  We study low-salt, binary aqueous suspensions of charged colloidal spheres of
size ratio Phi = 0.57, number densities below the eutectic number density n_E,
and number fractions of p = 1.00-0.40. The typical phase obtained by
solidification from a homogeneous shear-melt is a substitutional alloy of body
centred cubic structure. In strictly gas-tight vials, the polycrystalline solid
is stable against melting and further phase transformation for extended times.
For comparison, we prepare the same samples also by slow and mechanically
undisturbed deionization in commercial slit cells. These cells feature a
complex but well reproducible sequence of global and local gradients in salt
concentration, number density and composition as induced by successive
deionization, phoretic transport and differential settling of the components,
respectively. Moreover, they provide an extended bottom surface suitable for
heterogeneous nucleation of the beta-phase. We give a detailed qualitative
characterization of the crystallization processes using imaging and optical
microscopy. By contrast to the bulk samples, the initial alloy formation in
slit cells is not volume-filling, and we now observe also alpha- and
beta-phases with low solubility of the odd component. In addition to the
initial homogeneous nucleation route, the interplay of gradients opens various
further crystallization and transformation pathways leading to a great
diversity of microstructures. Upon subsequent increase in salt concentration,
crystals melt again. Wall-based, pebble-shaped beta-phase crystals and facetted
alpha-crystals melt last. Our observations suggest that the substitutional
alloys formed in bulk experiments by homogeneous nucleation and subsequent
growth are mechanically stable in the absence of solid-fluid interfaces but
thermodynamically metastable.
