Microstructure and Properties of AZ61 Ultra-Fine Grained Magnesium Alloy Prepared by Mechanical Milling and Powder Metallurgy Processing
ZHU Yunpeng1,2,3(), QIN Jiayu1,3, WANG Jinhui1,3, MA Hongbin1,3, JIN Peipeng1,3, LI Peijie2
1.Qinghai Provincial Key Laboratory of New Light Alloys, Qinghai University, Xining 810016, China 2.Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China 3.Qinghai Provincial Engineering Research Center of High Performance Light Metal Alloys and Forming, Qinghai University, Xining 810016, China
Cite this article:
ZHU Yunpeng, QIN Jiayu, WANG Jinhui, MA Hongbin, JIN Peipeng, LI Peijie. Microstructure and Properties of AZ61 Ultra-Fine Grained Magnesium Alloy Prepared by Mechanical Milling and Powder Metallurgy Processing. Acta Metall Sin, 2023, 59(2): 257-266.
The Mg-Al series alloys are well known for their low density, high specific strength, and superior damping capability. However, the application of Mg-Al series alloys is often limited by inadequate mechanical properties. To fulfill the growing demand for lightweight structural components, ultra-fine grained magnesium alloys with superior mechanical properties have gained attention. Mechanical milling and powder metallurgy were used to produce the AZ61 ultra-fine grained magnesium alloy with super-high tensile characteristics. The effects of mechanical milling on the grain size, precipitates, texture, and tensile properties of AZ61 ultra-fine grained magnesium alloy were investigated. The grain size of the AZ61 alloy produced by mechanical milling and powder metallurgy was reduced from 0.91 μm to 0.68 μm when compared to that produced by non-mechanical milling. Mechanical milling accelerated the dynamic precipitation and refining of Mg17Al12 while weakening the basal texture. The yield strength, tensile strength, and elongation of AZ61 ultra-fine grained magnesium alloy are 393.1 MPa, 431.9 MPa, and 8.5%, respectively, which are superior to the AZ61 alloy produced by other processes. The theoretical grain refinement and Orowan strengthening mechanisms of AZ61 alloy were calculated, and it shows that the grain refinement strengthening contributes more than 90% of the yield strength. The Orowan strengthening was overestimated when Mg17Al12 was distributed at grain boundaries. The weakening of basal texture resulted in a reduction in yield strength. The fracture mechanism of the AZ61 alloy without mechanical milling is dominated by oxidation on the surface of the AZ61 powder, which initiates interparticle debonding. The fracture mechanism of the AZ61 alloy with mechanical milling is dominated by deformation mismatch between the deboned powders and the stronger bonded powders, and the increase in the amount of Mg17Al12 along the grain boundary.
Fig.2 SEM images of raw AZ61 powder (a), mechanically milled AZ61 powder (b); and OM images of sintered raw AZ61 powder (c), sintered mechanically milled AZ61 powder (d)
Fig.3 XRD spectra of AZ61 powder (Inset shows the locally high magnification of Fig.3a) (a) and sintered AZ61 alloy (b)
Fig.4 XRD spectra (a) and SEM images of extruded AZ61 alloy without (b) and with (c) mechanical milling, and EDS elemental maps of Mg (d), Al (e), and Zn (f) in Fig.4c
Extruded AZ61 alloy
Grain size / μm
Diameter of Mg17Al12 / nm
Volume fraction of Mg17Al12 / %
Without mechanical milling
0.91
230
6.6
With mechanical milling
0.68
170
11.7
Table 1 Grain size, volume fraction of Mg17Al12, and size of Mg17Al12 of extruded AZ61 alloy
Fig.5 Low (a, b) and high (c, d) magnified TEM images of extruded AZ61 alloy without (a, c) and with (b, d) mechanical milling (Inset in Fig.5d shows the selected area electron diffraction pattern of the precipitates at grain boundaries)
Fig.6 {0001} and {} pole figures of extruded AZ61 alloy without (a) and with (b) mechanical milling (ED—extruded direction, TD—transverse direction)
Fig.7 Tensile stress-strain curves of extruded AZ61 alloy at room temperature
Fig.8 Comparisons of mechanical properties of this work and other reported[1,7,10,23-27] AZ61 alloys at room temperature (UTS—ultimate tensile strength; YS—yield strength; MM—mechanically milled; PM—powder metallurgy; FSP—friction stir process; MDF—multi-directionally forged; DMD—disintegrated metal deposition; ED—extruded; ECAP—equal channel angular pressing; HRS—hot rotary swaging) (a) UTS (b) YS
Fig.9 SEM fractographs of extruded AZ61 alloy without (a-c) and with (e, f) mechanical milling, and EDS along the line in Fig.9c (d)
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