Anisotropy and strain rate effects on the failure behavior of TWIP steel: A multiscale experimental study
Graphical abstract
Introduction
The demand for safety, durability and cost effectiveness of structural parts used in modern automobiles has increased tremendously over the years. To achieve superior crash performance, the major requirement is high strength and ductility. Steels widely used in automobiles require greater formability for the fabrication of complex body parts. High manganese twinning induced plasticity (TWIP) steels are such class of steels characterized by high work hardening capacity along with exceptional strength and ductility (Grässel et al., 2000; Lee et al., 2017). The excellent strain hardening rate (SHR) of TWIP steels originates from their deformation mechanisms such as dislocation glide, deformation twinning and ε-martensite transformation (TRIP) (Pierce et al., 2015; Kim et al., 2017). TWIP steels with different chemical compositions such as Fe-18Mn-0.6C, Fe-28Mn-0.3C, Fe-22Mn-0.6C and Fe-18Mn-0.6C-1.5Al, showed a uniform elongation (UE) of more than 50% and a ultimate tensile strength (UTS) of above 1000 MPa (Saeed-Akbari et al., 2012; De Cooman et al., 2018). Evaluation of the material performance solely based on quasi-static tensile tests and judging their suitability for safety critical parts may be inadequate. Since during the crash of automobiles, the parts are not deformed until failure and the deformation rates are very high (Bleck and Schael, 2000). Thus to check the crash-worthiness of TWIP steel, the mechanical properties and energy absorption (EA) capacity is investigated under both quasi-static and dynamic loading conditions.
The superior tensile properties of TWIP steels were attributed to the sustained high work hardening rates. Plastic deformation mechanisms such as deformation twinning and strain induced ε-martensite transformation occurred beside dislocation slip in these steels depending on the chemical composition, strain rate and temperature (Idrissi et al., 2009). Deformation twins and ε-martensite platelets increase the SHR by acting as obstacles to dislocation motion thereby reducing the effective dislocation mean free path (Renard and Jacques, 2012). Fe-Mn-C-Al alloys with stacking fault energy (SFE) in the range of 21–27 mJ/m2, exhibited enhanced strength and ductility compared to alloys with SFE above 42 mJ/m2. Thus in alloys where twinning prevailed over dislocation glide, displayed better SHR compared to the alloys where deformation occurred only via dislocation glide (Curtze and Kuokkala, 2010). The increase in SHR was due to increase in deformation twin density. However, with increase in plastic strain the twin density saturated resulting in a sudden drop of SHR leading to failure. The dynamic strain aging (DSA) occurred above room temperature, where twinning along with dislocation glide were the active deformation modes (Koyama et al., 2013). The DSA effectively modified the SHR but its contribution to SHR is rather limited, since the activation energy attained in Fe-22Mn-0.6C TWIP steel was quite low (Allain et al., 2008). Thus the high SHR in TWIP steels was solely due to the occurrence of deformation twinning along with dislocation glide at intermediate temperatures. The influence on the activation of different deformation mechanisms during deformation and its relation with the SHR is not very well understood. Thus in this study, the evolution of twinning with increasing macroscopic strain is studied and correlated with the SHR.
