Analyzing the scale of the bainitic ferrite plates by XRD, SEM and TEM
Introduction
Microstructures consisting of a matrix of bainitic ferrite with a second phase dispersion of other constituents, such as retained austenite or M-A (martensite-austenite) constituent have been developed by optimizing heat treatment schedules and chemical composition in Si rich steels [1], [2], [3], [4]. It has been proposed that like in the case of martensite, the growth of bainite is displacive, and thus it can be described by an invariant plane strain (IPS). The associated strain has two components, a large shear deformation with a value of about 0.46 for nanostrutured bainite and in the range of 0.22–0.26 for sub-micron bainite [5], [6]. The second component is a smaller dilatational strain (~ 0.03). The associated strain is so large that the transformation product is in the form of a thin plate, which minimizes the strain energy when the plate is elastically accommodated. As the large strains associated with the transformation cannot be sustained only elastically by the parent austenite, those are relaxed by the plastic deformation of austenite [5], [7]. Under such scenario, generated dislocations are inherited by the product phase, bainitic ferrite, limiting the final thickness of the formed bainitic ferrite plate [8], [9]. But, there are other important factor controlling the scale of the microstructure, thus, a strong austenite or a large driving force would also lead to a refinement of the bainitic ferrite plates, the former because there is a larger resistance to interface motion and the latter because an increased nucleation rate leads to microstructural refinement [10], [11]. The magnitude of the mentioned parameters, dislocation density, austenite strength and driving force, increases as temperature decreases, explaining the observed refinement of bainitic microstructures when the transformation temperature is lowered [7].
At this point it is necessary to clarify the different morphologies that bainite might present. One of the most recent and comprehensive classifications was given by Zajac et al. [12], [13], which may be applied for both low-carbon and high-carbon bainite. Thus, bainite is divided into groups according to bainitic ferrite morphology and the type and distribution of second phases: granular bainite with irregular bainitic ferrite grains containing globular island shaped second phases (either retained austenite or martensite); upper bainite with several lath-like bainitic ferrite crystals parallel to each other within sheaves and separated by a second phase (either cementite or M-A constituent); and lower bainite with bainitic ferrite plates containing a fine dispersion of precipitated carbide particles inside. It is important to remark that in granular bainite microstructures, the relatively coarse grains of ferrite present a well-developed substructure. In fact, they consist of sheaves of parallel plate-like subgrains of bainitic ferrite with very thin regions of austenite between the sub-units because of the low carbon concentration of the steels involved [14].
Carbide precipitation during bainitic transformation can be suppressed by suitable alloying element. It has been demonstrated that instead of the classical structure of bainitic ferrite laths separated by carbide crystals, a microstructure consisting of bainitic ferrite sub-units interwoven with thin films of untransformed retained austenite can be obtained by alloying the steel with about 1.5 wt.% of Si [7].
Like in the case of martensite, sub-units of bainitic ferrite would change its morphology from lath-like to plate-like with increasing carbon content, low transformation temperatures or in strong austenite [7].
It is well documented that most of the strength of this microstructure relies on the scale of the bainitic ferrite [1], [2], [3], [4] and it is the plate thickness rather than the length which primarily determines the mean free slip distance [15]. Therefore, accurate determination of plate thickness is fundamental for quantitative relating the microstructure to the mechanical properties. This paper deals with the determination of the bainitic ferrite plate thickness by means of SEM and TEM. For this purpose, two very different, in terms of the scale, bainitic microstructures were selected, one in the nano range the other submicron. As plate thickness determination from TEM or SEM micrographs can be tedious and time consuming, it has been recognized in previous works that the crystallite size determination from XRD peak profile analysis was an alternative [16]. But a direct comparison between the sizes obtained using an image analysis procedure with the crystallite size calculated from XRD patterns showed that indeed crystallite and plate size refer to two very different concepts.
Section snippets
Materials and Experimental Procedures
For the purpose of this work, two bainitic steels with chemical compositions given in Table 1 were used. They will be referred according to the scale of their final bainitic microstructure, i.e. as Nano and Sub for a nanostructured and submicron bainite, respectively. Both alloys were selected for having very different C contents, which also implies very distinctive Bs and Ms transformation temperatures, and also final scale of the bainitic microstructure, see Table 1. A common feature in both
General Description
Regardless of the steel and heat treatment, the microstructure in all cases consisted in bainitic ferrite and retained austenite. TEM observations, as reported in previous works, failed to reveal cementite precipitation [9], [22]. Examples of the microstructures at selected temperatures and magnifications can be found in Fig. 1, where both phases have been identified. XRD measurements come to support these results, showing that the predominant phase is ferrite, the remaining being retained
Summary
The method and applicable corrections when measuring the bainitic ferrite plate thickness on SEM and TEM micrographs have been explained in detail. The achieved plates size, on two bainitic steels treated at different isothermal temperatures to obtain a nano and submicron structure, were also compared with the crystallite size obtained from peak broadening analysis in X-ray. Results thus obtained clearly show, as expected, a very good agreement between measurements performed on SEM and TEM but
Acknowledgements
The authors gratefully acknowledge the support of the European Research Fund for Coal and Steel, the Spanish Ministry of Economy and Competitiveness and the Fondo Europeo de Desarrollo Regional (FEDER) for partially funding this research under the contracts RFSR-CT-2014-00019 and MAT2013-47460-C5-1-P respectively. The authors acknowledge the support of the following laboratories at CENIM, X Ray diffraction, Metallography and Phase Transformations. We would like to also express our gratitude to
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Present Address: B. L-E at CEIT and TECNUN, Paseo de Manuel Lardizabal 15, 20018 San Sebastian, Spain.
L. M-R at University of Kaiserslautern, Materials Testing, Gottlieb-Daimler-Str., 67663 Kaiserslautern, Germany.