To motivate and validate the new thermodynamic frameworks developed in T1 and T2, we will initially focus on several well-characterized ternary metallic systems: Ni-Al-Mo, Ni-Al-W, Co-Al-W and Cr-Mo-Nb.

These systems have been carefully selected to examine the hypothesis that the stable defect phases will exhibit local structure and chemistry that can be related quantitatively to nearby thermodynamically stable bulk phases. For example, we will examine whether a superlattice intrinsic stacking fault in the L1₂ Co3(Al,W) phase has a local crystal structure similar to the bulk DO19 Co3W phase (stable in the bulk ternary) with a composition that represents a minimum in the defect phase diagram.

With regard to dislocations, a benchmark experiment to be conducted is to bend a single crystal sample at ambient temperature (which introduces geometrically necessary dislocations (GNDs) that are not at a minimum energy state) and then heat the sample to let the dislocations and solute equilibrate into lower energy configurations predicted by T2. 

Much of the characterization work will involve detailed dislocation character determination, using a Medium and Low Angle Annular Dark Field technique (MAADF/LAADF) for which the image simulation capabilities have already been developed by the PI. UCSB has a full processing suite for fabrication of experimental materials, spanning from Bridgman furnaces for growing single crystals to laser powder bed printing systems for generating well-defined dislocation cell structures.

Another area we will explore connects to our understanding of terrestrial planetary interiors, in particular, Earth. Iron-oxide (Fe_1-xO wüstite) is thought to play a central role in the dynamics and thermal regulation of Earth ́s mantle base. The mineral is likely to be present in structures that build up from Earth’s core-mantle boundary and are in direct contact with Earth’s iron-alloy dominant core, so-called ultralow seismic velocity zones. However, many aspects of Fe_1-xO wüstite remain enigmatic, particularly its microstructure and its relationship to macroscopic properties, such as elasticity (propagation of seismic waves) and transport (viscosity, conductivity). 

Many of the unanswered questions will be studied in this MURI program:

• What is the exact nature of the defect structure (energetics, likely configurations)?
• Are the defects associated with a Fe³⁺₂O₃ sublattice?
• What is the relationship of defect structure and order-disorder transition to macroscopic elastic and transport properties, e.g., elasticity, viscosity, and conductivity?
• In particular, considering the likely presence of wüstite or similar iron-oxide-based phases at Earth’s mantle base, how do these aforementioned microscopic properties affect heat transfer from Earth’s core to the mantle (which drives convection, and in turn, tectonic plate motion at the surface)?