Tensile deformation behavior analysis of low density Fe–18Mn–10Al–xC steels

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Abstract

Three low density Fe–18Mn–10Al–xC steels containing 0.5, 0.8 and 1.2 C (wt%) were utilized to investigate the differences in microstructures and their influence on mechanical behaviors during plastic deformation. The 0.5C steel had a duplex ferritic–austenitic structure and the fraction of austenite was ~59.2%, while the fraction of austenite was ~84.8% for the 0.8C steel and some ordered phases existed in both ferrite and austenite. In contrast, a fully austenitic microstructure with some κ carbides was achieved in the 1.2C steel. Due to the existence of the ordered phases by the addition of C from 0.5% to 0.8%, the 0.8C steel exhibited a highest tensile strength of ~979 MPa, and a moderate strain hardening capacity. The 1.2C steel revealed a superior combination of strength and ductility (ultimate tensile strength of 946.7 MPa and elongation to failure of 56%), with a much more pronounced strain hardening than the other two steels. The differential Crussard–Jaoul (C–J) analysis demonstrated a two-stage strain hardening behavior in both 0.5C and 0.8C steels, while a three-stage one in the 1.2C steel. This difference in strain hardening behavior was further understood in terms of microstructural analysis at the different stages of plastic deformation.

Introduction

The target of high performance steels is to increase strength and maintain ductility of steels as to satisfy the needs for weight savings by reducing the thickness of structural automotive components. However, the requirement for the stiffness of structures cannot be satisfied when the thickness of the structure is thinned to a certain extent. That is why lowering the density of steels has become one of the research trends in the development of high performance steels.

In the previous research, it was revealed that the density of Fe–Mn–C steels can be reduced to 6.5–7.1 g/cm3 with an addition of 6.5–12% Al [1]. Generally, low density steels can be classified into three categories: (1) ferrite based steels [2], [3]; (2) austenite based steels [4], [5], [6] and (3) duplex or triplex steels [1], [7], [8], [9], [10], [11], [12]. The physical phenomena in duplex and triplex low density steels are quite complex due to the different deformation mechanisms in each phase and the interactions among the major phases and the ordered ones. Frommeyer et al. [1] investigated the high-strength Fe–(18–28)Mn–(9–12)Al–(0.7–1.2) C triplex steels exhibiting a strength of 700–1100 MPa and a total elongation up to 60% or more. Accordingly, the strain hardening exponents were determined to investigate the tensile deformation behaviors of the steels. Later, Sutou et al. [7] demonstrated that an addition of C to Fe–20Mn–10Al–xC alloys, ranging from 0.25 to 1.5% linearly increased the tensile strength, with a maximum of ~10921 MPa. However, the tensile ductility of the steels first increased, reached a maximum value (~59.1%) at 1.0% C and then declined again (only ~17.1%) at 1.5% C. The influence of cooling patterns on the working hardening curves of the steels was studied as well, though the microstructural analysis during deformation was not provided in this study. Hwang et al. [8] studied a duplex Fe–20Mn–9Al–0.6C steel exhibiting a tensile strength of ~800 MPa and an elongation of ~46%. It was pointed out that the strain hardening exponent of austenite was higher than that of ferrite. Recently, the strain hardening behavior of a duplex steel with the similar composition, Fe–18.1Mn–9.6Al–0.65C, was investigated and multi-stage strain hardening characteristics were reported [9]. Another work [10] demonstrated that the Fe–27Mn–12Al–0.8C steel contained ordered phases in both phases, B2 and DO3 in ferrite and κ carbide in austenite, in which a relatively high yield strength of 812 MPa and tensile strength of 955 MPa were achieved, with a moderate elongation of 42% due to a relatively low initial strain hardening rate. Meanwhile, our preliminary work [12] indicated that an addition of C ranging from 0.8 to 1.2% to the Fe–18Mn–10Al–xC steels caused a dramatic change in microstructures and hence in the final mechanical properties of the steels.

Although some studies have been conducted on the microstructure–mechanical properties relationship of such low density Fe–Mn–Al–C steels, the strain hardening behavior of duplex Fe–Mn–Al–C alloy system, especially those containing ordered phases has not been fully understood yet. It is well known that the Crussard–Jaoul (C–J) analysis (i.e. ln(dσ/dε) vs. lnε) is considered as a useful tool to indicate the relative changes in work hardening behaviors over a range of plastic strains, especially in materials that exhibit microstructural evolutions (e.g. mechanical twinning or martensitic transformation) upon loading. Meanwhile, as a strong austenite stabilizer, an addition of C to high Mn and high Al alloys can significantly increase the volume fraction of austenite and also lead to the formation of ordered phases. In the present work, therefore, a Fe–18Mn–10Al–xC alloy system containing three different C contents was chosen to investigate the effects of the microstructures on the mechanical behavior of the steels. A particular emphasis will be placed on the clarification of the differences in mechanical behaviors in the steels consisting of different phase constituents by C–J analysis, which can provide some useful data for alloy design and applications of low density Fe–Mn–Al–C steels.

Section snippets

Experimental

Three different Fe–18Mn–10Al–xC (0.5, 0.8 and 1.2C, wt%) steels were selected, and the chemical compositions of the steels are shown in Table 1. The densities of three steels were calculated [7] as 6.92, 6.88 and 6.78 g/cm3, respectively. The stacking fault energy (SFE) values of austenite in 0.5C and 0.8C steels are 73.4 and 77.9 mJ/m2 and that of 1.2C steel is 84.5 mJ/m2 according to the model we used previously [13]. The 50 kg ingots of steels were prepared by induction melting under an argon

Microstructures before tensile deformation

Fig. 1 shows the representative optical microstructures of the as-quenched steels, revealing that 0.5C and 0.8C steels have a duplex ferritic–austenitic microstructure, while 1.2C steel is fully austenitic with some annealing twins in grains. The volume fractions of austenite are determined to be 59.2% and 84.8% in 0.5C and 0.8C steels, respectively. Detailed microstructure analysis under TEM reveals that no precipitates exist in both austenite (Fig. 2(a)) and ferrite (Fig. 2(b)) phases of 0.5C

Discussions

There are several factors influencing the mechanical properties of the duplex, triplex or complex high Mn steels such as alloying elements, phase fraction, grain size as well as characteristics of each constituent phase. In Fe–Mn–Al–C system, Mn and C are austenite stabilizers and Al is a ferrite stabilizer. Meanwhile, various ordered phases appear in this system due to the heavy alloying of the elements even in the quenched state after solution treatment [10]. In the present work, the initial

Conclusions

The present work investigates the influence of microstructure differences on strain hardening behaviors during tensile deformation of Fe–18Mn–10Al–xC alloys containing 0.5, 0.8 and 1.2 C (wt%). The main findings can be summarized as follows.

  • 1.

    The 0.5C and 0.8C steels exhibited a duplex ferritic–austenitic structure, while ordered phases (B2 and DO3) and κ carbides formed in ferrite and austenite of 0.8C steel, respectively. The 1.2C steel was fully austenitic with some κ carbides.

  • 2.

    With increasing

Acknowledgment

This work is financially supported by the Natural and Scientific Foundation of China (Grant no. 51474062).

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