Grain boundary segregation in Fe–Mn–C twinning-induced plasticity steels studied by correlative electron backscatter diffraction and atom probe tomography
Introduction
Fe–Mn–C twinning-induced plasticity (TWIP) steels represent a novel grade of advanced high-strength and formable austenitic steels with high potential for automotive and related sheet-forming applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Knowledge of equilibrium grain boundary (GB) segregation in these austenitic steels is required to better understand the local concentration dependence of associated phenomena, e.g. the mobility of GBs, and thereby the kinetics of grain growth or recrystallization during thermomechanical treatment [13], [14]. Furthermore, GB segregation can affect the twinning behavior of TWIP steels since the critical stress for deformation twin formation is directly related to the stacking fault energy (SFE) [3], [15], [16], which is a function of the chemical composition [17], [18]. Therefore, a change in the local chemical composition at GBs should influence the critical stress necessary for the onset of twin nucleation at GBs and hence the strain hardening behavior.
In addition to the influence of GB segregation on twinning, better understanding is required of its effect on failure phenomena in TWIP steels. Room temperature fracture behavior of TWIP steels is normally ductile, with cup–cone dimples on the fracture surface [2], [19], though there are reports that higher Mn contents slightly increase the fraction of intergranular cracking [20], [21], which mainly nucleates at MnS inclusions [22]. Stress corrosion cracking is another relevant fracture mechanism [23], as is the corrosion behavior in general, since segregation causes local differences in the chemical potential [24]. A related aspect that is attracting great interest is the hydrogen embrittlement of high-Mn steels [25], [26], [27], [28], [29]. Hydrogen has been also shown to deteriorate mechanical properties in general, leading to intergranular fracture [30], [31], [32]. Hydrogen-induced delayed fracture of TWIP steels has been reported for cup-drawn specimens in air [25] and hydrogen-assisted cracking has been observed to affect all boundary types, surprisingly including Σ3 coherent annealing twins and deformation twins [29]. He et al. [33] investigated the segregation of hydrogen to Σ3 (1 1 1) boundaries in α-iron by first-principles calculations and concluded that the segregation of Cr reduces the segregation of hydrogen. This result indicates that the presence of other segregated elements at boundaries can potentially reduce the impact of hydrogen embrittlement. It is thus a motivation for the investigation of GB equilibrium segregation [34], [35] in TWIP steels.
Knowledge of the local grain boundary chemistry is needed to understand the above-mentioned phenomena. Atom probe tomography (APT), with its near-atomic spatial resolution and high detection sensitivity (in the range of a few parts per million, and equal for all elements), is a powerful tool for the characterization of segregation phenomena at buried interfaces that occur within a few angstroms around the boundary plane and often involve light elements [36], [37], [38], [39], [40], [41]. In the current work, segregation was evaluated by quantitative APT analyses of three boundary types: a general GB (also called a random GB), referring to a high-angle grain boundary (HAGB) the configuration of which does not correspond to a special boundary type according to coincident site lattice (CSL) theory; an annealing twin, created by diffusion during an extended heat treatment; and a deformation twin, created by mechanical deformation at room temperature [42], [43]. The first two were identified by electron backscattering diffraction (EBSD), the deformation twins by transmission electron microscopy. All boundaries were prepared by site-specific focused ion beam (FIB) lift-out techniques [44].
Section snippets
Experimental
A high-Mn TWIP steel with nominal composition Fe–28Mn–0.3C (wt.%) was cast into 140 × 140 mm ingots, followed by hot rolling in three passes at 1150 °C to 50 mm thickness, homogenization for 5 h at 1200 °C, hot rolling in 12–14 passes to 3 mm, cold rolling to 2 mm and air cooling. The sample was recrystallized by annealing for 24 h at 700 °C followed by water quenching, then ground to the middle layer of the sheet. All heat treatments were performed in an air circulation furnace. The long annealing time
Results
Fig. 4 illustrates the crystallographic analysis of the plane orientation of GB2. The analysis of the surface trace of GB2 in the EBSD dataset shows that the GB plane is parallel to the surface trace of a {1 1 1} plane in each of the abutting grains (Fig. 4a). As measured on the SEM micrograph of the lift-out sample, the GB plane inclination angle in depth is 16° (Fig. 4b). Fig. 4c shows the standard stereographic projection of the 〈1 1 1〉 poles of both grains illustrated using the TOCA software
Discussion
GB segregation was quantified (Table 2) from ladder diagram plots [49], examples of which are shown in Fig. 7. This approach takes only the integrated number of excess atoms per GB area into account. The shape of the concentration profile across the boundary, including the maximum peak height and the width of the segregation zone, is deliberately ignored as it is subject to well-known atom probe artifacts, such as the local magnification effect or preferential field retention/evaporation
Summary and conclusions
Grain boundary segregation in an Fe–Mn–C TWIP steel was investigated by APT in three representative cases: in specimens with a general GB, an annealing twin boundary and a deformation twin boundary. All boundaries were found to differ significantly in their local chemical composition. The general GB exhibited segregation of B, C, Si and P, and traces of Cu. The annealing twin also displayed segregation of B and P but at much lower Gibbs excess values than the general GB. Additionally, and in
Acknowledgements
The authors acknowledge financial support by the German Research Foundation (DFG) through funding of the SFB 761 “steel ab initio” project. R.K.W.M. acknowledges the support of the Alexander von Humboldt Foundation through the award of a Humboldt Postdoctoral Fellowship. M.K. gratefully acknowledges the ERC Advanced Grant “Smartmet” for funding. The authors further acknowledge Stefan Zaefferer for support with the EBSD analysis.
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