PSI - Issue 69

R. Surki Aliabad et al. / Procedia Structural Integrity 69 (2025) 69–75

71

In-situ HEXRD was employed to explore the microstructural evolution of the hot-rolled (HR) sample upon heating. The sample was heated at a rate of 1°C/s to 700°C, slightly above the Ac1 temperature (640 °C). This HEXRD analysis was performed at the Brockhouse High Energy Wiggler Beamline, located at the Canadian Light Source (CLS). The HR sample, with dimensions of 1 × 1 × 20 mm³, was housed in a quartz capillary while it was heated in a high temperature capillary furnace. A 70 keV monochromatic X-ray beam (wavelength: 0.1779 Å) was employed, with the sample-to-detector distance set at 1116 mm. Calibration was done at room temperature using a nickel standard. The resulting two-dimensional diffraction data were collected using an area detector, Varex XRD 4343CT detector, and radially integrated and analyzed using the Rietveld refinement method in GSAS-II software. Field emission scanning electron microscope (FESEM) couples with electron backscatter diffraction (EBSD) detector was utilized to analyze the microstructure. Samples were prepared through several steps, including sectioning, grinding, polishing with a 1 μm diamond paste, and chemical etching in a 2% Nital solution (2% nitric acid in ethanol) for about 10 seconds at room temperature. The analyses were performed using an InLens and EBSD detectors at 5 kV and 15 kV accelerating voltage, with a working distance of 4 mm and 15 mm, respectively. For detailed microstructural characterization, Scanning Transmission Electron Microscopy (STEM) was employed using a JEOL JEM-2200FS Energy Filtered Scanning Transmission Electron Microscope (EFTEM/STEM) at 200 kV. Thin lamella specimens measuring approximately 8 × 10 μm were prepared using a focused ion beam-field emission scanning electron microscope (FIB-FESEM) equipped with a Ga ion source. Thermodynamic and kinetic simulation was carried out using Thermo-Calc 2024a software. The phase equilibrium was modeled utilizing the TCFE13 thermodynamic database. 3. Results and Discussion The microstructure of the hot-rolled and subsequently air-cooled steel was analyzed using TEM and EBSD methods, as illustrated in Fig. 1. The microstructure consisted of mainly a lath martensitic matrix with plate-like carbides heterogeneously precipitated within the laths. These carbides, measuring 10–20 nm in thickness and less than 100 nm in length, are clearly visible obtained on a wide block of tempered martensite (Fig. 1a). In addition to the carbides, HEXRD results at room temperature detected approximately 7 vol.% RA. EBSD phase map (Fig. 1b) revealed RA with an average thickness of 300 nm.

Fig. 1. (a) TEM BF image of plate-like carbides within the martensite matrix; (b) EBSD phase and band contrast maps highlighting RA morphology and size; (c) SAED patterns of carbides and martensite; (d) key diagram for diffraction pattern, beam // [111] ! // [110] " ; (e) TEM DF image of -carbides with zone axis 2 -2 0 taken from the area shown in (a). In this study, carbides precipitated in the martensite matrix (Fig. 1a) were analyzed using TEM microscopy. The diffraction pattern revealed a structure consistent with the η-Fe₂C carbide, as shown in Fig. 1c, with the [111] ! //