On the mechanism of twin formation in Fe–Mn–C TWIP steels

Microstructure evolution and strengthening mechanisms of Fe–23Mn–0.3C–1.5Al TWIP steel during cold rolling

“…In general, the dislocation density in TWIP steels is one order of magnitude higher than that in other fcc metals deformed to the same strain [8]. The extensive mechanical twinning subdivides the initial grains into separate crystallites bounded by twin boundaries, which reduces the mean free path of lattice dislocation, leading to extraordinary accumulation of lattice dislocations [2,5,8,22,23]. In addition, the DSA and the slip-twin interactions hinder dynamic recovery [8].…”

“…Generally, an increase in the yield strength of TWIP steels by cold working is associated with grain boundary strengthening, strain hardening and dynamic strain aging (DSA) [2][3][4][5][6]8,9,[15][16][17]. In high-Mn TWIP-steels, DSA is a type of solid solution strengthening attributed to the formation of interstitial C -substitutional Mn dipoles interacting strongly with dislocations or stacking faults.…”

“…Austenitic steels with high Mn content exhibit the most attractive combination of tensile strength and superior ductility due to their extraordinary strain-hardening rate, which is interpreted in terms of twinning-induced plasticity (TWIP) [2][3][4][5][6]. It is known that in face-centered cubic (fcc) metals with low stacking fault energies (SFEs),extensive deformation twinning results in the formation of deformation twins with a plate-like shape as well as nanometer spacing and thickness [2,[7][8][9]. The twin boundaries, which are special (low ÀΣ coincidence site lattice) high-angle boundaries [7] and contain a high density of sessile dislocations [8], act as equally effective obstacles to dislocation gliding as conventional grain boundaries in polycrystalline austenitic steels [10].…”

“…It is known that in face-centered cubic (fcc) metals with low stacking fault energies (SFEs),extensive deformation twinning results in the formation of deformation twins with a plate-like shape as well as nanometer spacing and thickness [2,[7][8][9]. The twin boundaries, which are special (low ÀΣ coincidence site lattice) high-angle boundaries [7] and contain a high density of sessile dislocations [8], act as equally effective obstacles to dislocation gliding as conventional grain boundaries in polycrystalline austenitic steels [10]. The subdivision of initial grains into lamellas with thicknesses ranging from 10 to 40 nm [2][3][4][5][6]8] leads to a significant decrease in the effective grain size and, therefore, results in remarkable strengthening in accordance with the Hall-Petch relationship [11-13]: σ 0:2 ¼ σ 0 þK H d À 0:5 ð1Þ where σ 0.2 is the offset yield strength; d is the effective grain size, which is the distance between barriers to dislocation glide and can be estimated to be twice the width of lamellae [14]; σ 0 is the friction stress; and K H is the Hall-Petch coefficient.…”

“…The subdivision of initial grains into lamellas with thicknesses ranging from 10 to 40 nm [2][3][4][5][6]8] leads to a significant decrease in the effective grain size and, therefore, results in remarkable strengthening in accordance with the Hall-Petch relationship [11-13]: σ 0:2 ¼ σ 0 þK H d À 0:5 ð1Þ where σ 0.2 is the offset yield strength; d is the effective grain size, which is the distance between barriers to dislocation glide and can be estimated to be twice the width of lamellae [14]; σ 0 is the friction stress; and K H is the Hall-Petch coefficient. A "dynamic Hall-Petch effect" [2][3][4][5][6]8] attributed to the reinforcement of austenite by deformation twins was exploited to explain the extraordinary ductility of the TWIP steels [9,15]. However, it is apparent that this interpretation of the very high strain-hardening rate of TWIP steels in terms of the dynamic Hall-Petch effect is oversimplified and open to debate.…”