Gigacycle Fatigue Resistance and Cyclic Phase Separation in Metastable Nanocrystalline Cu–Ag Alloy Thin Films

Nanocrystalline metals derive their exceptional strength from a high density of grain boundaries (GBs), but under cyclic loading those same boundaries are unstable, driving abnormal grain growth that seeds fatigue cracking. Solute-mediated grain boundary stabilization is shown here to extend nanocrystalline fatigue life by orders of magnitude. A metastable nanocrystalline Cu-42 at.% Ag alloy thin film survives 1.2 billion cycles at strain amplitudes exceeding 1.1%. To our knowledge, no other metallic thin film has shown gigacycle fatigue resistance at such extreme strain amplitudes, while comparison with literature data requires care given differences in testing conditions. A combination of microresonator-based very-high-cycle fatigue testing, electron microscopy, and atomistic simulations reveals a composition-dependent transition in the governing mechanism. In dilute Cu-3Ag, solute clusters kinetically pin interfacial disconnections and slow boundary migration, extending fatigue life but not preventing eventual failure via abnormal grain growth. In contrast, concentrated Cu-42Ag exhibits stable grain boundaries for over a billion cycles. Fatigue damage is ultimately associated with a previously unreported fatigue-induced phase separation: strain-enhanced transport decomposes the metastable solid solution into semi-coherent Cu- and Ag-rich domains. Solute-stabilized nanocrystalline alloy thin films thus offer a route to gigacycle fatigue resistance in microdevices, and cyclic phase separation emerges as a distinct damage mode.