UC Berkeley

Chemical short-range order (SRO) of atoms within a nominally single-phase solid solution is known to significantly impact the mechanical properties of alloys. While associated phenomena have been indirectly revealed, direct observation of the SRO microstructure at the nanoscale remains unexplored. In the current dissertation we show the development of methods to directly observe SRO in several model alloys using energy-filtered Transmission Electron Microscopy (TEM). Our observations reveal the distribution of SRO domains and verified that the degree of SRO could be tailored by the thermomechanical history of the alloys. The SRO distribution is discussed in light of the deformation features formed both ex-situ and in-situ in the TEM. The ability to conduct quantitative analysis of SRO should give us a new method to engineer desirable performance of structural alloys. A synergistic study of the mechanical properties of Ti-Al and CrCoNi alloys were carried out to examine their correlations to the degree of SRO. The SRO induced deformation features, including SRO hardening, wavy to planar slip transition, diffuse anti phase boundary and planar slip assisted deformation twinning, were observed from bulk to nanometer scales. We demonstrated that with the presence of SRO, the dislocation dynamics during the deformation would exhibit a tendency of planar configuration with subsequent deformation localization and reduced work-hardening ability. Additionally, in the CrCoNi alloy, the local stacking fault energy (SFE), which is directly related to the deformation behaviors of alloys, could be significantly altered by changing the SRO state. Subsequent impact on the competition of dislocation and deformation twinning is probed in terms of the microstructures and density functional theory calculations. Novel characterization techniques based on nanobeam electron diffraction were developed to enable full strain tensor analysis. In summary, the current dissertation presents a set of methods to image SRO in alloys and correlate the SRO to mechanical properties. The results provide insight into future alloy development. The techniques developed could be potentially applied to the broader materials science community to facilitate materials characterization in emerging topics like high-entropy ceramics.