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Acta Metall Sin  2020, Vol. 56 Issue (3): 311-320    DOI: 10.11900/0412.1961.2019.00181
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Microstructural Evolution and Work Hardening Behavior of Fe-19Mn Alloy Containing Duplex Austenite and ε-Martensite
WANG Shihong1,LI Jian1(),GE Xin1,2,CHAI Feng1,LUO Xiaobing1,YANG Caifu1,SU Hang1
1. Department of Structure Steels, Central Iron and Steel Research Institute, Beijing 100081, China
2. School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China
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Abstract  

As the excellent combination of strength and ductility, the high manganese steel has been used in the manufacturing field of automobile, liquefied natural gas (LNG) ship and oil and gas exploitation. On the other hand, due to the good damping capacity within a certain Mn content range, it has also been used to make components on the machines to reduce vibration and noise. So high manganese steel is considered to be a structural and functional integrated material with great application prospects. Many factors can affect the mechanical properties and damping capacity, such as chemical composition, grain size and heat treatments. Among these, carbon concentration has a complicated influence on them. For example, a high carbon concentration will improve mechanical properties, but in return deteriorate damping capacity. In order to acquire a material with good damping capacity and suitable strength and ductility, ultralow carbon Fe-19Mn-0.0017C (mass fraction, %) alloy was designed. The microstructural evolution and mechanical properties of the alloy during tensile process were investigated by means of OM, EBSD, TEM, XRD and tension test. The results show that Fe-19Mn shows deformation-induced martensite transformation, which changes from γ-austenite→ε-martensite transformation to ε-martensite→α'-martensite transformation as the amount of deformation increases. Analysis of the strain hardening rate (ln(dσtrue/dεtrue)) combined with the fraction of constituent phases reveals that the transformation of ε-martensite→α'-martensite is more effective in improving work hardening rate than that of γ-austenite→ε-martensite. This is, on one hand, because of the lower strength of ε-martensite which is caused by the lack of carbon solution strengthening; and on the other hand, α'-martensite has higher hardness than ε-martensite, which can impede dislocation movement more effectively. In addition, {101?2}<1?011>ε deformation twins are formed to accommodate deformation of ε-martensite except for dislocation slip during tensile process. The combined action of transformation induced plasticity (TRIP) effects of γ-austenite→ε-martensite→α'-martensite transformation, dislocation slip of γ-austenite/ε-martensite/α'-martensite and {101?2}<1?011>ε deformation twinning makes Fe-19Mn with ultralow carbon concentration have an excellent combination of strength and ductility, whose tensile strength and total elongation can reach 722 MPa and 31%, respectively.

Key words:  high manganese steel      deformation-induced martensite transformation      twinning deformation      work hardening behavior     
Received:  03 June 2019     
ZTFLH:  TG142  
Fund: Naval Equipment Pre-research Foundation of China(302030122-0183-001)
Corresponding Authors:  Jian LI     E-mail:  lijianzy@cisri.com.cn

Cite this article: 

WANG Shihong,LI Jian,GE Xin,CHAI Feng,LUO Xiaobing,YANG Caifu,SU Hang. Microstructural Evolution and Work Hardening Behavior of Fe-19Mn Alloy Containing Duplex Austenite and ε-Martensite. Acta Metall Sin, 2020, 56(3): 311-320.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00181     OR     https://www.ams.org.cn/EN/Y2020/V56/I3/311

Fig.1  Tensile properties of Fe-19Mn(a) curve of engineering stress-strain(b) curves of true stress (σtrue) and work hardening rate (dσtrue/dεtrue) vs true strain (εtrue)
Fig.2  Microstructure of solution treated sample
Fig.3  TEM images and corresponding selected area electron diffraction (SAED) patterns of solution treated sample(a) TEM image of γ-austenite and ε-martensite(b, c) dark-field images of circle areas 1 and 2 in Fig.3a, respectively(d) stacking faults in γ-austenite (indicated by arrows)(e~g) SAED patterns for circle areas 1~3 in Fig.3a, respectively
Fig.4  EBSD analyses of samples after the deformations of 0% (a), 5% (b), 10% (c) and 15% (d) (Blue region is austenite, yellow region is ε-martensite, green region is α'-martensite and red line is austenite twin boundary, arrow in the illustration of Fig.4b indicates {101?2}<1?011>ε twin)Color online
Fig.5  Misorientation angle distributions of γ-austenite (a), ε-martensite (b) and α'-martensite (c)
Fig.6  TEM images and SAED patterns of microstructures of Fe-19Mn after deformation (Arrows in Figs.6a and b indicate ε-martensite)(a) TEM image after 5% deformation(b) dark-field image of ε-martensite(c) new ε-martensite plates formed in ε-martensite matrix after 5% deformation(d) TEM image after 10% deformation(e~h) SAED patterns corresponding to areas 1~4 in Figs.6a and c, respectively
Fig.7  TEM bright-field image of microstructures (a), dark-field image of ε-martensite (b) and SAED pattern corresponding to circle area in Fig.7a (c) after 10% deformation (Arrows in Fig.7a indicate ε-martensite, and the box area shows the dislocation pile-up)
Fig.8  XRD spectra (a, b) and phase fractions (c) of samples with different tensile deformations (Fig.8b shows the XRD spectra between 46°~54° in Fig.8a)
Fig.9  ln(dσtrue/dεtrue)-lnσtrue curve based on the strain hardening rate curve for the modified Crussard-Jaoul (C-J) analysis
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