Compressive Behavior of an Fe-Mn-Al-Mo-C Lightweight Steel at Different Strain Rates

Compression tests over a wide range of strain rates were performed on cold-rolled and annealed, as well as annealed-and-aged, Fe-26Mn-8Al-1.2C-3Mo steel to elucidate the roles of Mo2C precipitation and κ/Mo2C co-precipitation in microstructural evolution and compressive behavior. The annealed microstructure consists of partially deformed and equiaxed ultrafine recrystallized austenite grains, with Mo2C carbides uniformly dispersed throughout the matrix. Aging at 550 °C induces nanoscale spherical κ carbides, while the size and spacing of Mo2C particles remain essentially unchanged. Both conditions exhibit pronounced strain-rate strengthening, primarily attributed to intensified dislocation–carbide interactions. In the annealed state, deformation is dominated by dislocation bypassing of Mo2C carbides, resulting in discontinuous slip microbands. After aging, κ-carbide precipitation facilitates slip-band propagation and promotes interactions among adjacent slip bands. As the strain rate increases from 10−3 to 100 s−1, dislocation density increases, slip-band propagation is hindered, and the strain-hardening rate decreases. At 103 s−1, adiabatic thermal softening becomes significant, leading to a further reduction in strain hardening. Overall, aging increases strength but reduces strain-rate sensitivity in the low strain-rate regime due to κ-carbide-induced slip-plane softening and an increased effective slip distance.