Impact brittleness induced by rapid grain growth in Fe-23Co soft magnetic alloy

Abstract

Long- and short-range ordering (L/SRO) in low-cobalt iron-cobalt alloys significantly lead to notch and impact brittleness. High-temperature heat treatment suppresses LROs and SROs, but it suffers from rapid grain growth, which also deteriorates mechanical properties. A critical challenge in developing low-cobalt Fe-Co alloys lies in suppressing both ordering-induced embrittlement and grain coarsening during high-temperature processing. In this work, effects of grain growth on the mechanical properties of γ-quenched Fe-23 wt.% Co alloy were studied (free of LROs and SROs). After heat treatment for different time, both the average grain size and size distribution increased with increasing holding time, resulting in impact brittleness characterized by a cleavage fracture mode. In comparison to tensile properties, the deformation and fracture behavior of impact samples is significantly influenced by grain growth. When the average grain diameter exceeds 100 μm, the impact property transitions from ductile fracture to brittle fracture. However, when the average grain size does not exceed 190 μm, the tensile properties continue to exhibit favorable strength and plasticity. Our findings indicate that the brittleness observed in Fe-23Co samples quenched from the γ phase region is mainly caused by the rapid grain growth. The Fe-23Co alloy, characterized by good impact toughness and tensile plasticity, can be achieved by quenching after a short holding time from the γ phase region.