Effect of κ-carbides on the mechanical properties and superparamagnetism of Fe–28Mn–11Al-1.5/1.7C–5Cr lightweight steels

The effect of κ-carbides on the mechanical properties and superparamagnetism of Fe–28Mn–11Al-1.5/1.7C–5Cr (wt.%) lightweight steels in hot-rolled and solution-treated samples were investigated by scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), X-ray diffraction (XRD), transmission electron microscopy (TEM), vibratory sample magnetometer (VSM) and magnetic force microscopy (MFM). The grain sizes of the hot-rolled samples are similar. Compared with 1.5C steel, 1.7C steel not only has more and larger intragranular κ-carbides but also submicron-sized intergranular κ0-carbides, which leads to the higher strength. At the initial stage of deformation, the slip bands are gradually refined, and the smaller spacing of slip band in 1.7C steel results in a higher strain hardening rate than 1.5C steel. When the strain reaches or exceeds 25%, the microbands only arise in 1.7C steel, while three non-coplanar slip bands appear in 1.5C steel. After solution treatment at 1100 °C for 30min, the grain size of the solution-treated samples grows significantly, but is still close. The volume fraction and average size of intragranular κ-carbides are reduced, and the intergranular κ0-carbides are mostly dissolved in 1.7C steel. The strength of two lightweight steels is decreased but the elongation increased obviously. Meanwhile, the yield phenomenon of 1.5C steel disappears because the precipitation strengthening of κ-carbides is no longer the most significant strengthening mechanism. The results show that the volume fraction and average size of intragranular κ-carbides in 1.7C steel are both higher than that in 1.5C steel, and the intergranular κ0-carbides are only formed on the grain boundaries of 1.7C steel. The precipitation strengthening of κ-carbides is the most significant strengthening mechanism except for the solution-treated sample of 1.5C steel, which leads to yield phenomena. At the initial stage of deformation, the slip bands of the hot-rolled samples are gradually refined, and the spacing between slip bands of 1.7C steel is smaller than 1.5C steel. When the strain reaches or exceeds 25%, the microbands only arise in 1.7C steel, while three non-coplanar slip bands appear in 1.5C steel. The difference of deformation mechanism between the two samples is due to the presence of submicron-sized intergranular κ0-carbides and the size effect of intragranular κ-carbides. In addition, the hysteresis loops of two lightweight steels at room temperature exhibit superparamagnetic property due to the single domain structure of κ-carbides. Moreover, the values of saturation magnetization are proportional to the volume fraction of intragranular κ-carbides. The blocking temperature increases with the decrease of the intragranular κ-carbides volume fraction and average size because of the effect of shape anisotropy and surface anisotropy.