1 Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China 2 National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China
Cite this article:
Huijun KANG, Jinling LI, Tongmin WANG, Jingjie GUO. Growth Behavior of Primary Intermetallic Phases and Mechanical Properties for Directionally Solidified Al-Mn-Be Alloy. Acta Metall Sin, 2018, 54(5): 809-823.
Intermetallic compounds (including quasicrystals) have been widely employed as reinforced phases in many alloys due to their high strength, high hardness and good thermal stability. The growth behavior and growth pattern of these intermetallic compounds affect the mechanical properties of materials significantly. However, the intermetallic compound, which exhibits complex crystal structures and directional bonding usually shows a faceted growth pattern with strong anisotropy and forms crystals with a wide range of morphologies and coarse grains during solidification. The inappropriate morphology and size of the intermetallic compound will destroy the integrity of the matrix and thus deteriorate the mechanical properties of materials. In this work, the microstructural evolution, morphology evolution of intermetallic compounds and mechanical properties have been investigated in directionally solidified Al-3Mn-7Be (atomic fraction, %) alloy with a wide pulling rates of 1~1500 μm/s. The addition of Be results in the shift of Al-Mn binary phase diagram toward the Mn-rich side, the appearance of intermetallic compounds, namely λ-phase, T-phase, Be4AlMn, and icosahedral quasicrystal (I-phase) and significantly refines the microstructures of the as-cast and directionally solidified samples. With increasing pulling rates, a transition of primary phase is observed from λ-phase to T-phase, and then I-phase, accompanied by the formation of the primary Be4AlMn phase, which can be attributed to the increase of supersaturation and supercooling near the solid/liquid interface. Meanwhile, the morphology, size and growth pattern of primary phases vary with the increase of pulling rates. The mechanical properties of directionally solidified Al-3Mn-7Be alloy have been investigated. It is indicated that the room-temperature strength of this alloy decreases first and then increases as the pulling rates increase, and a larger elongation is presented at the lowest and highest pulling rates, which can be attributed to the microstructures of alloys, properties of strengthening phases and the interfaces between matrix and strengthening phase.
Fig.1 XRD spectra (a) and microstructures of as-cast Al-3Mn (b) and Al-3Mn-7Be (c) alloys, AES analyses of phases composition (d) and DSC curve (e) of as-cast Al-3Mn-7Be alloy
Fig.2 Microstructures of the longitudinal section of directionally solidified Al-3Mn-7Be alloy at pulling rates of 1 μm/s (a), 5 μm/s (b), 60 μm/s (c), 600 μm/s (d) and 1000 μm/s (e) (Insets are corresponding SAED patterns of primary phases)
Fig.3 3D morphologies of λ-phase obtained at a pulling rate of 1 μm/s (a) regular hexagonal prisms along longitudinal direction (b) regular hexagonal prisms along cross section direction (c) irregular prisms along cross section direction
Fig.4 Projections of λ-phase unit cell along the [0001] (a) and [1010] (b) directions, respectively
Fig.5 3D morphologies of T-phase at different pulling rates (a) 5 μm/s (b) 90 μm/s (c) 200 μm/s, along [0001] direction (d) 200 μm/s, along [1120] direction (e) 400 μm/s (f) 600 μm/s
Fig.6 Bright field TEM image of the Be4AlMn and T-phase (a), composite SAED pattern of Be4AlMn and T-phase (b), and SAED patterns of Be4AlMn (c) and T-phase (d)
Fig.7 Octahedral (a) and spinel twin (b) morphologies of Be4AlMn (Insets sketch the formation of a typical spinel twinned crystal, and the twin plane is shaded. bbb indicates the (111) twin plane)
Fig.8 Room-temperature tensile stress-strain curves of directional solidified Al-3Mn-7Be alloys at different pulling rates and corresponding microstructures (insets)
Pulling rate /(μms-1)
UTS / MPa
YS / MPa
Elongation / %
Elastic modulus / GPa
0
78
71
2.99
66.5
1
184
116
15.62
94.6
5
146
88
6.33
41.9
60
189
103
9.48
67.1
200
215
115
9.65
77.3
1000
244
123
12.01
64.4
Table 1 Room-temperature tensile properties of directional solidified Al-3Mn-7Be alloys at different pulling rates
Fig.9 Microstructures of deformational zone of directional solidified Al-3Mn-7Be alloys near fracture surface at pulling rates of 60 μm/s (a), 400 μm/s (b) and 1000 μm/s (c)
Fig.10 TEM analyses of directionally solidified Al-3Mn-7Be alloy at a pulling rate of 1000 mm/s (a) bright-field image (b, c) SEAD patterns of I-phase and α-Al in Fig.10a (d) HRTEM image of the interface between I-phase and α-Al (e, f) Fouier transform of regions "1" and "2" in Fig.10d
Fig.11 Bright field TEM image of the directionally solidified Al-3Mn-7Be alloy at a pulling rate of 1000 μm/s after tensile test at room temperature (High-density dislocations are observed and the dislocation lines terminating at I-phase particles)
Fig.12 Fractographs of as-cast and directional solidified Al-3Mn-7Be alloys at different pulling rates (Insets show the magnified images corresponding to the squares) (a) as-cast (b) 1 μm/s (c) 5 μm/s (d) 60 μm/s (e) 200 μm/s (f) 1000 μm/s
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