Abstract: The ​“phase space” for materials discovery is much larger in systems far from equilibrium due to the large number of metastable phases that can be accessed. Understanding and controlling phase and morphological selection for the numerous metastable phases requires probing the details of the atomic scale processes. Accomplishing this requires better integration of experimental and characterization tools with the development of theoretical methods that can account for nonequilibrium driving forces and kinetic effects. State-of-the-art aberration-corrected scanning transmission electron microscopy (AC/STEM) provides the requisite chemical and spatial sensitivity; however, in operando studies also require precise temperature control and positional stability during data acquisition.

Using the FEI NanoExTM-i/v MEMS microheater in a FEI Titan Themis 300 Cubed 300 STEM, we have been able to precisely measure the nucleation and growth of metastable phases and the atomic-scale attachment kinetics of an amorphous binary alloy. Subsequent solid-solid phase transformations were likewise characterized. A second topic will present in situ processing of a mixed-phase platinum and SnO nanoparticles to form a stable PtSn intermetallic. This was accomplished, in part, by employing a series of heating/cooling cycles of the sample where the positional reproducibility is within a few nanometers and the temperature variability is <0.1 K. This allows for precise measurements of interface mobility, solute segregation, and coupled growth involving nanoscale defects that were only revealed by direct observations. Challenges and opportunities for such detailed studies will be discussed, including the effects of sample preparation on the observed phase selection process.