Elsevier

Materials Characterization

Volume 94, August 2014, Pages 93-110
Materials Characterization

EBSD imaging of orientation relationships and variant groupings in different martensitic alloys and Widmanstätten iron meteorites

https://doi.org/10.1016/j.matchar.2014.05.015Get rights and content

Highlights

  • Analysis of different low and high carbon steels and Widmanstätten meteorites

  • Automatic color mapping of the classical orientation relationships in EBSD maps

  • Quantification of variant pairing and grouping tendencies

  • Crystallographic scenario for the formation of morphological packets

Abstract

An automatic method to colorize and quantify the classical Pitsch, Kurdjumov–Sachs, Greninger–Troiano and Nishiyama–Wasserman orientation relationships in the electron backscatter diffraction maps of martensitic/bainitic steels is detailed. Automatic analysis of variant grouping is also presented. The method was applied to low and high carbon steels, and to iron–nickel Widmanstätten meteorites. Many results of recent literature are confirmed. In low carbon steels the individual laths exhibit continuous orientation gradients between the classical orientation relationships, and the laths tend to be grouped by close-packed plane (morphological) packets. A crystallographic scenario describing the formation of the packets is proposed on the base of the one-step model. When the carbon content increases, the orientation spreading is reduced; and martensite tends to form plate groups and burst configurations. In iron–nickel meteorites, the centimeter long Widmanstätten laths do not exhibit continuous orientation gradients but are constituted of subgrains with uniform orientation relationship; the kamacite grains in the plessite regions are grouped into Bain zones, probably due to a recrystallization during the slow cooling of the meteorites.

Introduction

We recently proposed two models of martensitic phase transformations from face centered cubic (fcc) γ austenite to body centered cubic (bcc) α martensite or bainite [1], [2]. Both models are built from the same approach: the pole figures of martensite obtained by electron backscatter diffraction (EBSD) or X-ray diffraction exhibit continuous features that are considered as the plastic trace of the transformation. These figures can be simulated by using the Kurdjumov–Sachs (KS) variants and two continuous rotations: A(a) around <111>α//<110>γ and B(b) around <110>α//<111>γ of respective angles a and b varying between 0 and a limit of few degrees. These two rotations are sufficient to create a continuous link between the orientations usually reported in literature: KS, Nishiyama–Wasserman (NW), Greninger–Troiano (GT) and Pitsch ORs (some references are given in [1]). The classical phenomenological theory of martensite transformation/crystallography (PTMT or PTMC) developed in the 1950s is widely used in metallurgy (see for example [3], [4]), but is unable to explain these EBSD features. The two-step model [2] supposes the existence of an intermediate fleeting hexagonal close packed (hcp) ε phase; and the γ  ε and the ε  α steps could explain the rotations A and B, respectively. The one-step model [1] correlates the rotations A and B under the same mechanism: martensite nucleates by a Pitsch distortion along the close packed direction [110]γ = [111]α and grows by the same distortion in the austenite matrix deformed by the transformation itself, leading to the KS, GT and NW ORs. The Pitsch distortion respects the hard sphere packing of the iron atoms; its principal strains calculated in a non-orthonormal basis are lower than for the Bain distortion. This model makes us expect orientation gradients inside the martensitic laths starting from Pitsch OR in the middle and growing laterally to KS and NW ORs, as represented in Fig. 1.

The PTMC remains defended in the metallurgical community and criticisms were raised against the two-step model by Bhadeshia [5] (see our reply [6]), and more recently by Chintha et al. [7]. These last authors affirm that “for every material, where γ  α′ transformation is thermodynamically possible, there is a unique OR between the parent and the product phase” taking for argument that “the KS OR was determined on the investigations done in low-carbon martensitic steel and the NW OR was determined based on the work done on Fe–Ni alloys”. It seems that even when the OR deviates from KS or NW (as it is the case with GT), these authors assume that there is a unique OR for each alloy. To support their claim, they showed that the EBSD continuous features tend to disappear in high carbon steels. One can imagine that similar arguments could be opposed to the one-step model. Despite the affirmation of Chintha et al., the coexistence of different KS, NW and other ORs in a same alloy is widely recognized. This has been shown in FeCrC steels by transmission electron microscopy (TEM) [8], and in FeNi alloys [9], [10] and some FeNi meteorites [11], [12], [13], [14] by EBSD and X-ray diffraction. Low misorientations inside blocks of laths are also commonly observed in FeC bainite and martensite [16], [17] and Widmanstätten ferrite [18], which makes impossible the existence of a unique global and uniform OR. The KS/NW/Pitsch proportions have been quantified by Verbeken et al. [19] in Transformation Induced Plasticity (TRIP) steels by using some retained austenite in the EBSD maps. An average austenite/martensite OR can be obtained by filtering the pole figures [13], or, when there is no retained austenite, by advanced reconstruction methods that determine both the austenite orientation and the local OR [20], [21], [22], [23], [24], [25], [26]. The average OR is always found close to the GT OR for low or medium carbon content steels. We stress that this OR is an average calculated from the experimental orientation spreading. Until now, the different ORs are shown on the pole figures but not directly on the EBSD maps.

We present a method for coloring the orientation relationships in the EBSD maps. It will be shown that (i) there is not a unique OR in steels whatever the carbon content, (ii) the orientations inside the martensitic laths/plates gradually change between Pitsch, KS and NW ORs (with an average OR close to GT), and (iii) the proportions of the ORs depend on the transformation temperature.

