| 其他摘要 | In contrast to the conventional polycrystalline metals, metallic glasses, due to their atomic structure, exhibit superior properties, such as high strength, hardness, elastic limitation and specific strength as well as good corrosion resistance. The glass -forming ability (GFA) and macroscopic mechanical properties of bulk metallic glasses (BMGs) are key issues to develop BMGs as the potential engineering materials. Among the BMG families, Ni-based BMGs exhibit super-high fracture strength, high thermal stability, good corrosion resistance and wear resistance, while Hf-based BMGs have high mass density and relatively low-cost with respect to noble metals, as considered as the candidate materials for kinetic energy penetrators. The GFA and macroscopic plasticity of Ni- and Hf-based BMG, however, need to be further improved. In the present work, the GFA of the Ni–Nb–Sn-based, Hf–Ni–Al and Hf–Cu–Ni–Al-based alloys were systematically studied. Alloying elements effects were also discussed from the metallurgical consideration. While obtaining the BMG with high GFA, Compositional dependence of mechanical properties was systematically investigated, for developing the BMGs combined with good GFA and plasticity. The major contents and conclusions are shown as following:
1. A wide composition zone (54~62 at.% Ni, 32~36 at.% Nb and 3~11 at.% Sn) were revealed in the Ni–Nb–Sn ternary alloys, which can be cast into fully glassy rods of 2-mm in diameter. Based on the Ni–Nb–Sn ternary alloy, Ni was partially substitute by Co (Ni1-xCox, 0.05≤x≤0.15), the Dc of BMG formation was upgraded up to 3 mm with “3D pinpointing approach”. On further using partially substituting for Nb with Hf in the Ni56Co3Nb36Sn5 alloy, the Dc can be increased up to 4 mm at the Ni56Co3Nb28Hf8Sn5 quinary. Solidification microstructure of the BMG forming alloys cooled at slow cooling rate indicates that the addition of Co and Hf does not change the original ternary eutectic reaction. The beneficial effects of Co and Hf on the GFA can be attributed to their contribution to stabilize the supercooled liquid and suppress the formation of competing intermetallic phases such as Ni3Nb.
2. A quite wide composition region for the BMG formation with the Dc of 3 mm was discovered in the Hf-rich region of the Hf–Ni–Al ternary system (50~63 at.% Hf, 20~31 at.% Ni, and 12~25 at.% Al). The best glass former is located at Hf54Ni28Al18 with Dc of 4 mm. Solidification microstructure of slowly-cooled glass-forming alloys shows that the BMG-forming composition was correlated with eutectic reaction: L→Hf5Ni4Al＋Hf6NiAl2.
3. Zr-rich and Hf-rich BMGs in the Zr–Cu–Al and Hf–Ni–Al ternary system exhibit excellent compressive plasticity. Although their plastic deformation is inhomogeneous and dominated by a single shear band. As molded by computer simulation, the content of different kinds of icosahedrons in these glasses determines the barrier for the atomic shear transition. In a given alloy system, the BMGs with lower glass transition temperature and shear modulus (G) as well as higher Poisson’s ratio exhibit lower yield strength and higher plastic strain. These parameters associated with structure can be used as the indicator to search for the BMGs with good plasticity.
4. A quite wide composition region for the BMG formation with the Dc of 8 mm was discovered in the Hf–Cu–Ni–Al quarternary. At Hf47Cu29.25Ni9.75Al14 and Hf48Cu29.25Ni9.75Al13, the alloys can be cast into fully glassy rods of 10 mm in diameter. The combination of high GFA and mass density (>11 g/cc) makes these BMGs to be potential candidate materials for kinetic energy penetrator. Solidification microstructures of BMG-forming alloys show that the BMG forming compositions correlate with eutectic reaction of L→CuHf2+HfNiAl。
5. Under the guidance of the change of Tg and elastic properties with the composition, the BMG with good GFA together with compressive ductility was located at Hf51Cu27.75Ni9.25Al12 (Dc=8 mm, yielding strength=2380~2440 MPa,plastic strain=5~6%) in the Hf–Cu–Ni–Al quaternary. Elastic properties do not significantly change when partial substitution of 2 at.% Nb or Ta for Hf in the Hf48Cu29.25Ni9.75Al13 and Hf51Cu27.75Ni9.25Al12 alloys, respectively. With respect to the starting BMGs, BMG with Nb or Ta addition exhibits higher yield strength and slight decrease in plasticity. Data of the Tg and elastic properties for different kinds of BMG given by Johnson show that G has linear relationship of (Tg/Vm)[1-4/9(T/Tg)2/3] with a fit of G=4.9+672 (Tg/Vm)[1-4/9(T/Tg)2/3]. Similarly, the G and Tg of Hf-based BMGs exhibit linear relationship of a fit of G=9.9+576 (Tg/Vm)[1-4/9(T/Tg)2/3], indicating that Tg can be used as a parameter to roughly estimate the value of G, and then as a indicator to search for the BMGs with better plasticity. |
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