Rietveld refinement, microstructure and high-temperature oxidation characteristics of low-density high manganese steels

doi:10.1016/j.jmst.2017.09.012

摘要/Abstract

Abstract:

Three forged low-density high manganese steels Mn28Al10, Mn28Al8 and Mn20Al10 were used as experimental materials in this study. The forged microstructure and external oxidation characteristics at 1323 K and 1373 K for 5-25 h in air were investigated by microstructural observation and X-ray diffraction (XRD) technique. The phase compositions and abundance in the forged and oxidized samples were quantitatively obtained by Rietveld method on the basis of XRD pattern analysis. The results showed that an austenitic microstructure formed in steels Mn28Al10 and Mn28Al8, and 18.02 wt% ferrite could be found in Mn20Al10. The relative amount of ~5.28 wt% κ-carbide (Fe₃AlC_0.5) in Mn28Al10 was far greater than that in Mn28Al8 and Mn20Al10. The oxidation kinetics of forged steels oxidized at 1323 K for 5-25 h had two-stage parabolic rate laws; and the oxidation rate of the first stage was lower than that of the second stage. When they were oxidized at 1373 K for 5-25 h, the oxidation kinetics followed only a parabolic law and the oxidation rates were respectively greater than those at 1323 K for 5-25 h. When they were oxidized at 1323 K for 25 h, detached external scales contained Fe₂MnO₄ and α-Fe₂O₃ oxides. α-Al₂O₃ and (Fe, Mn)₂O₃ oxides could only be indexed in steels Mn28Al8 and Mn28Al10, respectively. When they were oxidized at 1373 K for 25 h, Fe₂MnO₄, Fe₃O₄, α-Fe₂O₃ and α-Al₂O₃ oxides could all be indexed in the external detached scales. The main phase of detached external scales was Fe₂MnO₄; and the relative amount of α-Al₂O₃ in steel Mn28Al8 was higher than that in steels Mn28Al10 and Mn20Al. The external oxidation layers of these three forged steels oxidized at 1323 K and 1373 K for 25 h were essentially followed the sequence of α-Al₂O₃, Fe₂MnO₄, Fe₃O₄, FeMnO₃, and Fe₂O₃ from the substrate to the outside surface. The forged Mn28Al10 steel with austenitic microstructure and a certain amount of κ-carbide (~5.28 wt% in the present work) possessed a better combination of strength, ductility, specific strength, and oxidation rate when compared to that of the forged Mn28Al8 and Mn20Al10 steels.

Key words: High manganese steel, Low-density steel, Rietveld method, Microstructure, Oxidation

. [J]. 材料科学与技术, 2017, 33(12): 1531-1539.

Huang Zhenyi, Jiang Yueshang, Hou Along, Wang Ping, Shi Qi, Hou Qingyu, Liu Xianghua. Rietveld refinement, microstructure and high-temperature oxidation characteristics of low-density high manganese steels[J]. J. Mater. Sci. Technol., 2017, 33(12): 1531-1539.

图/表 14

Table 1 Composition of the forged steels (wt%) and their density (g/cm3).

Steel	C	Mn	Al	Fe	Density
Mn28Al10	1.05	28.13	10.04	Bal.	6.76
Mn28Al8	1.04	27.87	8.26	Bal.	6.95
Mn20Al10	0.98	19.40	9.82	Bal.	6.77

Fig. 1. Microstructures for the forged Fe-Mn-Al-C steels: (a) Mn28Al10, (b) Mn28Al8, (c) Mn20Al10 and (d) κ-carbide in Mn28Al8 steel.

Fig. 2. XRD pattern for the forged Fe-Mn-Al-C steels with refined data obtained by Rietveld method. The experimental points are given as plus (×) and theoretical data are shown as solid line. There is a difference in curves at bottom of XRD patterns between theoretical and experimental data. The vertical lines represent the Bragg’s allowed peaks for γ(Fe, Mn), Fe3AlC0.5 (κ-carbide) and α(Fe, Mn) (from above), respectively: (a) Mn28Al10, (b) Mn28Al8 and (c) Mn20Al10.

Table 2 Phase constitution, structural parameters, and phase abundance of the forged steels as refined by XRD Rietveld method.

Steel	Phases	Space group	Lattice parameters (nm)			Abundance (wt%)
			a	b	c
Mn28Al10	γ(Fe,Mn)	Fm$\bar3$m	0.36836(8)			92.85
R_wp = 3.27%	Fe₃AlC_0.5	Pm$\bar3$m	0.37123(8)			5.28
S = 3.46	α(Fe,Mn)	Im$\bar3$m	0.29048(9)			1.87
Mn28Al8	γ(Fe,Mn)	Fm$\bar3$m	0.36773(2)			100
R_wp = 6.43%
S = 3.41
Mn20Al10	γ(Fe,Mn)	Fm$\bar3$m	0.36844(5)			81.98
R_wp = 8.64%	γ(Fe,Mn)	Fm$\bar3$m	0.36844(5)			81.98
S = 5.88	α(Fe,Mn)	Im$\bar3$m	0.29071(5)			18.02

Table 3 Mechanical properties of the forged steels.

Steel	UTS (GPa)	TE (%)	Density (g/cm³)	UTS × TE (GPa %)	UTS/density (GPa g/cm³)
Mn28Al10	1.21	45.08	6.76	54.55	0.18
Mn28Al8	1.06	65.32	6.95	69.24	0.15
Mn20Al10	1.21	35.20	6.77	42.59	0.18

Fig. 3. Oxidation kinetics curves (a) and (b) the square of weight gain per unit area against time for the Fe-Mn-Al-C steels at 1323 K.

