Serrated Flow and Dynamic Strain Aging in Fe-Mn-C TWIP Steel
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UCTION
TWINNING-INDUCED plasticity (TWIP) steel has been regarded as an attractive material for lightweight automotive structures due to their high strength, plasticity, and energy absorption performance. This, in turn, has a positive effect on reducing fuel consumption and emissions.[1–3] In the recent 5 years, TWIP steel in the Fe-Mn-C-Al system has been successfully commercialized with great efforts spent on chemical and physical metallurgy.[4,5] More research is still needed, however, to
PENG LAN and JIAQUAN ZHANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 10083, P.R. China. Contact e-mails: [email protected] and [email protected] Manuscript submitted March 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
bring TWIP steels into greater use in automotive applications. The first-generation TWIP steel was introduced by Grassel and Frommeyer[6,7] in the Fe-Mn-Si-Al system about 20 years ago. Twinning was observed to be the major deformation mechanism for Fe-25Mn-3Si-3Al TWIP steel with 60 to 90 GPa pct in the product of strength and elongation. About 10 years later, Scott and Allain[8,9] developed an interstitial-containing TWIP steel in the Fe-Mn-C system without the addition of Al and Si, which has been regarded as the second-generation TWIP steel. A tensile strength of 1000 MPa and an elongation of 50 pct were obtained in Fe-(18 to 22)Mn-(0.4 to 0.8)C TWIP steel. Large amounts of secondary twins among the primary ones were observed in these C-alloyed TWIP steels, which were not observed in the earlier Fe-Mn-Si-Al steels. Serrated flow related to dynamic strain aging (DSA) and Portevin–Le Chatelier (PLC) behavior was observed in Fe-Mn-C TWIP steels
under some conditions, as revealed by Chen[10] and Zavattieri.[11] The influence of twinning and DSA on work hardening has been widely studied in the second-generation Fe-Mn-C and Fe-Mn-C-Al TWIP steels. Barbier and Gey[12] observed the texture and microstructure evolution of Fe-22Mn-0.6C TWIP steel during tensile deformation and found that the prevention of dislocation movement by twins was the hardening mechanism. Gutierrez-Urrutia and Raabe[13] studied the deformation behavior of Fe-Mn-C TWIP steel by electron channeling contrast imaging method and concluded that the interaction between dislocation and twin substructure was the vital factor affecting mechanical performance. Allain[14] and Bouaziz[15] employed the Hall–Petch theory to elucidate the work hardening behavior of TWIP steel. They believed that the decrease in mean free distance of dislocations by twinning leads to the strengthening observed in TWIP steels. Another phenomenon, the serrated flow associated with DSA, has also been an important topic in interstitial-containing TWIP steels. Dastur and Leslie[16] and Chen and Kim[10] pointed out that twinning was not the dominant contribution to the excellent work hardening in high-Mn austenite steels, and that planar glide and DSA were also vital factors. Hong and Shin[17]
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