We present molecular dynamics calculations for a two-dimensional binary alloy composed of soft disks. We start the simulation with a system of 108 identical particles at low temperature arranged in a close-packed triangular lattice. Then the diameter ratio lambda is varied in steps while the temperature is kept constant. We define a set of order parameters suitable for detecting the existence of ordered binary domains over a wide range of A-values. The phase diagram thus obtained for the range 0.3<or= lambda <or= lambda 1 shows two ordered phases: one FCC structure below lambda =0.5 and a new structure, not previously reported, for 0.5<or= lambda <or=0.85. This is not expected from Hume-Rothery rules but is in remarkable agreement with experimental results for colloidal systems. Both transitions are first-order ones; we also study hysteresis and metastable states.