Nb solute behavior and its effect on grain size stabilization in Cu-Nb alloys was studied using a combination of Vickers hardness testing, x-ray diffraction measurements, transmission electron microscopy and atom probe tomography (APT). Cu-Nb alloys with concentrations in the range from 1 to 10 at. % Nb were studied after annealing at 400°C and 800°C. The grain growth resistance at both temperatures increased with an increase in Nb solute content. For instance, after annealing at 800°C (0.74 T m), Cu-1Nb, Cu-5Nb and Cu-10Nb have a grain size that is ~8, ~14 and ~14 times respectively smaller than that of unalloyed Cu. This resistance is attributed to the formation of Nb-oxide-based clusters, elemental Nb segregation zones and large elemental (Nb)-based precipitates as observed by APT. The Nb-oxide-based clusters are the precursors of phase separation and form due to a reaction with oxygen, which is a contaminant from the milling process. Once the oxygen is consumed, the process continues and the grain boundaries accumulate more solute and begin to thicken into elemental Nb segregation zones. Eventually, Nb solute phase separates and forms Nb-based precipitates. After annealing at 400°C and 800°C, Cu-5Nb has a hardness which is approximately 2.5 times and 3 times respectively that of the hardness of unalloyed Cu after an equivalent anneal. This increase has been attributed to Hall-Petch strengthening and precipitation strengthening.

A cross-correlative precession electron diffraction – atom probe tomography investigation of Cr segregation in a Fe(Cr) nanocrystalline alloy was undertaken. Solute segregation was found to be dependent on grain boundary type. The results of which were compared to a hybrid Molecular Dynamics and Monte Carlo simulation that predicted the segregation for special character, low angle, and high angle grain boundaries, as well as the angle of inclination of the grain boundary. It was found that the highest segregation concentration was for the high angle grain boundaries and is explained in terms of clustering driven by the onset of phase separation. For special character boundaries, the highest Gibbsain interfacial excess was predicted at the incoherent ∑3 followed by ∑9 and ∑11 boundaries with negligible segregation to the twin and ∑5 boundaries. In addition, the low angle grain boundaries predicted negligible segregation. All of these trends matched well with the experiment. This solute-boundary segregation dependency for the special character grain boundaries is explained in terms of excess volume and the energetic distribution of the solute in the boundary.

Microstructure and phase evolution in magnetron sputtered nanocrystalline tungsten and tungsten alloy thin films are explored through in situ TEM annealing experiments at temperatures up to 1000°C. Grain growth in unalloyed nanocrystalline tungsten transpires through a discontinuous process at temperatures up to 550°C, which is coupled to an allotropic phase transformation of metastable b-tungsten with the A-15 cubic structure to stable body centered cubic (BCC) a-tungsten. Complete transformation to the BCC a-phase is accompanied by the convergence to a unimodal nanocrystalline structure at 650°C, signaling a transition to continuous grain growth. Alloy films synthesized with compositions of W-20 at.% Ti and W-15 at.% Cr exhibit only the BCC a-phase in the as-deposited state, which indicate the addition of solute stabilizes the films against the formation of metastable b-tungsten. Thermal stability of the alloy films is significantly improved over their unalloyed counterpart up to 1000°C, and grain coarsening occurs solely through a continuous growth process. The contrasting thermal stability between W-Ti and W-Cr is attributed to different grain boundary segregation states, thus demonstrating the critical role of grain boundary chemistry in the design of solute-stabilized nanocrystalline alloys. transition with a focus on harsh environment sensors produced by additive manufacturing processes. Professor Trelewicz's research is on the science of interface engineered alloys with particular emphasis on high-strength and radiation-tolerant nanomaterials for extreme environment applications. His group couples in situ and analytical characterization tools with large-scale atomistic simulations to explore the thermal stability, mechanical behavior, and radiation tolerance of solute-stabilized nanocrystalline alloys, crystalline-amorphous nanolaminates, metallic glass matrix composites, and other unique hierarchical metallic structures. Professor Trelewicz is a recipient of the 2017 DOE Early Career

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A transition of deformation modes from shear banding to co-deformation subject to nanoindentation was revealed by a systematic experimental study of multilayers of amorphous-Cu45Zr55 (at/%)/crystalline-Cu. The Cu45Zr55 was fixed at 150 nm where the Cu layers varied from 5 nm to 150 nm. At the 5 nm Cu layer thickness, the shear bands propagated through both layer types physically splitting the Cu layers. Upon increasing the Cu to 25 nm, the shear bands were able to propagate through the amorphous layer but only locally bend the Cu layers. At the 150 nm Cu layer thicknesses, the two phases co-deformed without clear evidence of shear propagation through the multilayer structure. Using molecular dynamics simulations, the spatial correlation of the shear transformation zones in the amorphous layers as a function of various Cu thicknesses was investigated. The simulations revealed a percolation created by the indent impression of the strain localization initiated in the amorphous layers above and below the Cu layer prior to shear banding. This spatial correlation condition was suspected to shear the Cu layer from both sides if the Cu layer is sufficiently thin.

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