Nanocrystalline Ag-W alloys lose stability upon solute desegregation from grain boundaries

Abstract

Alloying has proven an enabling strategy to stabilize nanocrystalline materials against grain growth, especially in cases where the solute segregates to grain boundaries and lowers their energy. Among such materials reported to date, most all are stable up to some temperature at which second phases precipitate, depleting solute from the boundaries. Here in contrast we present a system that loses stability by thermal desegregation of solute back into solution in the grains. Specifically, we explore minor additions of W (0, 0.3, 1.3, and 1.9 at.%) on the grain structure, grain boundary segregation, and thermal stability of nanocrystalline Ag using transmission electron microscopy and atom probe tomography. W is shown to segregate at grain boundaries in electrodeposited nanocrystalline Ag, pushing the onset temperature for grain growth from ∼200 °C up to ∼300 °C. Upon such heating we observe the dissolution of W off the grain boundaries and back into the FCC host lattice, at a temperature in line with thermodynamic expectations on the basis of the segregation isotherm.