Precipitation and hardening behaviour of the Fe20Co18W alloy aged at 800 °C

The hardness and the microstructure evolution are studied in the Fe20Co18W (mass %) alloy aged at 800 °C after a solution treatment at 1380 °C. The hardness strongly increases at the onset of the annealing treatment then decreases. The precipitation sequence is characterized using X-ray diffraction and transmission electron microscopy. Several phases are identified during the annealing: nanometric W rich clusters, FeCoW ordered domains, nanometric plate shaped precipitates of Laves phase then micrometric Laves phase precipitates. The specific effect of Co addition on mechanical properties of FeCoW alloys is likely due to the formation of the FeCoW ordered compound at the beginning of aging and to the larger stability domain of the Laves phase in these alloys.

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► Annealing the Fe20Co18W alloy at 800 °C significantly increases its hardness.
► W-rich clusters, ordered domains and nano-sized then micron-sized C14 precipitates form on aging.
► The measured peak in hardness is related to nano-sized phases.
► The precipitates are identified owing to transmission electron microscopy and X-ray diffraction.
► Co addition increases the stability domain of the C14 phase in the Fe-Co-W system.