In situ synthesis of a porous high-Mn and high-Al steel by a novel two-step pore-forming technique in vacuum sintering

doi:10.1016/j.jmst.2019.09.008

摘要/Abstract

Abstract:

In this work, a novel in-situ two-step pore-forming process in vacuum sintering (ITPVS) technique combining low-temperature processing to produce open pores through the interdiffusion among the intrinsic components in the base steel, and subsequent high-temperature processing to further improve the porosities by the sublimation of Mn via previously formed open pores, was proposed to produce a lab-scale porous FeMnAl steel. For the first time, a high-Mn and high-Al steel with open and overall porosities of ~59.6 vol.% and ~63.7 vol.% (percent in volume, vol.%) was synthesized by isothermal holding of the quaternary elemental Fe/Mn/Al/C powder mixture at 640 ℃ for 1 h and the subsequent sintering at 1200 ℃ for 1 h. Elemental Al partly incorporated into/reacted with α-Fe and α-Mn after sintering at 640 ℃ for 1 h, leading to the overall and open porosities promoting by ~26.6 vol.% and ~25.6 vol.%. After sintering at 1200 ℃, FeMnAl steel with increased porosities mainly comprising of austenite and α-Fe obtained. The compression strength and corresponding strain of the 1200 ℃-sintered porous specimen without crack on the surface was ~75 MPa and ~25%. The ITPVS technique takes advantage of using the intrinsic components like Al, Mn and Fe in steels to produce porous structure. This is beneficial to avoiding the contamination of the FeMnAl steel matrix caused by the employment of the foreign pore-forming agents.

Key words: Porous metal, High manganese steel, Two-stage sintering, Reaction-diffusion, Sublimation method

. [J]. 材料科学与技术, 2020, 39(0): 82-88.

Chuanbing Zhuang, Zhigang Xu, Shangyu Huang, Yu Xia, Chuanbin Wang, Qiang Shen. In situ synthesis of a porous high-Mn and high-Al steel by a novel two-step pore-forming technique in vacuum sintering[J]. J. Mater. Sci. Technol., 2020, 39(0): 82-88.

图/表 9

Fig. 1. Micrographs and EDS elemental mapping analysis of the Fe-Mn-Al-C powder mixture after 5 h ball milling: (a) general morphology of the ball milled powder, (b) elemental mapping result of (a), (c) fine powder without agglomeration, (d) the agglomerated fine powder, (e) enlarged portion in (d), (f)?(i) EDS mapping results of each element in (d). Note: the dark red in (i) is due to the carbon conductive adhesive tape.

Fig. 2. X-ray diffraction patterns of the Fe-Mn-Al-C compacts: (a) green compacts after 5 h ball milling, (b) compacts sintered at 640 ℃ for 1 h, (c) compacts sintered at 1200 ℃ for 1 h.

Fig. 3. Microstructure and elemental mapping results of the cross-section of the 640 ℃-sintered Fe-Mn-Al-C compacts for 1 h: (a) microstructure of the sintered sample, (b) EDS elemental mapping result of the sintered sample, (c) enlarged portion in the rectangular box ‘A’ of (a), (d) enlarged portion in the rectangular box of (c), (e) enlarged portion in the rectangular box ‘B’ of (a), (f) enlarged portion in the rectangular box ‘C’ of (a). Note: The EDS mapping cannot actually measure carbon content due to both its light weight in nature and low content of <1 wt%. Thus, carbon was not put in (c). ‘S’ in Fig. 3(c), (e) and (f) denote as the start point of EDS line scan.

Fig. 4. EDS line scan results of the 640 ℃-sintered samples for 1 h: (a) chemical composition distribution on the distance marked with blue line in Fig. 3(c), (b) chemical composition distribution on the distance marked with blue line in Fig. 3(f), and (c) chemical composition distribution on the distance marked with blue line in Fig. 3(e). Note: S denotes as the start point of the analysis, the blue dash lines were presented to separate different phases.

Fig. 5. Microstructure and elemental mapping results of the cross-section of the 1200 ℃-sintered Fe-Mn-Al-C compact for 1 h: (a) microstructure of the sintered samples at low magnification, (b) morphology at the center of the sintered compacts, (c) elemental mapping result of the sintered sample, (d) region containing possible Al4C3 intermetallic phase, (e) enlarged area in the rectangular box of (d), (f) region contains possible k-carbide phase, (g) the cross view of the sintered sample without grinding, (h) enlarged area in the rectangular box of (g). Note: The EDS mapping cannot actually measure carbon content due to both its light weight in nature and especially the low content of <1 wt%. Thus, carbon was not mapping in (c).

Table 1 Results of the average chemical compositions at points illustrated in Fig. 5.

Point	Average chemical composition (at.%)^a				Possible phases
	Fe	Mn	Al	C
1	75.32	1.43	19.74	3.51	α-Fe
2	70.36	9.26	16.26	4.12	α-Fe + Austenite
3	54.06	26.40	15.22	4.32	Austenite
4	-	-	65.1	34.9	Al₄C₃ + C
5	55.95	9.26	18.71	16.08	k-carbide

Fig. 6. Macroscopic images (a), weight loss fraction (b), expansion rate (c) and porosities (d) of the sintered samples at different status. Note: 640 ℃ denotes as the samples pre-sintered at 640 ℃ for 1 h, 1200 ℃ denotes as the samples final-sintered at 1200 ℃ for 1 h. Porosities of the green compacts were given in (d) for comparison purpose.

Fig. 7. Stress?strain curve of the 1200 ℃-sintered porous samples.

Fig. 8. Compression photographs of the 1200 ℃-sintered porous steel at different strain stage: (a) initial, (b) 10% strain, (c) 15% strain, (d) 20% strain, (e) 25% strain, (f) 32%, (g) final, (h) comparison of the uncompressed sample and final compressed samples.

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