It has recently been established that vacancy loops of ~100 nm in maximum size could form in hydrogen pre-implanted α-Fe (Huang et al. Fusion Eng. Des., 2010). The formation temperature of these large loops is composition dependent, varying from ~500 °C in α-Fe, to ~450 °C in Fe[sbnd]Ni and ~ 620 °C in Fe[sbnd]Cr (Wan et al. J. Nucl. Mater., 2014; Du et al. Acta Metall. Sin., 2019). In this work, the effect of Mn addition upon vacancy loop formation has been investigated. Pure Fe and Fe-1.4 wt% Mn alloy specimens were irradiated with 30 keV H+ ions at room temperature. Radiation-induced loop production, size, population and nature were characterized based on diffraction contrast imaging using a combination of 1 MV high voltage electron microscope (HVEM) and 200 kV conventional transmission electron microscope (CTEM). Results of observation indicate that Mn addition gives rise to the increase of vacancy loop formation temperature in α-Fe, similar to the effect of Cr. A new criterion has been thus proposed, to classify the common alloying elements (Cr, Ni, Mn etc) found in Fe.
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