The alpha Zr-ZrB2 eutectic, a model system for metal-boride interfaces, was prepared by r.f. induction melting from high-purity zirconium ingots and zirconium diboride powders. At the eutectic composition and depending on the cooling rate, the formation of either the ZrB phase or a Zr(B) solid solution has been observed in addition to the expected compound alpha Zr and ZrB2. For slow cooling rates, the formation of the compound ZrB by a peritectoid reaction and most likely stabilized by light elements (carbon, nitrogen, oxygen) has been observed. After rapid quenching, TEM investigations revealed the formation of a zirconium-based metastable phase; this new phase, with a nearly fee structure, has been found in thin foils and is directly related to hexagonal alpha Zr by a Shoji-Nishiyama orientation relationship. The structure at interfaces with habit planes featured by trigonal symmetry ({0001} for hexagonal and {111} for fcc), has been investigated using weak-beam diffraction contrast and high-resolution transmission electron microscopy. The interfaces with a small difference in lattice parameter are accommodated by a misfit dislocation network, whilst those with a large difference in lattice parameter exhibit a more complex structure with ledges and facets.