In addition, variant selection and variant grouping tendencies will be statistically quantified and the results will be compared to literature. Indeed, interesting results on variant selection have been published in the last years. In low carbon steels, martensite forms morphological packets of quasi-parallel laths lying on the habit planes (HP) of type {557}γ//{165}α which are few degrees away from the common close-packed plane (CPP) (111)γ//(110)α of the KS OR [16], [17]. In these packets, the laths are grouped into three blocks (i.e. pairs a low-angle misoriented variants) linked by the 60° rotations around the common [111]γ//[110]α normal to the CPP [17]. Bainite forms groups of intricated low-angle misoriented laths with non-parallel HPs which exhibit only one Bain circle in the <100>α pole figure; they are called Bain groups, or crystallographic packets [16]. In the rest of the paper, we call CPP group the morphological packet, and Bain group the crystallographical packet. The intricacy of the laths in the different packets results from a self-accommodation process [27]. The sequence of lath formation was observed in-situ by using a high-temperature laser scanning confocal microscope coupled with EBSD mapping [28]. It was recently shown that CPP and Bain groups tend to form at low and high temperatures, respectively [29]. In high carbon steels, martensite appears at low temperature as interconnected lenticular or thin plates forming nice figures such as spear, diamond, wedge, kink and zigzag [30]. The martensite in such assemblies is constituted of four variants with {3 10 15}γ HP oriented such that their normal directions are clustered about a common [110]γ axis. Such assemblies are called “plate” groups; they are formed by an autocatalytic burst mechanism [31]. Stormvinter et al. [32] could recently show by statistical analysis of EBSD maps that there is a progressive transition from CPP grouping at low carbon content to plate grouping at high carbon content; the carbon limit for the change is around 0.8%. Plastic deformation in the austenite state (ausforming) has an important impact on variant selection. In lenticular or plate martensite, plate groups with habit plane nearly perpendicular to the active slip plane are favored [31]. In lath martensite, CPP groups with the (111)γ//(110)α common plane parallel to the primary slip plane are preferentially formed. When two slip planes are activated, martensite forms groups of four variants elongated along the <110>γ direction which is intersection of the two {111}γ slip planes [33], [34], [35]. Such groups, called close-packed direction (CPD) group [1], contain the four variants that share a common <110>γ//<111>α axis. The CPP or CPD tendencies were recently studied by Chiba et al. [36]; the authors could attribute the difference of variant selection to a difference of deformation capacity of austenite. It can also be noticed that CPD grouping was also observed in steels containing ε and α martensites, the four α variants in a CPD group being formed at the intersection of two ε martensite plates [37].

The other aim of the paper is to quantify variant selection in different steels and iron–nickel meteorites in order to reinvestigate/confirm some of the results recently reported in literature. We will also take the opportunity of this paper to propose a simple representation of the different variant groups. A new assembly of variants based on the plate group and called “burst configuration” will be also introduced. The methods for the OR coloring and statistical analysis of the variant grouping are fully automatic with no need of retained austenite. These observations will bring new information to explore theoretical models (for example Fig. 1 will be corrected) or to quantify martensitic/bainitic microstructures in industrial steels.

Section snippets

Materials

The materials under study are low and medium carbon commercial steels (EM10, APX4 and FeCSi) and a high purity high carbon steel (Fe1.8C). More information on the EM10 and Fe1.8C steels are given in [38], [32], respectively. Two iron–nickel meteorites were also included in our analysis. Some details are given in Table 1. The Muonionalusta and Gibeon meteorites are IVA octahedrites that have been chosen for their nice Widmanstätten patterns visible to the naked eye. They are formed by nucleation

Method

The theoretical background and the method for coloring the orientation relationships are described in Fig. 2 with a simple example chosen for didactic reasons: it is a parent square crystal γ with its daughter variants αi which are triangles oriented such that γ and αi share a common mirror symmetry.

Results

Low carbon steels are constituted of interconnected martensitic laths or plates. When the carbon content increases, the laths become more lenticular and the content of retained austenite increases. The habit planes of the martensite in the low carbon steel of Fig. 6 are close to {111}γ and close to {557}γ in APX4 (Fig. 7). They are less easy to identify in the medium carbon steel FeCSi (Fig. 8). The interlaced Widmanstätten laths in the meteorites are elongated along the common <110>γ//<111>α

Discussion

This work shows that it is possible to automatically and systematically quantify the OR spreading and variant grouping in martensitic steels and other iron alloys. Some tests of sensitivity on the choice of the initial OR and reconstruction parameters have been performed. The precise shape of the reconstructed grains can vary but the parent orientations are stable with deviations lower than 2°. The percentages of Pitsch, KS, NW and GT ORs can change according to the choice of the initial OR,

Conclusions

An automatic semi-quantitative method to colorize in RGB or pure color mode the classical orientation relationships in the EBSD map of martensitic/bainitic steels was detailed and applied to different materials, such as low and high carbon steels and iron–nickel Widmanstätten meteorites. In addition, automatic variant grouping analysis was performed. Many results recently published by other researchers were confirmed. In low carbon steels, the individual laths exhibit continuous orientation

Acknowledgments

We would like to thank Denis Venet for the sample preparation, Romain Soulas for the heat treatments of the FeCSi alloys, and Albin Stormvinter for our discussions and for providing the EBSD map of the Fe1.8C steels.

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