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Ke, Xing

Graduate Program in Materials Science

2019

Ph. D.

Silver (Ag) is a precious metal with a low stacking fault energy that is known to form copious nanoscale coherent twin boundaries during magnetron sputtering synthesis. Nanotwinned Ag metals are potentially attractive for creating new interface-dominated nanomaterials with unprecedented mechanical and physical properties. Grain-boundary segregation of solute elements has been found to increase the stability of interfaces and hardness of nanocrystalline metals. However, heavily alloying inevitably complicates the underlying deformation mechanisms due to the hardening effects of solutes, or a change of stacking fault energies in Ag caused by alloying. For the above reasons, we developed a microalloying (or doping) strategy by carefully selecting Cu as the primary impurity--a solute that is predicted to have no solid-solution strengthening effect in Ag when its content is below 3.0 wt.%. Neither will Cu affect the stacking fault energy of Ag at a concentration <1.0 wt.%. Moreover, Cu atoms are ~12% smaller than Ag ones, and Ag-Cu is an immiscible system, which facilitates the segregation of Cu into high-energy interface sites such as grain-boundaries and twin-boundary defects. In this thesis, large-scale hybrid Monte-Carlo and molecular dynamics simulations are used to study the unexplored mechanical behavior of Cu-segregated nanocrystalline nanotwinned Ag. First, the small-scale mechanics of solute Cu segregation and its effects on incipient plasticity mechanisms in nanotwinned Ag were studied. It was found that solute Cu atoms are segregated concurrently to grain boundaries and intrinsic twin-boundary kink-step defects. Low segregated Cu contents (< 1 at.%) are found to substantially increase twin-defect stability, leading to a pronounced rise in yield strength at 300 K. Second, atomistic simulations with a constant grain size of 45 nm and a wide range of twin boundary spacings were performed to investigate the Hall-Petch strength limit in nanocrystalline nanotwinned Ag containing either perfect or kinked twin boundaries. Three distinct strength regions were discovered as twin boundary decreases, delineated by normal Hall-Petch strengthening with a positive slope, the grain-boundary-dictated mechanism with near-zero Hall-Petch slope, and twin-boundary defect induced softening mechanism with a negative Hall-Petch slope. Third, by systematically studying smaller grain sizes, we find that the "strongest" size for pure nanotwinned Ag is achieved for a grain size of ~16 nm, below which softening occurs. The controlling plastic deformation mechanism changes from dislocation nucleation to grain boundary motion. This transition decreases to smaller grain sizes when Cu contents are segregated to the interfaces. Our simulations show that continuous Hall-Petch strengthening without softening, down to grain sizes as small as 6 nm, is reached when adding Cu atoms up to 12 at. %. For Cu contents [greater than or equal to] 15 at. %, however, the predominant plastic deformation mechanism changes to shear-band induced softening. The present thesis provides new fundamental insights into solute segregation, and strengthening mechanisms mediated by grain boundaries and twin boundaries in face-centered cubic Ag metals, which is expected to motivate experimental studies on new nanotwinned metals with superior mechanical properties controlled by microalloying.