The constituent phases, the microstructure, and the mechanical properties of a series of Fe87–xTi7Zr6Bx (x = 0, 2, 4, 6, 8, 10, and 12) alloys produced by copper mold casting were investigated. Partial substitution of iron by boron in the Fe87Ti7Zr6 ultrafine eutectic alloy induces phase/microstructural evolution and simultaneously changes the mechanical properties. In the composition range of 2 ≤ x ≤ 6, the typical lamellar structure slightly changes into a spherical cellular-type eutectic. For 8 ≤ x ≤ 12, multiphase composites containing a glassy phase form. The ultrafine eutectic composites exhibit a high compressive strength of ~2.9–3.1 GPa and a distinct plasticity of ~2–8%, whereas the glassy matrix composites show a high strength of ~3.1–3.3 GPa but no observable macroscopic plasticity before failure. These findings reveal that the plasticity of heterogeneous multiphase composites is strongly related to the length scale variables and the crystallinity of the constituent phases.