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X-ray powder diffraction evidence for the incorporation of W and Mo into M 23 C 6 extracted from high-temperature alloys

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X-ray powder diffraction evidence for the incorporation of W and Mo into M 23 C 6 extracted from high-temperature alloys

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X-ray powder diffraction patterns of M23C6 where M=(Cr, Mo) and M=(Cr, W) have been recorded accurately using a focusing Guinier camera and a diffractometer. The improved resolution of the modern focusing diffraction equipment made it possible to detect new features in the powder diffraction patterns, showing that M23C6 carbides extracted from W- and Mo-containing alloys differ from pure Cr23C6 and (Cr, Fe)23C6 met in more conventional stainless steels. The distinctions concern the lattice parameters, as well as the intensities of specific diffraction maxima. These differences can be explained in terms of the theoretical considerations of Goldschmidt who predicted that W and Mo, when incorporated in Cr23C6, will preferentially occupy specific positions in the unit cell. Diffraction intensity calculations indeed confirm that the introduction of W and Mo atoms on these particular positions will modify the diffracted intensities in the same way as was observed experimentally.
... Based on the fitting results, it can be derived that the lattice parameter of the Cr-rich M 23 C 6 carbides in experimental steels falls in a range of 1.061~1.065 nm, which is similar to the value of a pure Cr 23 C 6 phase (1.0658 nm) from Erdos [29], The slight difference between them could be induced by the solution of Fe atoms into Cr 23 C 6 [30]. ...
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[[ Free access to this paper provided by the publisher Elsevier until Nov. 07, 2020. See: https://authors.elsevier.com/c/1bmKp,FwinMyY ]] In this work, the essential mechanism of “loss of passivation” (LOP) effect for a 13 wt. % Cr-type martensitic stainless steel (MSS) is studied via electrochemical and microstructure approaches. The MSSs were quenched at 1030 °C and then tempered at 440−600 °C. Electrochemical results show that the tempering range for “LOP” is 460−540 °C. Severe corrosion occurred on 520 °C-tempered MSS after being immersed in Cl− containing solution for 35 min. Transmission electron microscopy (TEM) observation towards the corroded area of 1030-520 sample reveals the massive precipitation of nano-sized Cr-rich carbides, which is the essential cause for the “LOP” of MSSs.
... The obtained parameter could be regarded as the same based on the measurement precision of the present experiments, which are slightly smaller than the value of a pure Cr 23 C 6 phase (1.0658 (1) nm) in Erdos's research [20]. This could be induced by the solution of Fe atoms into the Cr 23 C 6 [21]. In addition, the intensity of the Cr-rich M 23 C 6 peaks of the 1030-650 sample is much stronger than that of the other samples, indicating that the precipitation of M 23 C 6 is promoted by increasing the tempering temperature to 650 C. Fig. 3 shows the SEM morphologies of the experimental samples. ...
... The diffraction angles fitted by MDI Jade version 5.0 fitting software were 48.514(4)°, 50.881(4)°, and 51.659 (5) Fitting results reveal that the lattice parameter of Cr-rich M 23 C 6 (FCC) in annealed, 980 and 1,030 samples are 1.061(1), 1.063(1), and 1.064(1) nm, respectively. Based on the measurement precision of the present experiments, the obtained lattice parameters can be regarded as the same, however, they are slightly smaller than the value of 1.0658(1) nm of pure Cr 23 C 6 noted in Erdos's research [19], perhaps due to the solution of Fe into the pure Cr 23 C 6 [20]. In addition, the peak intensity of Cr-rich M 23 C 6 of the annealed sample was much greater than that of the quenched samples. ...
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