Microstructure Evolution Mechanism of New Low-Alloyed High-Strength Mg-0.2Ce-0.2Ca Alloy During Extrusion
LI Jingren, XIE Dongsheng, ZHANG Dongdong, XIE Hongbo, PAN Hucheng(), REN Yuping, QIN Gaowu
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
Cite this article:
LI Jingren, XIE Dongsheng, ZHANG Dongdong, XIE Hongbo, PAN Hucheng, REN Yuping, QIN Gaowu. Microstructure Evolution Mechanism of New Low-Alloyed High-Strength Mg-0.2Ce-0.2Ca Alloy During Extrusion. Acta Metall Sin, 2023, 59(8): 1087-1096.
This study utilizes the Pandat software to design a novel ternary alloy, Mg-0.2Ce-0.2Ca (mass fraction, %). The Mg alloy samples are extruded conventionally and provide high strength and low alloying with yield strength of approximately 364 MPa and total content of only approximately 0.4%. The microstructures at different stages of extrusion are characterized, revealing the existence of twin in the Mg-0.2Ce-0.2Ca alloy throughout the extrusion process, indicating high twin migration resistance. In the middle and later stages of extrusion, dynamically recrystallized grains nucleate at regions of intersected twinning variants, leading to a significant reduction in the proportion of twinning interfaces. Moreover, during the early stage of extrusion, a large number of <c + a> dislocations are stored in the Mg-0.2Ce-0.2Ca alloy, and the dislocation-dominated recovery/recrystallization mechanism is functional until the late stage of extrusion due to the high slipping resistance of dislocations. This mechanism directly contributes to the formation of ultrafine grains in present Mg alloy. The results show that the addition of Ca increases the resistance of twinning motion in the Mg matrix, while the addition of Ce and Ca induces multisystem slip, which are the main mechanisms for regulating the microstructure evolution of Mg-Ce-Ca alloy during extrusion. These findings have significant implications for the development of new high-strength, low-alloyed Mg alloys.
Fund: National Key Research and Development Program of China(2021YFB3701000);National Natural Science Foundation of China(U2167213);National Natural Science Foundation of China(51971053);Young Elite Scientists Sponsorship Program(2019-2021QNRC001);Young Elite Scientists Sponsorship Program(2019-2021QNRC002);Young Elite Scientists Sponsorship Program(2019-2021QNRC003);Fundamental Research Funds for the Central Universities(N2202020)
Corresponding Authors:
PAN Hucheng, associate professor, Tel:13166643462, E-mail: panhc@atm.neu.edu.cn
Fig.1 Vertical sectional phase diagram of Mg-0.2Ce-xCa alloy calculated by Pandat software (x—mass fraction of Ca (0~2%), T—temperature)
Fig.2 Mechanical properties and microstructure of as-extruded Mg-0.2Ce-0.2Ca alloy (DRX—dynamic recrystallization, ED—extrusion direction, IPF—inverse pole figure) (a) engineering stress-strain curve of as-extruded Mg-0.2Ce-0.2Ca alloy (b, c) distribution maps of yield stress (YS) (b) and YS increment per weight (c) vs elongation of Mg alloys based on diverse alloy system[20,21] (d) OM image (e) EBSD IPF map (f) mean DRXed grain size distribution of Mg-0.2Ce-0.2Ca alloy (Inset shows the IPF of DRXed grains) (The color strip represents intensity represented as multiple of random distribution, the same in Figures below)
Alloy
Composition / (mass fraction, %)
State*
s / MPa
σuts / MPa
Elongation / %
Mg
Pure Mg
F
87
185
5
M1
Mg-1.5Mn
F
189
255
12
AZ10
Mg-1.5Al-0.2Zn-0.2Mn
F
155
240
10
AZ31
Mg-3Al-1Zn-0.6Mn
F
200
260
15
AZ61
Mg-6Al-1Zn-0.6Mn
F
230
310
16
AZ80
Mg-8Al-0.5Zn-0.1Mn
T5
275
380
7
ZK21
Mg-2Zn-0.6Zr
F
195
260
4
ZK60
Mg-6Zn-0.5Zr
F
260
340
11
WE43
Mg-4Y-3RE
F
214
296
14
AXM100
Mg-0.6Al-0.28Ca-0.25Mn
T6
253
277
8
MgCeCa
Mg-0.2Ce-0.2Ca
F
364
374
2.5 (this work)
Table 1 Mechanical properties of commercial Mg alloys and Mg-0.2Ce-0.2Ca alloy[20,21]
Fig.3 OM images of the interrupted extrusion sample of Mg-0.2Ce-0.2Ca alloy (a) low magnified microstructure near the die exit (Inset shows the view surface diagram) (b-d) magnified microstructures right below the die exit at the locations as marked by rectangles in Fig.3a
Fig.4 Typical EBSD and TEM images at the position of 17.5 mm below extrusion die exit of Mg-0.2Ce-0.2Ca billet (a) the IPF map (b) grain boundary (GB) and twin boundary (TB) (θ—crystallographic misorientation angle between the two sides of the boundary) (c) enlarged zone of Fig.4b (The schematic insert in Fig.4c illustrates characteristic position of the billet below extrusion die exit in Figs.4-7) (d-f) existence of <a> dislocations (d) and <c + a> dislocations (e, f) (Insets in Figs.4e and f show the two-beam conditions of g = 0002 and g = 110, respectively)
Fig.5 Typical EBSD and TEM images at the position of 12.5 mm below extrusion die exit of Mg-0.2Ce-0.2Ca billet (a) EBSD IPF map (b) GB and TB (c) IPF (d) IPF map of selected grains from Fig.5a (e-j) TEM images showing twinning (e, f), low-angle grain boundary (LAGB) (g, i), <c>-component dislocations (h), and nano second phases (j) (Arrows in Figs.5g and h represent <c>-component dislocations)
Fig.6 Typical EBSD and TEM images at the position of 7.5 mm below extrusion die exit of Mg-0.2Ce-0.2Ca billet (a) EBSD IPF map (b) GB and TB (c) IPF (d) selected area from Fig.6a (e-j) TEM images showing dislocation tangles (e), DRXed grains (f), LAADF-STEM image (g), <c>-component dislocation (h, i), and HAADF-STEM image (j) (LAADF-STEM, HAADF-STEM—low and high angle annular dark-field scanning transmisson electron microscopy, respectively)
Fig.7 Typical EBSD results at the position of 2.5 mm below extrusion die exit of Mg-0.2Ce-0.2Ca billet, showing the IPF map (a), GB and TB (b), IPF (c), and selected area in Fig.7a (d)
Fig.8 Distribution diagram of misorientation angle at different positions from extrusion die exit
Fig.9 Proportion distribution of low angle grain boundary and twin boundary at different positions from extrusion die exit
Fig.10 KAM + GB overlapped map for as-extruded Mg-Ce-Ca alloy (KAM—kernel average misorientation)
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