High Mn steels show a unique characteristic behavior in the form of nucleation and propagation of narrow deformation shear bands during tensile loading. This results in serrated/jerky flow known as Portevin-Le Chatelier (PLC) effect. The formation and propagation of shear bands during deformation was mainly due to DSA associated with the negative strain rate sensitivity (SRS) (Chen et al., 2007). The shear bands traverse the gage length of deforming tensile specimen between the successive serrations and the shape of the serrations is related to the mode of band propagation. The serrations of type A are known to be associated with repetitive nucleation of deformation bands from one end of the gage length and their continuous propagation to the other end. Each of the load drop/increase corresponds to the initiation/disappearance of the shear band and the plateau between the two consecutive serrations peaks corresponds to the nucleation and propagation of shear band within the gage length (Gopinath et al., 2009). The type A bands nucleated at the gripping ends or in the gage section center and the band propagation commenced at the end of each nucleation event with inhomogeneous strain distribution (Zavattieri et al., 2009). The deformation bands in TWIP steel begin with small strain heterogeneity and develop into pronounced deformation bands accompanied with serrated flow. Lee et al. (2011) investigated TWIP steel with and without Al and stated that DSA is the result of an interaction between Mn-C point defect complexes and stacking faults. It was stated that DSA can occur by a single diffusive jump of C atom in the stacking fault region. DSA occurs after deforming the material to critical strain (εc), when the C atom of the Mn-C complex has enough time to interact with a stacking fault irrespective of the Mn-C reorientation time. The presence of Mn-C octahedral short range order (SRO) or short range clusters (SRC) was proved experimentally by small-angle neutron scattering (SANS) technique (Kang et al., 2014; Song and Houston, 2018). Both the studies investigated Fe-18Mn-0.6C alloy and it was stated that the size of the Mn-C clusters decreased and the number density of the Mn-C clusters increased with increasing macroscopic strain. Seol et al. (2017) has used multiscale characterization techniques to understand DSA and showed that SRO caused by SRC was responsible for DSA and also claimed that high deformation resulted in increase in C concentration of SRC, which led to the suppression of DSA. Kang et al. (2017) compared the local deformation behavior of TWIP steel under quasi-static and dynamic tensile loading and reported that twinning in the early stage of deformation resulted in higher strength and ductility along with similar local strain at failure even under dynamic loading. Bian et al. (2017) investigated the influence of on the occurrence of DSA. DSA was suppressed with increasing and the local strain concentration within the shear band was more severe at higher . The εc for the onset of DSA increased with increase in showing negative SRS and the velocity of bands propagation (Vb) decreased with increasing macroscopic strain. It is important to understand the role of DSA on the plastic strain localization and failure initiation to use TWIP steel for automotive applications. Hence this study focuses on elucidating the failure initiation mechanism due to the propagation of shear bands.
Dynamic tensile tests was carried out on Fe-25Mn-3Al-3Si TWIP/TRIP steel by Benzing et al. (2018) showed an increase in yield strength (YS), UTS and elongation with increase in . The material mainly deformed by dislocation glide along with TWIP and TRIP effect. The specimen temperature increased by about 60 ∘C due to adiabatic heating, which resulted in a change of SFE from 21 to ∼23 mJ/m2. It was stated that, did not influence the microstructure morphology such as thickness and spacing of deformation twins or ε-martensite laths. In a similar study on Fe-15Mn-0.6C-1.2Al TWIP steel by Kang et al. (2017), serrated flow behavior was observed under quasi-static tests and homogenous flow behavior without serrations under dynamic tests. Dynamic loading resulted in increase of UTS, slight reduction of elongation and profuse necking. This is mainly due to the adiabatic heating during deformation which led to increase in SFE resulting in decrease of twinning rate. Park et al. (2017) studied the same alloy and found differences in slip characteristics and twin formation due to variation of . Under quasi-static loading, dislocation cell structures formation showed a transition from combined planar and wavy slip to wavy slip, whereas under dynamic loading combined planar and wavy slip was activated from the early stages of deformation. In the initial stages of deformation, stacking faults were observed and after deforming to 20%, 30% strain, primary twins could be seen under quasi-static test, while in dynamic tests both primary and secondary twinning were quite active in the later stages of deformation. Shen et al. (2016) has investigated the influence of grain size and strain rate on the hardening behavior in Fe-20Mn-0.6C steel. For all the investigated grain sizes in the range from 3.5 to 25 μm, YS increased with increase in , whereas UTS and elongation decreased with increase in . It was reported that strain localization within the shear bands and adiabatic heating has led to the reduction of mechanical properties. Microstructure investigations has revealed dislocation slip along with primary twins in the early and middle stages of deformation, whereas in the later stages of deformation rapid nucleation of twins was observed at high . Rahman et al. (2014) has investigated Fe-15Mn-0.7C-2Al-2Si steel at various and observed a positive SRS of YS and decrease in deformation twinning rate at high . At low , activation of multiple twin systems and large intergranular misorientations was observed in contrast to high . In the current study, the influence of adiabatic heating at different on the activation of deformation mechanisms is investigated. The influence on the deformation behavior and mechanical properties is studied in detail, with a special emphasis on strain localization and failure.
