Crystallization behavior and machining properties of annealed Fe–Si–B–Cr amorphous alloys

We have systematically prepared diverse microstructures by annealing B-rich Fe–Si–B–Cr amorphous sheets to obtain the optimum mechanical property and cutting machinability of the alloy. Thermal, structural, and mechanical analyses showed that the early reaction sequence of the amorphous alloy upon annealing is characterized by structural relaxation, heterogeneous nucleation of the α-Fe(Si) phase at the surface, and homogeneous nucleation of metastable Fe3B core, which is enclosed by the α-Fe shell. The development of the core–shell structure, approximately 200 nm, is governed by the repeated partitioning out of Si and B from the Fe3B and α-Fe phases, respectively. The hardness and specific cutting resistance force (SCRF) were found maximum for the alloy annealed to 873 K, which is filled with the aforementioned core–shell crystalline phases. On the other hand, the best cutting performance with minimum burrs and chips, and the lowest SCRF, was realized for the alloy annealed at 763 K, which exhibits a heterogeneous microstructure, where the core–shell units are finely dispersed in the amorphous matrix.