  Solid-state homogeneous precipitation of nano-sized precipitates is one of
the most effective processes to strengthen metal alloys, where the final
density and size distribution of precipitates are largely controlled by the
precipitation kinetics. Here, we report a strategy to inoculate the homogeneous
precipitation of coherent precipitates to enhance the precipitation
strengthening. Using the technologically important dilute Al-Zr alloys as an
example, we demonstrate that an addition of a trace level of economical and
readily available, non-L1$_{2}$ phase forming impurity atoms, X (X= Sn, Sb, Bi
or Cd) and Si, can significantly enhance the diffusivity of Zr atoms and
overturn the precipitation of L1$_{2}$-structured Al$_{3}$Zr nanoparticles from
the classical homogeneous nucleation and growth pathway into a nonclassical
nucleation pathway: Al$_{3}$Zr forms through the spontaneous formation of
nano-scale local concentration fluctuations of Zr atoms on Zr-X(-Si)-vacancy
clusters followed by a continuous increase of the concentration and chemical
short-range ordering (CSRO). Such an impurity atoms induced heterogeneous
nucleation based on a "spinodal decomposition like" mechanism dramatically
accelerates the precipitation kinetics, leading to an order of magnitude higher
number density of precipitates and a record high hardening efficiency of solute
Zr atoms. By formulating the generalized selection principles for inoculating
impurity elements, this inoculation strategy should be extendable to a broader
range of materials to further explore the precipitation strengthening
potentials.