TWIP showed characteristic differences in the failure behavior when the material deformed by only SLIP or SLIP in combination with TWIP or SLIP in combination with TRIP (Madivala et al., 2018). Fracture surface analysis depicted the formation of fine dimples with quasi-brittle islands where ε-martensite was present, fine or shallow dimples where twinning was the dominant deformation mode and larger dimples along with coalescence of voids at MnS inclusions where deformation occurred by only dislocation glide . Yu et al. (2017) showed the formation of micro-cracks at the grain boundary junctions and the mechanical twin boundaries. These cracks were mainly found at the edge and side surfaces of tensile specimen where local deformation shear bands met. Yang et al. (2017) has reported that sudden failure or drop of strain hardening in TWIP steels was due to exponential increase of the macroscopic void volume fraction. Fabrègue et al. (2013) investigated the damage evolution in TWIP steel by 3D X-ray tomography experiments and showed that average void diameter and triaxility remained constant throughout the deformation. The damage process involved intense nucleation of tiny voids combined with significant growth of the biggest cavities. Lorthios et al. (2010) examined the void size distribution close to the fracture surface in a sheared specimen. Most of the voids found were tiny secondary dimples with few elongated cavities, indicating localized failure. The factors such as temperature, strain rate and stress state have a direct influence on the active deformation modes, which in turn influence the damage and failure in TWIP steels. This work focuses on the mechanism of micro-crack formation in TWIP steel and its relation with deformation mechanisms.
This paper discuses the influence of anisotropy and strain rate on the strain hardening and fracture behavior of X60Mn22 TWIP steel. Uni-axial tensile tests were carried out at different strain rates ranging from 0.00001 to 250/s for investigating the mechanical behavior under both quasi-static and dynamic conditions. The serrated flow caused due to the DSA was studied by carrying out tensile tests in conjunction with digital image correlation (DIC) over the entire deformation range until failure. Analyzing local plastic strain evolution during deformation using DIC measurements assisted in understanding the inhomogeneous flow behavior resulted due to the propagation of shear bands and their subsequent influence on the failure initiation. Synchronous temperature measurements during deformation at various strain rates aid in the accurate estimation of SFE at different elongations, enabling the prediction of different active deformation mechanisms. The correlation of predicted mechanisms with microstructure evolution processes was verified by electron back scatter diffraction (EBSD) measurements. Detailed deformation mechanisms were examined by analyzing how twinning and dislocation glide were varied under the quasi-static and dynamic loading conditions. The role of inclusions present on the mechanical properties and failure initiation along rolling, transverse and shear (45∘) directions was investigated in detail. The mechanism of micro-cracks formation and their evolution at different stages of deformation was studied by interrupted micro tensile tests in the scanning electron microscope (SEM).
Section snippets
Material and experimental methods
The material investigated in this study is a high manganese TWIP steel, produced by industrial processing route, designated as X60Mn22 (Fe-22Mn-0.6C). Initial microstructure analysis such as the grain size or morphology and the evolution of twinning with increasing macroscopic strain was characterized by using EBSD technique. The qualitative and quantitative analysis of inclusions type, size and their distribution was investigated by techniques such as optical and field-emission SEM with energy
Macroscopic response
The influence of anisotropy and strain rate dependency on work hardening behavior are of crucial importance for the crash performance of the material and also play a large role in selecting the optimal forming processes for manufacturing of components. Understanding tensile properties under quasi-static and dynamic loading conditions will aid in choosing TWIP steel for a specific application.
Serrated flow
Tensile tests carried out in this study over a wide range of strain rates and temperatures (Madivala et al., 2018) revealed repeated “jerks” or “serrations” during the plastic deformation at intermediate temperatures and over a range of strain rates. Serrations are observed on the σ–ε curves at all quasi-static strain rates and they begin to diminish with increasing strain rates above 0.5/s as shown in Fig. 5. The jerky flow behavior is quite similar in characteristics along RD, TD and 45∘ in
Conclusions
The X60Mn22 TWIP steel was investigated by uni-axial tensile tests under both quasi-static and dynamic conditions. For investigating the anisotropy and strain rate effects on the mechanical properties and failure behavior, various in-situ tensile testing methods were applied. The local strain evolution during deformation was monitored by carrying tests in conjunction with digital image correlation and synchronous temperature measurements using thermocamera. Interrupted tensile tests were
Acknowledgments
The authors would like to acknowledge the financial support of the German Research Foundation (DFG) within the Collaborative Research Center SFB 761 “Steel-ab initio”. The authors thank ThyssenKrupp Steel Europe AG for the material supply.
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Institut für Eisenhüttenkunde (IEHK), RWTH Aachen University, Intzestraße 1, 52072, Aachen, Germany.