Macro to nanoscale deformation of transformation-induced plasticity steels: Impact of aluminum on the microstructure and deformation behavior

doi:10.1016/j.jmst.2017.11.011

摘要/Abstract

Abstract:

This work aims to elucidate the impact of aluminum-content on microstructure and deformation mechanisms of transformation-induced plasticity (TRIP) steels through macroscale and nanoscale deformation experiments combined with post-mortem electron microscopy of the deformed region. The solid-state transformation-induced mechanical deformation varied with the Al contents, and influenced tensile strength-ductility combination. Steels with 2-4 wt% Al were characterized by TRIP effect. In contrast to 2Al-TRIP and 4Al-TRIP steels, twinning-induced plasticity (TWIP) was also observed in conjunction with strain-induced martensite in 6Al-TRIP steel. This behavior is attributed to the increase in stacking fault energy with the increase of Al content and stability of austenite, which depends on the local chemical variation. The study addresses the knowledge gap with regard to the effect of Al content on austenite stability in medium-Mn TRIP steels. This combination is expected to potentially enable cost-effective alloy design with high strength-high ductility condition.

Key words: Medium-manganese steels, Aluminum content, Electron microscopy, Mechanical properties, Nanoscale deformation, TRIP steel, Twinning

. [J]. 材料科学与技术, 2018, 34(5): 745-755.

V.S.Y. Injeti, Z.C. Li, B. Yu, R.D.K. Misra, Z.H. Cai, H. Ding. Macro to nanoscale deformation of transformation-induced plasticity steels: Impact of aluminum on the microstructure and deformation behavior[J]. J. Mater. Sci. Technol., 2018, 34(5): 745-755.

图/表 13

Table 1 Chemical compositions of three experimental steels (wt%).

Steel	C	Mn	Al	Fe
Fe-11Mn-0.2C-2Al	0.22	11.2	1.95	86.63
Fe-11Mn-0.2C-4Al	0.18	11.02	3.81	84.99
Fe-11Mn-0.2C-6Al	0.21	10.75	6.08	82.96

Scheme 1. (a) Schematic illustration of experimental jet-polishing procedure for nanoindentation experiments and (b) nanoindentation performed in the center of a austenite grain.

Fig. 1. Micrographs of three steels: (a) 2Al steel; (b) 4 Al steel; (c) 6Al steel.

Fig. 2. Measured ferrite and austenite fraction in 2Al, 4Al and 6Al steels.

Fig. 3. Engineering stress-strain curves of 2Al, 4Al and 6Al steels.

Table 2 Summary of tensile properties of experimental steels (UTS: ultimate tensile strength; TE: total elongation; PSE: product of strength and elongation).

	UTS (MPa)	TE (%)	PSE (GPa%)
2Al-TRIP steel	1400	32	44.8
4Al-TRIP steel	884	40	35.4
6Al-TRIP steel	650	65	42.3

Fig. 4. Work hardening rates of 2Al, 4Al, and 6Al steels. (S1: stage 1; S2: stage 2; S3: stage 3).

Fig. 5. Transmission electron micrographs of tensile deformed region in vicinity of fracture surface: (a) 2Al steel; (b) 4Al steel; (c, d) 6Al steel with presence of martensite and twins, respectively; (e) diffraction pattern of twins occurring in (d).

Fig. 6. Representative load-displacement plots for 2Al (a), 4Al (b) and 6-Al (c) steels acquired via nanoindentation experiments in load-controlled mode with a Berkovich indenter of radius 20 nm (maximum load of 0.5 mN) and loading rate of 2 μN s-1.

Fig. 7. Representative TEM images in different regions illustrating strain-induced martensite in plastic zone surrounding indentation after indentation experiments at a loading rate of 2 μN s-1 for 2Al steel.

Fig. 8. Representative TEM images in different regions illustrating strain-induced martensite in plastic zone surrounding indentation after indentation experiments at a loading rate of 2 μN s-1 for 4Al steel.

Fig. 9. Representative TEM images in different regions illustrating strain-induced martensite and twinning in plastic zone surrounding indentation after indentation experiments at a loading rate of 2 μN s-1 for 6Al steel.

Table 3 Free energy differences and interaction parameter differences between austenite and martensite phases (T: Temperature).

Free energy difference (ΔGγ^-ε)		Interaction parameter (ΔΩγ^-ε)
Fe	-822 + 1.7T + 2.2E-3T²	Fe(Fe)	-10836 + 22886X_Mn
Mn	3970-1.666T	Fe(Mn)	1780
Al	5481.04-1.799T	Fe(Al)	3323
C	-24595.12	Fe(C)	42500

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