UC Irvine

Nanocrystalline metals achieve strengths far beyond those of their coarse-grained counterparts, but pushing them to even higher performance is complicated by the fact that several chemical and structural strengthening mechanisms operate at once. Solute atoms can strengthen the grain interiors, segregate to grain boundaries, or develop nonuniform intragranular distributions, while nanoscale twins introduce additional internal interfaces, and these pathways are coupled rather than independent. This dissertation uses model nanocrystalline Ni-based thin films, together with nanoindentation, transmission electron microscopy, atomic-resolution scanning transmission electron microscopy and energy dispersive X-ray spectroscopy, and an adapted nanocrystalline solid solution strengthening model, to disentangle how these pathways interact and to identify the variables that ultimately set the achievable strength. First, the combined effects of grain-interior solid solution strengthening and grain boundary segregation are examined in Ni-Cr-Y films, in which Cr strengthens the lattice and Y segregates to the boundaries. Y segregation strengthens the interfaces and suppresses grain-boundary-mediated deformation, extending the range over which Cr remains effective, but it also alters dislocation emission geometry and thereby reduces the efficiency of lattice strengthening; the optimized composition reaches a hardness of 11.0 GPa, among the highest reported for Ni-based alloys. Next, the role of solute spatial distribution is examined in Ni-Re-Y films, where hardness rises from 8.84 GPa without Re to a maximum of 12.14 GPa at 8.7 at.% Re and then falls to 10.48 GPa at 14.0 at.% Re despite continued lattice expansion. High-resolution elemental mapping reveals increasingly heterogeneous intragranular Re distributions at higher Re contents, indicating that nominal concentration alone is an insufficient strengthening descriptor and supporting an effective-solute-availability interpretation. Finally, the influence of nanotwins is studied in a high-throughput Ni-Cr library in which twin thickness varies from 0.8 to 2.2 nm at nearly constant grain size; the strengthening response is captured by a modified model with an effective obstacle spacing of approximately 12 nm set by twin-confined volumes, while the very finest twins instead produce softening. Together, these studies show that solute chemistry, interfacial chemistry, and internal twin structure act through a common length scale, the effective spacing of obstacles to confined dislocation motion, and must be coordinated rather than optimized in isolation when designing high-strength nanocrystalline alloys.