TWIP Effect and Plastic Instability Condition in an Fe^|^ndash;Mn^|^ndash;C Austenitic Steel

Abstract

We investigated the correlation among deformation twin density, work hardening, and tensile ductility in an Fe-18Mn-1.2C twinning-induced-plasticity (TWIP) steel, and discussed the correlation with the plastic instability condition. The deformation twin density was varied by changing the deformation temperature from 123 to 523 K. An important factor for the uniform elongation is the work hardening rate in a later deformation stage. The increase in the deformation twin density enhanced the work hardening rate significantly but not monotonically just before the fracture, since the deformation twin density is saturated against plastic strain. In addition, dynamic strain aging in a later deformation stage and ε-martensitic transformation were found to accelerate the fracture due to the localized deformation and the premature fracture, respectively. Accordingly, the relationship between uniform elongation and deformation twin density was not simple. The optimum conditions for the TWIP effect were concluded to be (1) considerable amount of deformation twinning in a later deformation stage, (2) suppression of dynamic strain aging in a later deformation stage, and (3) inhibition of ε-martensitic transformation.KEY WORDS: high carbon; high Mn steel; austenitic steel; twinning-induced plasticity; work hardening; dynamic strain aging.

Deformation Twinning Behavior of Twinning-induced Plasticity Steels with Different Carbon Concentrations – Part 1: Atomic Force Microscopy and Electron Backscatter Diffraction Measurements

“…These steels are TWIP steels, which exhibit an excellent combination of elongation and ultimate tensile strength (UTS), mainly because of deformation twinning. 16,17,34) The solution-treated bars were cut by spark machining to obtain the specimen geometries required for the following experiments.…”

“…8,[15][16][17] The stacking fault energy increases with increasing deformation temperature, 18) and the critical stress and strain for twinning increase with increasing stacking fault energy. 11,15,18) .…”

“…In addition, even in the Fe-18Mn-1.2C steel, which has the highest stacking fault energy, an extremely high amount of deformation twins appears after exceeding 40% plastic strain. 16,17) Because relatively-near-< 111 > orientations, such as < 144 > and < 110 > , rotate to < 111 > tensile orientations, 50,51) the initial crystallographic orientation dependence of deformation twinning behavior becomes weak in the late-deformation stage.…”

Deformation Twinning Behavior of Twinning-induced Plasticity Steels with Different Carbon Concentrations – Part 2: Proposal of Dynamic-strain-aging-assisted Deformation Twinning

“…These steels are TWIP steels that exhibit an excellent combination of elongation and ultimate tensile strength (UTS) mainly due to deformation twinning. 3,25,26) The solution-treated bars were cut by spark machining to achieve the specimen geometries required for the following experiments.…”

“…The plate-like products in the IQ images are deformation twins. 25,26) Figures. 4(a) and 4(b) reveal that the total number of deformation twins increased by decreasing the deformation temperature from ambient temperature to 223 K. In contrast, Figs.…”