Stability of binary nanocrystalline alloys against grain growth and phase separation

Grain boundary segregation has been established through both simulation and experiments as a successful approach to stabilize nanocrystalline materials against grain growth. However, relatively few alloy systems have been studied in this context; these vary in their efficacy, and in many cases the stabilization effect is compromised by second phase precipitation. Here we address the open-ended design problem of how to select alloy systems that may be stable in a nanocrystalline state. We continue the development of a general “regular nanocrystalline solution” model to identify the conditions under which binary nanocrystalline alloy systems with positive heats of mixing are stable with respect to both grain growth (segregation removes the grain boundary energy penalty) and phase separation (the free energy of the nanocrystalline system is lower than the common tangent defining the bulk miscibility gap). We calculate a “nanostructure stability map” in terms of alloy thermodynamic parameters. Three main regions are delineated in these maps: one where grain boundary segregation does not result in a stabilized nanocrystalline structure, one in which macroscopic phase separation would be preferential (despite the presence of a nanocrystalline state stable against grain growth) and one for which the nanocrystalline state is stable against both grain growth and phase separation. Additional details about the stabilized structures are also presented in the map, which can be regarded as a tool for the design of stable nanocrystalline alloys.United States. Army Research Office (Contract W911NF-09-1-0422)United States. Dept. of Energy. Office of Science (Solid-State Solar-Thermal Energy Conversion Center DE-SC0001299

Grain boundary networks in nanocrystalline alloys from atom probe tomography quantization and autocorrelation mapping

A local spatial autocorrelation-based modeling method is developed to reconstruct nanoscale grain structures in nanocrystalline materials from atom probe tomography (APT) data, which provide atomic positions and species, with minimal noise. Using a nanocrystalline alloy with an average grain size of 16 nm as a model material, we reconstruct the three-dimensional grain boundary network by carrying out two series of APT data quantization using ellipsoidal binning, the first probing the anisotropy in the apparent local atomic density and the second quantifying the local spatial autocorrelation. This approach enables automatic and efficient quantification and visualization of grain structure in a large volume and at the finest nanoscale grain sizes, and provides a means for correlating local chemistry with grain boundaries or triple junctions in nanocrystalline materials. Nanoscale grain boundary networks are reconstructed from atom probe tomography data, which provide atomic positions and species for a fraction of atoms within a nanocrystalline material with an average grain size of 16 nm, using a quantization and local spatial autocorrelation-based approach.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Grant W911NF-14-1-0539

Elasticity of Random Multiphase Materials: Percolation of the Stiffness Tensor

Topology and percolation effects play an important role in heterogeneous materials, but have rarely been studied for higher-order tensor properties. We explore the effective elastic properties of random multiphase materials using a combination of continuum computational simulations and analytical theories. The effective shear and bulk moduli of a class of symmetric-cell random composites with high phase contrasts are determined, and reveal shortcomings of classical homogenization theories in predicting elastic properties of percolating systems. The effective shear modulus exhibits typical percolation behavior, but with its percolation threshold shifting with the contrast in phase bulk moduli. On the contrary, the effective bulk modulus does not exhibit intrinsic percolation but does show an apparent or extrinsic percolation transition due to cross effects between shear and bulk moduli. We also propose an empirical approach for bridging percolation and homogenization theories and predicting the effective shear and bulk moduli in a manner consistent with the simulations.National Science Foundation (U.S.) (Contract CMMI-1332789)National Science Foundation (U.S.) (Contract DMR-0346848

Accelerated sintering in phase-separating nanostructured alloys

Sintering of powders is a common means of producing bulk materials when melt casting is impossible or does not achieve a desired microstructure, and has long been pursued for nanocrystalline materials in particular. Acceleration of sintering is desirable to lower processing temperatures and times, and thus to limit undesirable microstructure evolution. Here we show that markedly enhanced sintering is possible in some nanocrystalline alloys. In a nanostructured W–Cr alloy, sintering sets on at a very low temperature that is commensurate with phase separation to form a Cr-rich phase with a nanoscale arrangement that supports rapid diffusional transport. The method permits bulk full density specimens with nanoscale grains, produced during a sintering cycle involving no applied stress. We further show that such accelerated sintering can be evoked by design in other nanocrystalline alloys, opening the door to a variety of nanostructured bulk materials processed in arbitrary shapes from powder inputs.United States. Defense Threat Reduction Agency (Grant HDTRA1-11-1-0062)United States. Army Research Office (Grant W911NF-09-1-0422)United States. Army Research Office (Grant W911NF-14-1-0539)Kwanjeong Educational Foundation (Korea

