Microstructure, mechanical properties and toughening mechanism of directional Fe2B crystal in Fe-B alloy with trace Cr addition

doi:10.1016/j.jmst.2020.03.058

Abstract

Abstract:

Exploring the effective way to improve Fe₂B’s toughness has always been the hot topic in the researches of Fe-B wear resistant alloys. In the present work, the effects of Cr on the microstructure, mechanical properties and lattice structure of directional Fe₂B have been investigated. The affecting mechanism of Cr addition has been discussed according to the experimental and first-principle calculation analysis. The results show Cr addition can improve the toughness of directional Fe₂B on the longitudinal sections perpendicular to (002) crystal plane, without sacrificing the hardness distinctly. The toughening mechanism by Cr substitution has been revealed: Cr addition enriches the electron density between the B atoms along [002] direction, contributing to the shrinkage of the bond length and the enhancement of the bond strength of B—B bonds. The obtained results provide insight into the intrinsic reason for toughening Fe₂B by Cr doping.

Key words: Directional Fe₂B, Cr addition, Lattice evolution, Toughening mechanism

Yongxin Jian, Zhifu Huang, Xiaoting Liu, Jialin Sun, Jiandong Xing. Microstructure, mechanical properties and toughening mechanism of directional Fe₂B crystal in Fe-B alloy with trace Cr addition[J]. J. Mater. Sci. Technol., 2020, 57: 172-179.

Figures/Tables 10

Fig. 1. SEM microstructures of DS Fe-B cast alloy: (a) transversal section of 0Cr; (b) longitudinal section of 0Cr; (c) transversal section of 2.5Cr; (d) longitudinal section of 2.5Cr; (e) deep etching of transversal section; (f) deep etching of longitudinal section.

Fig. 2. XRD patterns of the transversal section of DS Fe-B cast alloy.

Fig. 3. EPMA mapping analysis of DS Fe-B cast alloy: (a) transversal section of the analyzed area; (b) longitudinal section of the analyzed area; (c) Cr distribution on transversal section; (d) Cr distribution on longitudinal section; (e) Fe distribution on transversal section; (f) Fe distribution on longitudinal section.

Fig. 4. Mechanical properties from nano-indentation: (a) hardness and Young’s modulus; (b) H/E ratio.

Fig. 5. Load-displacement curves of DS Fe-B cast alloy: (a) transversal section; (b) longitudinal section.

Fig. 6. HRTEM analysis of directional Fe2B crystal: (a) bright-filed microstructure; (b) interplanar spacing of (002) plane; (c) HRTEM microstructure of 0Cr; (d) HRTEM microstructure of 1Cr; (e) HRTEM microstructure of 2Cr; (f) HRTEM microstructure of 2.5Cr.

Table 1 Lattice and energy parameters of Fe8B4 and Fe7CrB4.

Species	a (?)	b (?)	c (?)	Volume (?³)	ρ (g/cm³)	Total energy (eV)	E_coh (eV/atom)	ΔH_r (eV/atom)
Fe₈B₄	5.014	5.014	4.206	105.719	7.697	-7236.271	-8.899	-2.627
Fe₇CrB₄	5.032	5.031	4.177	105.724	7.636	-8838.483	-8.925	-2.609

Table 2 Mechanical properties and anisotropic parameters of Fe8B4 and Fe7CrB4.

Species	B (GPa)	G (GPa)	E (GPa)	ν	B/G	H (GPa)
Fe₈B₄	246.9	162.1	398.9	0.231	1.52	20.9
Fe₇CrB₄	252.5	161.8	400.1	0.236	1.56	20.3

Table 3 Population analysis results calculated by Mulliken’s method.

Species	Atom	s	p	d	Total	Charge	Bond	Population	Length (?)
Fe₈B₄	B	1.13	2.55	0	3.68	-0.68	B-B	0.71	2.10
Fe₈B₄	Fe	0.44	0.53	6.69	7.66	0.34	Fe-B	0.15	2.14
Fe₇CrB₄	B	1.11	2.54	0	3.65	-0.65	B-B	0.73	2.07
	Fe	0.44	0.56	6.69	7.69	0.31	Fe-B	0.17	2.14
	Cr	2.52	6.03	5.02	13.57	0.43	Cr-B	0	2.19

Fig. 7. Electron density difference distributions on (1-20) plane of Fe8B4 (a) and Fe7CrB4 (b).

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Microstructure, mechanical properties and toughening mechanism of directional Fe₂B crystal in Fe-B alloy with trace Cr addition

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