy which is calibrated with the help of a thermodynamic database. The model
has been first applied to the classical core-shell (w\"ustite-iron) structure.
Simulations show that the phase transformation from w\"ustite to alpha-iron can
result in high stress and rapidly decelerating reaction kinetics. Mechanical
stresses can contribute elastic energy to the system, making phase
transformation difficult. Thus slow reaction kinetics and low metallization are
observed. However, if the stress becomes comparatively high, it can shift the
shape of the free energy from a double-well to a single-well case and speed up
the transformation. The model has been further applied to simulate an actual
iron oxide specimen with its complex microstructure, characterized by electron
microscopy. The simulation results show that isolated pores in the
microstructure are filled with water vapor during reduction, an effect which
influences the local reaction atmosphere and dynamics.
