Materials Design by Constructing Phase Diagrams for Defects.

Phase transformations and crystallographic defects are two essential tools to drive innovations in materials. Bulk materials design via tuning chemical compositions is systematized using phase diagrams. It is shown here that the same thermodynamic concept can be applied to manipulate the chemistry at defects. Grain boundaries in Mg-Ga system are chosen as a model system, because Ga segregates to the boundaries, while simultaneously improving the strength and ductility of Mg alloys. To reveal the role of grain boundaries, correlated atomic-scale characterization and simulation to scope and build phase diagrams for defects are presented. The discovery is enabled by triggering phase transformations of individual grain boundaries through local alloying, and sequentially imaging the structural and chemical changes using atomic-resolution scanning transmission electron microscopy. Ab initio simulations determined the thermodynamic stability of grain boundary phases, and found out that increasing Ga content enhances grain boundary cohesion, relating to improved ductility. The methodology to trigger, trace, and simulate defect transformation at atomic resolution enables a systematic development of defect phase diagrams, providing a valuable tool to utilize chemical complexity and phase transformations at defects.