Influence of Bi addition on dynamic recrystallization and precipitation behaviors during hot extrusion of pure Mg

doi:10.1016/j.jmst.2019.10.036

Abstract

Abstract:

Low material cost and high extrudability for ensuring price competitiveness with Al alloys, as well as excellent mechanical properties, are essential for expanding the application range of Mg extrudates. Bi is a promising alloying element for developing extruded Mg alloys that satisfy such requirements. Bi is inexpensive, exhibits a high solubility limit, and forms a thermally stable Mg₃Bi₂ phase, which improves the commercial viability and enables the high-speed extrusion of Mg-Bi alloys. In this study, the effects of Bi addition on the dynamic recrystallization (DRX) and dynamic precipitation behaviors during hot extrusion of a pure Mg and the resultant microstructure and mechanical properties of the extruded materials were investigated. The addition of 6 wt% and 9 wt% Bi to a pure Mg yielded numerous fine Mg₃Bi₂ precipitates during the early stage of hot extrusion. Consequently, the area fraction of dynamic recrystallized (DRXed) grains decreased because of DRX-behavior suppression by the Zener pinning effect. However, the DRXed grain size was substantially reduced through the grain-boundary pinning effect. The size and number of undissolved Mg₃Bi₂ particles in the homogenized billets increased when the Bi content was increased, which resulted in increased DRX fractions owing to the enhanced levels of particle stimulated nucleation. Bi addition yielded considerable strength improvement of the extruded pure Mg. However, the extruded Mg-Bi binary materials were less ductile than the extruded pure Mg material. This lower ductility resulted from the cracking at twins formed in the coarse unDRXed grains of the Mg-6Bi material and the cracking at large undissolved Mg₃Bi₂ particles in the Mg-9Bi material.

Key words: Magnesium, Bi addition, Extrusion, Recrystallization, Precipitation

Jongbin Go, Jong Un Lee, Hui Yu, Sung Hyuk Park. Influence of Bi addition on dynamic recrystallization and precipitation behaviors during hot extrusion of pure Mg[J]. J. Mater. Sci. Technol., 2020, 44: 62-75.

Figures/Tables 17

Table 1 Chemical compositions of materials used in this study (wt%).

Material	Bi	Si	Mn	Fe	Mg
Pure Mg	-	0.027	0.021	0.002	Bal.
Mg-6Bi	5.93	0.028	0.019	0.001	Bal.
Mg-9Bi	8.61	0.031	0.018	0.002	Bal.

Fig. 1. DSC curves of as-cast Mg-Bi binary alloys and commercial AZ80 alloy.

Fig. 2. (a-c) Optical and (d-f) SEM micrographs of homogenized (a, d) pure Mg, (b, e) Mg-6Bi, and (c, f) Mg-9Bi billets; davg, fMg-Si and fMg3Bi2 denote the average grain size and area fractions of undissolved Mg-Si and Mg3Bi2 particles, respectively.

Fig. 3. XRD patterns of (a) homogenized billets and (b) extruded materials.

Fig. 4. Size distribution and number density of undissolved particles in homogenized Mg-6Bi and Mg-9Bi billets; Davg and N denote the average diameter and number density of particles, respectively.

Fig. 5. Equilibrium phase diagram for Mg-xBi (0 ≤ x ≤ 12 wt%), as calculated using FactSage software; Thomo. and Text. denote the homogenization temperature (500 °C) and extrusion temperature (350 °C), respectively.

Fig. 6. Inverse pole figure maps and grain size distributions of extruded (a) pure Mg, (b) Mg-6Bi, and (c) Mg-9Bi materials; fDRX denotes the area fraction of recrystallized grains.

Fig. 7. Inverse pole figure maps of (a, c) DRXed region and of (b, d) unDRXed region of extruded (a, b) Mg-6Bi and (c, d) Mg-9Bi materials; AGS denotes the average grain size.

Fig. 8. SEM micrographs showing precipitates and undissolved particles of extruded (a) pure Mg, (b) Mg-6Bi, and (c) Mg-9Bi materials; fppt and fundis denote the area fractions of Mg3Bi2 precipitates and undissolved Mg3Bi2 particles, respectively.

Fig. 9. (0001) pole figures corresponding to (a-c) entire region, (d, e) DRXed region, and (f, g) unDRXed region of extruded (a) pure Mg, (b, d, f) Mg-6Bi, and (c, e, g) Mg-9Bi materials.

Fig. 10. (a) Photos showing remaining billet after extrusion (i.e., extrusion butt) and extrusion butt cross-sectioned for microstructural observation; (b) Optical micrograph obtained at position B (see (a)) in extrusion butt of pure Mg material; (c) Microstructure observed at position A (see (a)) in extrusion butt of Mg-9Bi material; this image is obtained by superimposing an inverse pole figure map showing the grain structure onto a SEM micrograph showing the Mg3Bi2 precipitates; (d) High-magnification view of region C in (c); Blue arrows in (d) indicate the fine Mg3Bi2 precipitates that pin initial grain boundary.

Fig. 11. EBSD and SEM micrographs in extrusion butt of Mg-6Bi material: (a) superimposed EBSD image of inverse pole figure map and image quality map; (b) SEM image showing Mg3Bi2 precipitates that pin initial grain boundary of billet.

Fig. 12. (a) Tensile stress-strain curves and (b) tensile properties of extruded pure Mg, Mg-6Bi, and Mg-9Bi materials.

Table 2 Microstructural characteristics and mechanical properties of extruded materials.

Material	Microstructural characteristics*								Mechanical properties**
Material	f_DRX (%)	d_avg (μm)	d_DRX (μm)	d_unDRX (μm)	f_undis. (%)	f_ppt (%)	SF_basal	KAM	TYS (MPa)	UTS (MPa)	EL (%)
Pure Mg	100	51.6	51.6	-	0.1	-	0.20	0.54	88	177	8.6
Mg-6Bi	76.4	41.3	8.8	147	0.9	11.8	0.15	1.51	129	196	4.1
Mg-9Bi	89.9	15.7	7.5	89	2.2	12.2	0.16	1.23	141	203	5.1

Fig. 13. (a) Variation in area fraction of DRXed and unDRXed regions as a function of c-axis deviation angle from extrusion direction of extruded Mg-6Bi material. Distribution of Schmid factor for basal slip under tension along extrusion direction of extruded (b) pure Mg, (c) Mg-6Bi, and (d) Mg-9Bi materials.

Fig. 14. EBSD kernel average misorientation maps of DRXed and unDRXed regions in extruded (a) pure Mg, (b) Mg-6Bi, and (c) Mg-9Bi materials. KAMDRX and KAMunDRX denote the average KAM values of DRXed and unDRXed regions, respectively.

Fig. 15. SEM images showing (a-c) cross-section and (d-f) fracture surface of fractured tensile specimens of extruded (a, d) pure Mg, (b, e) Mg-6Bi, and (c, f) Mg-9Bi materials.

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