The composition and temperature dependencies of deformation response of TWIP and XIP steels were investigated under high-velocity impact loading with a focus on micro-scale deformation mechanisms. The promotion of twinning deformation under high-velocity loading over the slip-twin interactions usually observed in low-velocity loading conditions was comprehensively examined with scanning electron microscopy and transmission electron microscopy. In addition, thermal analyses of plastic deformation were carried out by in situ thermal imaging. The current findings demonstrate that the deformation of TWIP steel is dictated by two major twin systems at elevated temperatures, while nano-twin formation within one primary twin system dominates at subzero temperatures. The XIP steel, on the other hand, deforms mainly by slip at elevated temperatures, while competing slip and twin activities, and eventually nano-twin formation within primary twins dominates as the temperature decreases. Overall, the current findings shed light on the complicated work hardening mechanisms prevalent in high-manganese austenitic steels utilizing high-velocity deformation experiments. (C) 2015 Elsevier B.V. All rights reserved.