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Abstract Refractory complex concentrated alloys (RCCAs) show potential as the next-generation structural materials due to their superior strength in extreme environments. However, RCCAs processed by metal additive manufacturing (AM) typically suffer from process-related challenges surrounding laser material interaction defects and microstructure control. Multimodalin situtechniques (synchrotron X-ray imaging and diffraction and infrared imaging) and melt pool-level simulations were employed to understand rapid solidification pathways in two representative RCCAs: (i) multi-phase BCC + HCP Ti_0.4Zr_0.4Nb_0.1Ta_0.1and (ii) single-phase BCC Ti_0.486V_0.375Cr_0.111Ta_0.028. As expected, laser material interaction defects followed similar systematic trends in process parameter space for both alloys. Additionally, both alloys formed a single-phase (BCC) microstructure after rapid solidification processing. However, significant differences in microstructure selection between these alloys were discovered, where Ti_0.4Zr_0.4Nb_0.1Ta_0.1showed a mixture of equiaxed and columnar grains, while Ti_0.486V_0.375Cr_0.111Ta_0.028was dominated by columnar growth. These behaviors were well described by the influence of undercooling effects on columnar-to-equiaxed transition (CET). Distinct microstructure formation in each alloy was verified through CET predictions via analytical melt pool simulations, which showed a ~ 5 × increase degrees in undercooling for Ti_0.4Zr_0.4Nb_0.1Ta_0.1compared to Ti_0.486V_0.375Cr_0.111Ta_0.028. Overall, these results show that microstructure control based on modulating the freezing range must be balanced with process considerations which resist defect formation, such as solidification crack formation in RCCAs. Graphical abstract 

Despite the critical role of sintering phenomena in constraining the long-term durability of nano-sized particles, a clear understanding of nanoparticle sintering has remained elusive due to the challenges in atomically tracking the neck initiation and discerning different mechanisms. Through the integration of in-situ transmission electron microscopy and atomistic modeling, this study uncovers the atomic dynamics governing the neck initiation of Pt-Fe nanoparticles via a surface self-diffusion process, allowing for coalescence without significant particle movement. Real-time imaging reveals that thermally activated surface morphology changes in individual nanoparticles induce significant surface self-diffusion. The kinetic entrapment of self-diffusing atoms in the gaps between closely spaced nanoparticles leads to the nucleation and growth of atomic layers for neck formation. This surface self-diffusion-driven sintering process is activated at a relatively lower temperature compared to the classic Ostwald ripening and particle migration and coalescence processes. The fundamental insights have practical implications for manipulating the morphology, size distribution, and stability of nanostructures by leveraging surface self-diffusion processes. 

One-dimensional lepidocrocite, 1DL, titania, TiO2, is a recently discovered form of this ubiquitous oxide that is of interest in a variety of applications ranging from photocatalysis to water purification, among others. The fundamental building blocks of these materials are snippets (30 nm long) of individual 1DLs that self-assemble into nanobundle, NB, structures. These NBs can then be driven to self-assemble into quasi-two-dimensional, 2D, sheets, films, or free-flowing mesoscopic particles. Here, we use analytical atomic-resolution scanning transmission electron microscopy (STEM) and first-principles density functional theory (DFT) calculations to demonstrate that the arrangement of the neighboring NFs can be altered through ion exchange with Li, Na, and tetramethylammonium hydroxide (TMA) cations. Moreover, using cryogenic electron energy-loss spectroscopy (EELS), we show that the introduction of different ion species results in a change in the local occupancy of the TiO2 t2g and eg orbitals. Both experimental findings are predicted by ground-state energy simulations of two-dimensional lepidocrocite TiO2.