Crystallization-driven structural evolution and its role in corrosion resistance of amorphous Fe–Cr–Ni–(Mo)–B alloys

Fe-based amorphous alloys have attracted considerable interest due to their excellent corrosion resistance, although crystallization may significantly affect their performance. This work investigates the influence of crystallization on the microstructural evolution and corrosion behavior of melt-spun Fe–26Cr–2.5Ni–3.5Mo–4.5B and Fe–33Cr–1Ni–4.5B (wt%) alloys. Rapidly solidified ribbons were heat-treated at 600 and 900 °C and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and electrochemical corrosion tests. Both alloys exhibited fully amorphous structures in the as-spun condition. Upon heat treatment at 600 °C, primary crystallization led to the formation of a ferritic matrix with Fe 3 B borides, whereas at 900 °C the microstructure evolved into a duplex ferrite–austenite matrix containing (Cr,Fe) 2 B. Crystallization followed a non-equilibrium pathway, with metastable Fe 3 B forming at lower temperatures, transforming into (Cr,Fe) 2 B at higher temperatures. The high-Cr alloy, Fe–33Cr–1Ni–4.5B, retains partial passivation even after crystallization and boride formation, in contrast to the Fe–26Cr–2.5Ni–3.5Mo–4.5B alloy. Although the simultaneous differences in Ni and Mo contents prevent attribution of this behavior exclusively to Cr, the retention of passivation despite the absence of Mo is consistent with the proposed role of a larger Cr reservoir in mitigating the effects of Cr consumption during phase transformation.