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Acta Metall Sin  2016, Vol. 52 Issue (7): 811-820    DOI: 10.11900/0412.1961.2016.00039
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Shuaiju MENG1,Hui YU1,2(),Huixing ZHANG3,Hongwei CUI4,Zhifeng WANG1,Weimin ZHAO1
1 School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China.
2 Light Metal Team, Korea Institute of Materials Science, Changwon 51508, Republic of Korea .
3 Mechanical and Material School, Tianjin Sino-German University of Applied Sciences, Tianjin 300350, China .
4 School of Materials Science and Engineering, Shandong University of Technology, Zibo 255049, China.
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Due to the increasing demand of low density and high strength Mg alloys for the automobile, railway, and aerospace industries, the exploration of cost-effective RE-free Mg alloys becomes more and more attractive. Instead of Mg-Sn based system, the Mg-Bi alloy system seems to satisfy this requirement as a potential candi date, since it shows typical precipitation-type phase equilibrium and contains thermal stable Mg3Bi2 phases, which exhibit a high melting temperature of 821 ℃ comparable to those in RE-bearing Mg alloy. In addition, the fine Mg3Bi2 plates on the prismatic plane were reported to be more effective than the more commonly observed basal plates for precipitation-hardening. In this work, pure Mg with/without 6% Bi (mass fraction) additions were extruded, and the corresponding microstructure and mechanical properties were investigated. The results show that dynamic recrystallization (DRX) occurs in both alloys after extrusion and these two kinds of specimens exhibit similar extrusion texture. However, the as-extruded Mg-6Bi alloy represents finer and homogenous microstructure, and the average grain size (AGS) decreases from 30 μm to 4 μm when 6% Bi added. In addition, the Mg-6Bi alloy contains strip-like fragmented Mg3Bi2 particles along the extrusion direction and fine Mg3Bi2 precipitates, and demonstrates superior mechanical properties with tensile yield strength of 189 MPa, ultimate tensile strength of 228 MPa, and an elongation of 19.9%. There is a large number of nano-scale Mg3Bi2 particles in the tensile fracture surface of Mg-6Bi alloy. And there is a large number of twins in the microstructure of compression fractured pure Mg sample; while for the Mg-6Bi alloy specimen, with a large number of second phase particles on the α-Mg matrix, little twins are observed. Moreover, the Mg-6Bi alloy also gives a low tension-compression yield asymmetry with yield asymmetric ratio of 1.01. These significantly improvement of mechanical properties are mainly attributed to the combined effects of grain refinement and large quantity of co-exist micro/nano-size Mg3Bi2 particles.

Key words:  Mg alloy      extrusion      microstructure      mechanical property     
Received:  26 January 2016     
Fund: Supported by Research Foundation of Higher Education School Scientific Research Program from Hebei Education Department (No.QN2015035), Graduate Student Innovation Project of Hebei Province (No.220056), Outstanding Youth Scholar Science and Technology Innovation Program of Hebei University of Technology (HEBUT) (No.2015-002), Natural Science Foundation of Hebei Province (No.E2016202130), Research Foundation of Introduction Talent, HEBUT (No.208002) and Young Scientist Exchange Program between Korea and China (No.201510)

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Point Mass fraction / % Atomic fraction / %
Mg Bi Mg Bi
A 42.65 57.35 86.47 13.53
B 83.08 16.92 97.69 2.31
C 97.21 2.79 99.67 0.33
Table 1  EDS analysies of points A, B and C in Fig.1c
Fig.1  XRD spectra of as-cast pure Mg (a) and Mg-6Bi alloy (b), and SEM image of Mg-6Bi alloy (c)
Fig.2  OM images of as-cast pure Mg (a) and Mg-6Bi alloy at low (b) and high (c) magnification, and solution treated Mg-6Bi alloy at 500 ℃ for 5 h (d)
Fig.3  Microstructures of as-extruded pure Mg (a, b) and Mg-6Bi alloy (c, d) at low (a, c) and high (b, d) magnification (ED—extrusion direction)
Fig.4  TEM bright-field images of Mg-6Bi alloy at low (a) and high (b, c) magnification and EDS analysis (d) of second phase in Fig.4b (Inset in Fig.4c shows SAED pattern of Mg3Bi2)
Fig.5  EBSD images (a, b), grain size distributions (c, d) and inverse pole figures (e, f) of as-extruded pure Mg (a, c, e) and Mg-6Bi alloy (b, d, f) (Inset in Fig.5a shows grain orientation)
Fig.6  Tensile (a) and compressive (b) stress-strain curves of the extruded pure Mg and Mg-6Bi alloy
Table 2  Microstructural characteristics and mechanical properties of as-extruded pure Mg and Mg-6Bi alloy
Fig.7  Fracture morphologies of pure Mg (a, b) and Mg-6Bi alloy (c, d) at low (a, c) and high (b, d) magnifications after tensile test
Fig.8  OM images of as-extruded pure Mg (a, b) and Mg-6Bi alloy (c, d) at low (a, c) and high (b, d) magnifications after compressive test
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