Mechanism for step-mediated partitioning in Si-Ge rapidly solidified from its parent melt

We investigate the partitioning behaviour of a Si-30 wt% Ge alloy solidified from its parent melt, both close to equilibrium and under rapid solidification conditions. Contrary to the isomorphous equilibrium phase diagram, we observe step-mediated partitioning, with a number of discrete, Ge-rich, compositions occurring in the grain boundary region. The extent of this heterogeneity becomes greater as the cooling rate increases, with little evidence for solute trapping. Three such step-mediated interfaces are investigated using transmission electron microscopy across a range of cooling rates from < 1 s−1 to in excess of 25000 s−1. Selected area electron diffraction patterns indicate that in all cases the more Ge-rich phase is chemically ordered, again at variance with the equilibrium phase diagram. High-resolution transmission electron microscopy imaging and geometric phase analysis are used to investigate the strain state of these interfaces, leading us to believe that step-mediated partitioning is a means of minimising the lattice strain arising from the atomic size difference between Si and Ge. This is known behaviour in Si-Ge thin films but has gone largely unrecognised in bulk Si-Ge material.