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Cryogenic mechanical alloying is a viable method to synthesize nanostructured alloys that exhibit improved mechanical properties without using highly contaminating, process control agents. However, cryogenically milled alloys still contain impurities introduced from the milling media and cryogenic fluid, and it is unclear how these milling parameters can be tailored to optimize alloy design. Here, milling media and cryogenic fluid were systematically varied and studied to quantify differences in impurity concentrations, microstructural evolution, and microhardness. Four derivatives of a Mo25Nb25Ta25W25 high entropy alloy were mechanically alloyed using tool steel or tungsten carbide milling media with either liquid N2 or Ar. Alloys were annealed for 100 h at 1000 °C or 1200 °C. Aberration-corrected scanning transmission electron microscopy (STEM) was primarily used to characterize as-milled and annealed specimens, and Vicker’s microindentation was used to compare hardness. Depending on the milling parameters, total impurity concentrations varied between 12 and 44 at.%, different impurity nitrides or carbides were identified in annealed specimens, and differences in hardness of up to 5 GPa were measured amongst the alloys. Overall, milling with LN2 led to the precipitation of ultrahard nitride phases that when combined with optimized heat treatments, improved the hardness beyond 17 GPa.

Mathematical models in process engineering are becoming an essential part of accelerated materials by design, drastically shortening time to discovery by minimizing the vast experimental matrix. In this paper a combination of thermodynamic and analytical models have been used to develop an algorithm for visually guiding the rapid design of a nontoxic (cobalt free) liquid metal binder for WC based cermets. A method based on rapid screening method of key processing parameters is developed that significantly reduces the design space (here, to 27% of the initial alloy composition space). The parameters included represent heretofore unexamined considerations such as chemistry effects on capillary flow, viscosity, surface tension, and melting point which are less computationally intensive than the traditional thermodynamic modeling used in this problem to date. The results are presented in a series of easy to interpret atlases that enable quick identification of systems of interest for further study. Keywords: Cermet, Capillary, Materials by design

An immiscible nanocrystalline (NC) copper-tantalum (Cu-Ta) alloy is shown to exhibit a stable microstructure under thermo-mechanical loading conditions with exceptional mechanical strength (i.e., 1200 MPa strength at 298 K) indicating anomalous deformation mechanisms as compared to microstructurally unstable nanocrystalline materials. Therefore, in this work, various aspects of strength partitioning in such NC Cu-Ta alloys are discussed and the role of tantalum nanoclusters on the dominant deformation mechanism is presented as a function of temperature. Toward this, initially, the mechanical responses of NC Cu-Ta alloy were measured under uniaxial compression experiments at various temperatures. Later, atomistic simulations were performed along with the high-resolution electron microscopy to identify and validate the rate limiting mechanism behind the plastic deformation in NC Cu-Ta alloys. In general, the observed trend through experiments and simulations identify a transition from a dislocation – nanocluster interaction mediated deformation mechanism to one controlled by grain boundary strengthening as the temperature increases. The former mechanism is shown here to have a crucial role in the observed strengthening behavior of microstructurally stable NC materials. Overall, the paper demonstrates that through effective nano-engineering techniques, it is expected to extend the scope of nanocrystalline materials to a number of engineering design applications. Keywords: Nanocrystalline, Deformation, Transmission electron microscopy, Atomistic

Under intense heating and/or deformation, pure nanocrystalline (NC) metals exhibit significant grain coarsening, thus preventing the study of length scale effects on their physical response under such conditions. Hence, in this study, we use in-situ TEM heating experiments, atomistic modeling along with elevated temperature compression tests on a thermally stabilized nanostructured Cu–10 at.% Ta alloy to assess the microstructural manifestations caused by changes in temperature. Results reveal the thermal stability attained in NC Cu-10 at.% Ta diverges from those observed for conventional coarse-grained metals and other NC metals. Macroscopically, the microstructure, such as Cu grain and Ta based cluster size resists evolving with temperature. However, local structural changes at the interface between the Ta based clusters and the Cu matrix have a profound effect on thermo-mechanical properties. The lattice misfit between the Ta clusters and the matrix tends to decrease at high temperatures, promoting better coherency. In other words, the misfit strain was found to decrease monotonically from 12.9% to 4.0% with increase in temperature, leading to a significant change in flow stress, despite which (strength) remains greater than all known NC metals. Overall, the evolution of such fine structures is critical for developing NC alloys with exceptional thermo-mechanical properties. Keywords: In situ TEM, Nanocrystalline, Atomistic, Misfit strain