Strain Hardening Associated with Dislocation, Deformation Twinning, and Dynamic Strain Aging in Fe–20Mn–1.3C–(3Cu) TWIP Steels

doi:10.1007/s40195-014-0100-0

Abstract

Abstract:

The effects of Cu on stacking fault energy, dislocation slip, mechanical twinning, and strain hardening in Fe–20Mn–1.3C twinning-induced plasticity (TWIP) steels were systematically investigated. The stacking fault energy was raised with an average slope of 2 mJ/m² per 1 wt% Cu. The Fe–20Mn–1.3C–3Cu steel exhibited superior tensile properties, with the ultimate tensile strength reached at 2.27 GPa and elongation up to 96.9% owing to the high strain hardening that occurred. To examine the mechanism of this high strain hardening, dislocation density determination by XRD was calculated. The dislocation density increased with the increasing strain, and the addition of Cu resulted in a decrease in the dislocation density. A comparison of the strain-hardening behavior of Fe–20Mn–1.3C and Fe–20Mn–1.3C–3Cu TWIP steels was made in terms of modified Crussard–Jaoul (C–J) analysis and microstructural observations. Especially at low strains, the contributions of all the relevant deformation mechanisms—slip, twinning, and dynamic strain aging—were quantitatively evaluated. The analysis revealed that the dislocation storage was the leading factor to the increase of the strain hardening, while dynamic strain aging was a minor contributor to strain hardening. Twinning, which interacted with the matrix, acted as an effective barrier to dislocation motion.

Key words: Twinning-induced plasticity (TWIP), Strain hardening, Mechanical twinning, Dislocation density, Dynamic strain aging

Lingyan Zhao, Dingyi Zhu, Longlong Liu, Zhenming Hu, Mingjie Wang. Strain Hardening Associated with Dislocation, Deformation Twinning, and Dynamic Strain Aging in Fe–20Mn–1.3C–(3Cu) TWIP Steels[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(4): 601-608.

Figures/Tables 8

Table 1 Chemical composition of the TWIP steels (in wt%)

Steel	C	Mn	Cu	S	Fe
Fe–20Mn–1.3C	1.30	19.84	0.02	0.006	Bal.
Fe–20Mn–1.3C–3.0Cu	1.28	20.03	3.04	0.013	Bal.

Fig. 1 Measured value using XRD method based on Fourier analysis and calculated value using modified thermodynamic model based on the Olson–Cohen model SFE of austenite in Fe–Mn–C–Cu TWIP steels

Fig. 2 Engineering stress–engineering strain curves a and true stress–true strain (σ - ?) curves and strain-hardening rate b of the Fe–20Mn–1.3C and Fe–20Mn–1.3C–3Cu TWIP steels; \( { \ln }\left( {{{{\text{d}}\sigma } \mathord{\left/ {\vphantom {{{\text{d}}\sigma } {{\text{d}}\varepsilon }}} \right. \kern-0pt} {{\text{d}}\varepsilon }}} \right) - { \ln }\sigma \) plots for modified Crussard–Jaoul analysis based on the Swift equation for annealed Fe–20Mn–1.3C c and Fe–20Mn–1.3C–3Cu d TWIP steels

Fig. 3 TEM micrographs showing the microstructures of the Fe–20Mn–1.3C–3Cu steel at the true strain of 0.02: a dislocations existed near the anneal twins; b dislocations distribute in high density throughout the matrix

Fig. 4 TEM images showing the deformation twins of the Fe–20Mn–1.3C–3Cu at the true strains of 0.33 a, 0.54 b

Table 2 Calculated values of dislocation density of TWIP steel samples at different levels of true strain via the XRD patterns

True strain	Average crystallite size (nm)		Average microstrain		Dislocation density (10¹⁵/m²)
True strain	Fe–20Mn–1.3C	Fe–20Mn–1.3C–3Cu	Fe–20Mn–1.3C	Fe–20Mn–1.3C–3Cu	Fe–20Mn–1.3C	Fe–20Mn–1.3C–3Cu
0	32.0	37.9	0.0202	0.0193	0.667	0.514
0.18	25.9	28.6	0.0325	0.0318	2.132	1.885
0.48	18.6	–	0.0586	–	9.677	–
0.67	–	20.8	–	0.0558	–	7.837

Fig. 5 TEM image taken from the Fe–20Mn–1.3C–3Cu TWIP steel at the strain of 0.18

Fig. 6 Contributions of all factors to the strain hardening of the Fe–20Mn–1.3C and Fe–20Mn–1.3C–3Cu TWIP steels deformed at the true strain of 0.18

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