Abnormal notch brittleness induced by short-range ordering in low-cobalt iron-cobalt alloys under tensile and impact loading: A combined experimental and molecular dynamics simulation study

YL Li and DB Yang and WJ Qiang, JOURNAL OF ALLOYS AND COMPOUNDS, 931, 167588 (2023).

DOI: 10.1016/j.jallcom.2022.167588

According to the binary Fe-Co phase diagram, Fe-21.5/23 wt% Co alloys undergo an order-disorder transition in the temperature range of 430-470 degrees C. Above the order-disorder temperature, Fe-21.5/23 wt% Co alloys are generally considered as disordered solid solutions with excellent ductility and toughness. In this work, short-range ordered (SRO) structures in Fe-21.5 wt% Co alloy quenched from alpha phase region were directly observed via scanning transmission electron microscope (STEM). SROs are mainly the B2 structure, with a very small amount of L60Fe3Co structure. The mechanisms of SRO-induced notch brittleness were systematically investigated by experiments and molecular dynamics (MD) simulations. Under tensile loading, SROs change slip mode from cross slip to planar slip, which significantly leads to the strengthening effects and large work hardening rates. For the notch tensile tests, different notch depths were set to investigate the effects of SROs on the notch brittleness. The Fe-23 wt% Co samples quenched from 550 degrees C were surprisingly found to exhibit the notch brittleness induced by SROs. Under impact loading, SROs significantly lead to impact brittleness with the cleavage fracture. MD results of notch tensile and impact tests showed that SRO-induced poor dislocation and twinning plasticity leads to larger concentrated stresses and stress concentration factor near the notch, directly resulting in the easier nucleation and rapid propagation of intergranular and intracrystalline microcracks. Our findings suggest that SRO-induced notch brittleness must be considered in engineering applications for the low-cobalt iron- cobalt alloys. (C) 2022 Elsevier B.V. All rights reserved.

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