Effect of Mn content on the austenite–martensite phases and magnetic properties in Fe–Mn–Co alloys

The influence of the Mn content on the magnetic properties and microstructure of the Fe–Mn–Co alloys has been investigated by means of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Mössbauer spectroscopy. Experiments reveal that two types of thermal-induced martensites, ɛ (h.c.p.) and α′ (b.c.c.) martensites, form in the as-quenched alloys. ɛ and α′ martensites coexist in the Fe–Mn–Co alloys with 13.2–17.3 wt% Mn content while only ɛ martensite appears in the case of 20.7 wt% Mn. The amount of α′ martensite decreases significantly while the amount of ɛ martensite increases with an increase in Mn content. The lattice parameters of austenite increase also with increasing Mn content. In addition, Mössbauer spectra of the alloys reveal a paramagnetic character with a singlet for the γ (f.c.c.) austenite and ɛ martensite phases and a ferromagnetic character with a broad sextet for α′ martensite phase. The magnetic character of the Fe–Mn–Co alloys changes as Mn content increases and the ferromagnetic character disappears completely in case of 20.7 wt% Mn.