Table 4 Parabolic rates for the forged steels oxidized at 1323 K for 5-25 h.

Steel	Rate constant (mg²/cm⁴/h)
	k_pi	k_pf
Mn28Al10	17.54 (5-10 h)	32.50 (10-25 h)
Mn28Al8	36.81 (5-10 h)	457.17 (10-25 h)
Mn20Al10	9.94 (5-20 h)	284.51 (20-25 h)

Fig. 4. Oxidation kinetics curves (a) and (b) the square of weight gain per unit area against time for the Fe-Mn-Al-C steels at 1373 K.

Fig. 5. XRD pattern for the Fe-Mn-Al-C steels oxidized at 1323 K for 25 h with refined data obtained by Rietveld method. The experimental points are given as plus (×) and theoretical data are shown as solid line. There is a difference in curves at bottom of XRD patterns between theoretical and experimental data. The vertical lines represent the Bragg’s allowed peaks for Fe2MnO4 (Fd$\bar3$m), α-Fe2O3 (R$\bar3$c) and (Fe,Mn)2O3 (Ia$\bar3$) in Mn28Al10 (a), Fe2MnO4 (Fd$\bar3$m), Fe3O4 (Fd$\bar3$m), α-Fe2O3 (R$\bar3$c) and α-Al2O3 (R$\bar3$c) in Mn28Al10, and Fe2MnO4 (Fd$\bar3$m) and α-Fe2O3 (R$\bar3$c) in Mn20M10 (c) (from above), respectively.

Table 5 Phase constitution, structural parameters, and phase abundance of the oxides in the detached scales of the steels oxidized at 1323 K for 25 h as refined by XRD Rietveld method.

Steel	Phases	Space group	Lattice parameters (nm)			Abundance (wt%)
			a	b	c
Mn28Al10	Fe₂MnO₄	Fd$\bar3$m	0.84777(10)			77.79
R_wp = 5.29%	α-Fe₂O₃	R$\bar3$c	0.50247(9)		1.37007(9)	15.35
S = 5.52	FeMnO₃	Ia$\bar3$	0.94024(10)			6.86
Mn28Al8	Fe₂MnO₄	Fd$\bar3$m	0.85089(9)			41.76
R_wp = 8.23%	Fe₃O₄	Fd$\bar3$m	0.84388(8)			7.65
S = 2.72	α-Fe₂O₃	R$\bar3$c	0.50297(8)		1.37118(9)	35.45
	α-Al₂O₃	R$\bar3$c	0.47932(10)		1.30936(8)	15.14
Mn20Al10	Fe₂MnO₄	Fd$\bar3$m	0.84685(6)			70.35
R_wp = 5.57%
S = 2.02	α-Fe₂O₃	R$\bar3$c	0.50281(7)		1.37094(8)	29.65

Fig. 6. XRD pattern for the Fe-Mn-Al-C steels oxidized at 1373 K for 25 h with refined data obtained by Rietveld method. The experimental points are given as plus (×) and theoretical data are shown as solid line. There is a difference in curves at bottom of XRD patterns between theoretical and experimental data. The vertical lines represent the Bragg’s allowed peaks for Fe2MnO4 (Fd$\bar3$m), Fe3O4 (Fd$\bar3$m), α-Fe2O3 (R$\bar3$c) and α-Al2O3 (R$\bar3$c) (from above), respectively: (a) Mn28Al10, (b) Mn28Al8 and (c) Mn20Al10.

Table 6 Phase constitution, structural parameters, and phase abundance of in the detached scales of the steels oxidized at 1373 K for 25 h as refined by XRD Rietveld method.

Steel	Phases	Space group	Lattice parameters (nm)			Abundance (wt%)
			a	b	c
Mn28Al10	Fe₂MnO₄	Fd$\bar3$m	0.84895(8)			56.21
R_wp = 4.35%	Fe₃O₄	Fd$\bar3$m	0.84362(7)			6.01
S = 2.84	α-Fe₂O3	R$\bar3$c	0.50299(8)		1.37084(9)	22.49
	α-Al₂O₃	R$\bar3$c	0.47912(9)		1.30847(9)	15.29
Mn28Al8	Fe₂MnO₄	Fd$\bar3$m	0.85101(7)			47.61
R_wp = 8.15%	Fe₃O₄	Fd$\bar3$m	0.84320(8)			10.23
S = 2.73	α-Fe₂O3	R$\bar3$c	0.50303(8)		1.37148(9)	24.47
	α-Al₂O₃	R$\bar3$c	0.47915(9)		1.30803(9)	17.69
Mn20Al10	Fe₂MnO₄	Fd$\bar3$m	0.84711(9)			40.43
R_wp = 7.97%	Fe₃O₄	Fd$\bar3$m	0.84211(7)			5.53
S = 2.25	α-Fe₂O₃	R$\bar3$c	0.50253(5)		1.36999(9)	37.80
	α-Al₂O₃	R$\bar3$c	0.47917(8)		1.30707(9)	16.24

Fig. 7. Al2p3/2 and O1s XPS spectra of Al2O3 formed between the substrate and the detached scales formed in the steels oxidized at 1323 K for 25 h: (a) Al2p3/2 spectra, (b) O1s spectra.

Fig. 8. Al2p3/2 and O1s XPS spectra of Al2O3 formed between the substrate and the detached scales formed in the steels oxidized at 1373 K for 25 h: (a) Al2p3/2 spectra, (b) O1s spectra.

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