Fig. 1. Engineering stress-strain curves of Fe-31Mn-3Al-3Si for the mean grain size 0.79 μm and 15.4 μm. The discontinuous yielding characterized by a clear yield drop is indicated by a black arrow.

Fig. 2. EBSD maps of fully recrystallized coarse-grained and UFG samples: (a, b) grain boundary map, (c, d) misorientation distribution histogram, (e, f) grain orientation distribution in the rolling direction. (a), (c), and (e) are from the coarse-grained (15.4 ± 5.2 μm) sample; (b), (d), and (f) are from the UFG (0.79 ± 0.39 μm) sample. Red lines in (a, b) represent high angle boundaries with rotation angle (θ), 15° ≤ θ < 60°, and green lines represent $\text{ }\!\!\Sigma\!\!\text{ }$ 3 boundaries. The number fraction in (c, d) is defined as the ratio of a certain grain boundary to total number of the grain boundaries such as 0.45 (out of 1.0) for $\text{ }\!\!\Sigma\!\!\text{ }$ 3 boundaries in the coarse-grained sample, 0.3 for Σ 3 boundaries in the UFG sample. The maximum texture intensity is relatively small for both samples (e, f) indicating that no strong texture is present.

Fig. 3. Bright field (BF) TEM images show the microstructures and defects in the coarse-grained sample deformed to engineering strain = 0.02. (a) Planar array of dislocations gliding and generating slip transfers across an annealing twin boundary, taken in a two-beam condition with the operative reflection${{\text{g}}_{\bar{1}\bar{1}1}}$. Some dislocations cross-slipped on the annealing twin boundary are indicated by black arrow. (b) Piled-up dislocations being impinged on a twin boundary, taken in a two-beam condition with the operative reflection ${{\text{g}}_{11\bar{1}}}$. (c) A selected area electron diffraction (SAED) pattern from the upper grain showing a streak contrast due to the shape factor of the planar fault. (d) A SAED pattern from the twin boundary. The diffraction spots of the matrix and twin were indexed according to [011]fcc zone axis.

Fig. 4. A series of $\text{g}\centerdot \text{b}$ analyses performed to a grain in the coarse-grained sample deformed to engineering strain = 0.02. Images were taken under five different diffraction conditions around a [011]fcc zone axis. The dark linear contrasts from the partial dislocations on the tip of stacking fault are indicated by arrows filled with dots. The piled-up dislocations are designated to be perfect dislocations with the burgers vector of ${}^{\text{a}}/{}_{2}\text{ }\!\![\!\!\text{ 0}\bar{1}\text{1 }\!\!]\!\!\text{ }$ while the burgers vector of the emitted partial dislocations is ${}^{\text{a}}/{}_{6}\left[ \bar{2}\bar{1}1 \right]$. A Thompson tetrahedron added in (c) indicates two emitted stacking faults from the grain boundary to the adjacent grain. The inclined stacking fault is found to be on plane ACD or ABD, while the edge-on stacking fault is found to be on plane BCD. The operative reflections are (a) ${{\text{g}}_{\bar{1}\bar{1}1}}$, (b) ${{\text{g}}_{0\bar{2}2}}$, (c) ${{\text{g}}_{\bar{1}1\bar{1}}}$, (d) ${{\text{g}}_{\bar{2}00}}$ and (e)${{\text{g}}_{\bar{3}\bar{1}1}}$.

Fig. 5. BF TEM images showing the microstructure and defects in the UFG sample deformed to engineering strain = 0.03. (a) Piled-up dislocations observed in an over-1 μm grain. (b) Two stacking faults generated at a grain boundary (arrows filled with dots) and dislocations formed at a grain boundary (striped arrow) in an under-1 μm grain (size nearly 700 nm). (c) Overlapping stacking faults near a grain boundary and few dislocations in the grain interior in an around-1 μm grain (size nearly 1.2 μm). (d) A magnified view of (c) indicating partial dislocations slipping on fault planes. The operative reflections are (a)${{\text{g}}_{111}}$, (b)${{\text{g}}_{111}}$, (c) ${{\text{g}}_{\bar{2}00}}$ and (d)${{\text{g}}_{\bar{1}3\bar{3}}}$.

Fig. 6. BF TEM images showing the microstructure and defects in the UFG sample deformed to engineering strain = 0.046. (a) Dislocations (striped arrows) appear to be generated from grain interior Frank-read sources in an over-1 μm grain (size nearly 2 μm). (b) A nearly 1 nm thick deformation twin in an around-1 μm grain (size nearly 1.4 μm). The inset HRTEM image shows the atomic structure of the thin deformation twin. (c) Grain boundaries decorated by several deformation twins in an under-1 μm grain (size smaller than 500 nm), (d) a magnified view of a deformation twin (black arrow) in (c), and a corresponding SAED pattern taken near a ${{\text{ }\!\![\!\!\text{ 0}\bar{1}\text{1 }\!\!]\!\!\text{ }}_{\text{fcc}}}$ zone axis showing reflections from the deformation twin.

Fig. 7. The microstructures and defects in the UFG sample deformed to engineering strain = 0.062. (a) Two beam BF TEM image shows wavy and tangled dislocations on multiple slip planes in an around-1 μm grain. The operative reflection is ${{g}_{111}}$. (b) BF TEM image shows various deformation twins in another around-1 μm grain. The inset [011]fcc zone axis SAED pattern is taken from a fine twin. Deformation twins induced by the impingement (white circle) occurred between deformation twin and annealing twin were observed. (c) BF TEM image shows high density of overlapping stacking faults (arrow filled with dots) in an under-1 μm grain (size nearly 500 nm). Streak contrast (white arrows) in the corresponding SAED pattern taken from a ${{\left[ 01\bar{1} \right]}_{\text{fcc}}}$ zone axis is due to the shape factor of the overlapping stacking faults. (d) BF TEM image shows multiple stacking faults (arrow filled with dots) in an under-1 μm grain (size nearly 300 nm). The inset HRTEM image shows a two-layer thick stacking fault (thickness is 0.448 nm, i.e., 2 × {111} planes).

Fig. 8. A schematic illustration shows the precursor of deformation twin initiated at the area near grain boundary as a result of piled-up dislocations and an annealing twin boundary interaction. The Burgers vectors b and ${{\text{b}}_{\text{p}}}$ were designated to be ${}^{\text{a}}/{}_{2}{{\text{ }\!\![\!\!\text{ 0}\bar{1}\text{1 }\!\!]\!\!\text{ }}_{111}}$ and ${}^{\text{a}}/{}_{6}{{\left[ \bar{2}\bar{1}1 \right]}_{\bar{1}1\bar{1}}}$, respectively. The Fujita-Mori twinning model associated with cross-slip of partial dislocations appears to the main twinning mechanism in this coarse-grained TWIP sample.

Fig. 9. The representative initial deformed microstructure in the UFG sample shows a clear correlation between the plastic deformation mechanisms and grain size: (a) grain size > 1 μm; slip dominant, (b) grain size ～1 μm: slip and twinning mixture, and (c) grain size ≤ 1 μm: twinning dominant.