Revealing orientation-dependent martensitic transformation in a medium Mn steel by micropillar compression
Introduction
Martensitic transformation in steels is a strong first order solid-state phase transformation (Krumhansl and Yamada, 1990; Levitas, 2013). The transformation from parent austenite to product martensite accompanies with a large transformation eigenstrain which can be resolved into the dilatation component and shear component (Clapp, 1995). Such transformation eigenstrain should be accommodated by deformation of both austenite and martensite phases (Greenwood and Johnson, 1965). Moreover, the transformation eigenstrain could be minimized if the preferred martensite variants are selected during mechanical loading (Magee, 1966). The martensitic transformation has been employed to effectively improve the work hardening behavior of advanced high strength steels (AHSS) (Fischlschweiger et al., 2012; Shi et al., 2010). The martensitic transformation triggered by mechanical loading can generate new dislocations and therefore increase the dislocation density and enhance the work hardening behavior (Jacques et al., 2001). Therefore, the martensitic transformation can offer the transformation-induced plasticity (TRIP) effect and enhances the mechanical performance of AHSS containing metastable retained austenite grains (He et al., 2016; Kubler et al., 2011; Mahnken et al., 2009; Yang et al., 2018).
It is widely accepted that the orientation of austenite grain affects the martensitic transformation in steels (Kwon et al., 2010; Lu et al., 2011). However, different studies could have sharply different conclusions. For instance, it is reported that the martensitic transformation is more favorable under <010> austenite crystallographic orientation parallel to the tensile loading direction (Creuziger and Foecke, 2010; De Knijf et al., 2014). Such grain orientation dependence of martensitic transformation can be explained by the concept of mechanical driving force which is the mechanical work acting on the martensitic habit plane (Zhang and Kelly, 2002; Zhang et al., 1999b). In contrast, it is found that the austenite grains with an orientation of [110] parallel to the tensile axis demonstrate higher volume fraction of α′-martensite as compared to those in [001] grain orientations (Gey et al., 2005; Kireeva and Chumlyakov, 2008). The above finding can be rationalized by considering the favorable partial dislocation slips in austenite grains with orientation of [110], generating the potential martensite nucleation sites at the intersections of partial slip bands (Liu et al., 2011). It is noted that the above experimental investigations are performed by the macroscopic tensile tests. Therefore, in addition to the orientation effect, other factors such as grain size (Yang and Bhadeshia, 2009), morphology (Xiong et al., 2013), stress/strain partitioning (Ryu et al., 2010) could also affect the martensitic transformation, and the effect of orientation is challenging to be decoupled during tensile tests.
The micropillar compression experiments have attracted much attention in the material science community owing to the interesting observation of sample dimension effect (Gu and Ngan, 2014; Lin et al., 2016; Liu and Dunstan, 2017; Lu et al., 2019; Shao et al., 2014; Uchic et al., 2004a). Moreover, the micropillar compression experiments have several unique characteristics, including the distinct compression axes and ease of grain orientation selections (Uchic et al., 2006; Wheeler et al., 2013; Zhang et al., 2017). These exceptional features enable the micropillar experiments as ideal method to capture the effect of orientation on the resultant mechanical behavior of single crystal without involving the possible influence of local stress concentration (i.e., stress partitioning) and grain rotation (De Knijf et al., 2014; Eskandari et al., 2016; Gey et al., 2005). Therefore, in this contribution, the micropillars compression tests are employed to reveal the effect of orientation on the martensitic transformation in a medium Mn steel. Two austenite grains with one orientation favors full dislocation slip while the other one favors partial dislocation slip are selected. Although an extensive martensitic transformation is observed in both orientations, the critical compressive stress to trigger martensitic transformation is sharply different among these two orientations. A theoretical study is performed to understand the different critical compressive stress in initiating the martensitic transformation among micropillars with various orientations.
Section snippets
Experimental procedure
A medium Mn steel with a chemical composition of Fe-0.6C-9.5Mn (wt. %) is employed as a model material for experimental investigation. The as-received medium Mn steel is subjected to austenisation at 1273 K for 1 h and then quenched by water down to room temperature. A fully austenitic microstructure is achieved after water quenching due to its high Mn and C contents (Fig. 1 (a)). The average grain size of austenite is found to be around 150 μm based on linear intercept method (Fig. 1 (a)). The
Results
Fig. 2 (a) shows the engineering stress-strain curves of [100] and [110] micropillars during compression tests. The engineering stress and strain are calculated using the mid-length diameter and height of the pillar, respectively. Note that eight micropillars for each pillar orientation are subjected to compression tests. For clarity purpose, only four out of eight of the engineering stress-strain curves for each orientation are shown in Fig. 2 (a). For both [100] and [110] micropillars, the
Discussion
It has been demonstrated that the martensitic transformation is responsible for the occurrence of a single large strain burst in deformed [100] micropillar with a diameter of 2.5 μm, which is confirmed by correlating the magnitude of strain burst to the amount of martensite formed (Wu et al., 2014). Moreover, the larger magnitude of average strain bursts could be ascribed to the more extensive martensitic transformation (Nimaga et al., 2017). Therefore, the large strain bursts in both [100] and
Conclusion
In this study, the effect of orientation on the critical stress for martensitic transformation in a medium Mn steel is investigated by micropillar compression experiments. Despite varied orientations, all of the micropillars demonstrate the strain burst behaviors during compression tests. Nevertheless, the micropillars with different orientations have obviously different critical stress to initiate the martensitic transformation. A theoretical model is developed to understand the orientation
Acknowledgments
M.X. Huang acknowledges the financial support from National Key Research and Development Program of China (No.2017YFB0304401), National Natural Science Foundation of China (No. U1764252, U1560204), and Research Grants Council of Hong Kong (No. 17203014, 17255016, 17210418). H.W. Yen acknowledges the financial support from Ministry of Science and Technology, Republic of China (MOST-104-2218-E-002-022-MY3).
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