Nanoscale segregation behavior and high-temperature stability of nanocrystalline W-20 at.% Ti

Nanocrystalline W powders with ∼20 nm average grain size are produced by high-energy ball milling and exposed to a target consolidation temperature of 1100 °C. After 1 week, unalloyed W exhibits substantial grain growth, whereas a W alloy with 20 at.% Ti retains its nanoscale structure. A heterogeneous distribution of Ti is observed by independent characterization methods, including scanning transmission electron microscopy, energy dispersive spectroscopy and atom probe tomography. This heterogeneous solute distribution is different from the expected homogeneous solid solution based on bulk W–Ti phase diagrams. Using a Monte Carlo simulation that includes the possibility of grain boundary segregation and allows grain boundaries as potential equilibrium states, a complex nanoscale structure of Ti around W-rich crystallites is explicitly reproduced. This simulated structure has both grain size and extrema in local Ti content in line with the experimental observations.United States. Defense Threat Reduction Agency (Grant No. HDTRA1-11-1-0062)United States. Army Research Office (Grant No. W911NF-09-1-0422

Anomalous grain refinement trends during mechanical milling of Bi2Te3

The structural evolution of nanocrystalline bismuth telluride (Bi2Te3) during mechanical milling is investigated under different milling energies and temperatures. After prolonged milling, the compound evolves toward a steady-state nanostructure that is found to be unusually strongly dependent on the processing conditions. In contrast to most literature on mechanical milling, in Bi2Te3 we find that the smallest steady-state grain sizes are attained under the lowest energy milling conditions. An analysis based on the balance between refinement and recovery in the steady state shows that two regimes of behavior are expected based on the thermo-physical properties of the milled powder. Bi2Te3 lies in a relatively unusual regime where greater impact energy promotes adiabatic heating and recovery more than it does defect accumulation; hence more intense milling leads to larger steady-state grain sizes. Implications for other materials are discussed with reference to a “milling intensity map” that delineates the set of material properties for which this behavior will be observed.United States. Dept. of Energy. Office of Basic Energy Sciences (Award Number DE-SC0001299

The triple junction hull: Tools for grain boundary network design

Grain boundary engineering (GBE) studies have demonstrated significant materials properties enhancements by modifying the populations and connectivity of different types of grain boundaries within the grain boundary network. In order to facilitate rigorous design and optimization of grain boundary networks, we develop theoretical tools that are based upon a spectral representation of grain boundary network statistics. We identify the connection between a local length scale, embodied by triple junctions, and a global length scale, associated with the grain boundary network configuration as a whole. We define the local state space for triple junctions, A(3)A(3), and enumerate its symmetries. We further define the design space for grain boundary networks, View the MathML sourceMH(3), characterize its important geometric properties, and discuss how its convexity permits grain boundary network design. We also investigate the extent to which the control of texture alone allows one to probe the full design space.United States. Dept. of Energy. Office of Basic Energy Sciences (Award no. DE-SC0008926)United States. Department of Defense (National Defense Science & Engineering Graduate Fellowship Program

Surface Roughness-Controlled Superelastic Hysteresis in Shape Memory Microwires

Superelasticity in Cu–Zn–Al shape memory alloy microwires is studied as a function of surface roughness. Wires with a rough surface finish dissipate more than twice as much energy per unit volume during a superelastic cycle than do electropolished wires with smooth surfaces. We attribute the increased damping in wires with large surface roughness to the increased density of surface obstacles where frictional energy is dissipated as heat during martensitic phase transformation.United States. Army Research Office. Institute for Soldier Nanotechnologie

Correlation-Space Description of the Percolation Transition in Composite Microstructures

We explore the percolation threshold shift as short-range correlations are introduced and systematically varied in binary composites. Two complementary representations of the correlations are developed in terms of the distribution of phase bonds or, alternatively, using a set of appropriate short-range order parameters. In either case, systematic exploration of the correlation space reveals a boundary that separates percolating from nonpercolating structures and permits empirical equations that identify the location of the threshold for systems of arbitrary short-range correlation states. Two- and three-dimensional site lattices with two-body correlations, as well as a two-dimensional hexagonal bond network with three-body correlations, are explored. The approach presented here should be generalizable to more complex correlation states, including higher-order and longer-range correlations