Complex Behavior of Supercooled Metallic Liquids—Composition-Dependent Dynamics and Two-Stage, Non-Classical Crystal Nucleation

Supercooled metallic liquids exhibit complex behavior both in equilibrium and as nucleate and grow crystals. In equilibrium, we have used momentum-resolved electron correlation microscopy to measure the structural relaxation time as a function of composition for two noble metal / transition metal / metalloid alloys, Pt_57.5Cu_14.7Ni_5.3P_22.5 and Pd₄₃Cu₂₇Ni₁₀P₂₀. The experimental results point to a strong role for chemical bonding and composition in determining local spatially heterogeneous dynamics and a weak role for topological structure including high symmetry structures like icosahedra. During isothermal annealing of Pd_77.7Cu₄Si_16.3 in the supercooled liquid state, we observe two-stage, non-classical nucleation using in situ 4D STEM. Instead of a crystal embryo forming directly from the liquid, creating a liquid-crystal boundary, we observe first the formation of a nanometer-diameter object with long persistence time but disordered structure as observed in nanodiffraction. A crystal then forms inside this disordered precursor, consumes it, then grows larger by consuming the liquid. This behavior is similar to previous reports of amorphous precursor particles in crystal nucleation from aqueous solution, but we believe this may be the first experimental observation of non-classical nucleation of a metal crystal from a liquid of the same composition.