Study On The Formation, Structure And Rheological Properties Of Bulk Metallic Glasses

Bulk metallic glass（BMG） have attracted a lot of research interest because of their novel physical, chemical and mechanical properties. BMG have wildly application prospects in defense, aerospace, machinery, electric power and other fields. Meanwhile, it provides an idea model system for studies of scientific problems in amorphous materials, e.g. structure, glass forming and deformation mechanism etc. To date, a number of BMGs have been successfully developed and some of them have application in engineering utilizing their excellent properties. However, the obstacles for the use of these amorphous alloys is still the low glass-forming ability（GFA） and lack of room-temperature ductility of many systems. Get insight into the microstructure of BMG is the key of understanding the nature of glass formation and deformation and developing new BMGs with improved properties.In this dissertation, the microstructure, glass formation and rheological properties of Zr-Cu-Al, Zr-Ni-Al and Zr-Cu-Al-Nb BMGs have been systematically investigated by ab initio molecular dynamics（AIMD） method with experimental verification. The main contents and results are summarized as follows:1. According to the classical crystallization theory, the influences of rheological properties on GFA for BMGs has been investigated. The theoretical analysis shows that GFA is in proportion to the viscosity of “nose” temperature（T_n） and GFA is inversely proportional to the viscosity of onset crystallization temperature（Tx） or it is proportional to the viscosity of liquidus temperature（T_l） on reheating in time–temperature–transformation（TTT） curve, and therefore a new GFA parameter, ψ0=（T_g-T₀）/（T_n-T₀）+（T_g-T₀）/（T_l-T₀） was proposed（wherein T_g and T₀ are glass transition temperature and Vogel temperature respectively）.2. Local structural changes from liquid to amorphous state in three Zr_55-xCu₄₅Al_x（x = 3, 7, 12） ternary metallic glasses have been investigated by AIMD method,and the atomic structure of the glasses has been analyzed by means of bond-type index method in Honeycutt-Andersen and Voronoi tessellation method. The simulation results indicated that Al-centered icosahedral clusters are more stable than the other clusters, and they play a key role in the structural heterogeneity and glass-forming ability of the Cu-Zr-Al bulk metallic glasses. Al-centered icosahedral clusters are identified as the basic local structural units when x=3 and x=7 for the Zr_55-xCu₄₅Al_x（x = 3, 7, 12） metallic glasses. While x=7, these icosahedral clusters are connected to each other by vertex-sharing（VS）, edge-sharing（ES）, face-sharing（FS） and intercrossed（IS） atoms and form the icosahedral medium range ordering（IMRO） “backbone” structure, and thus increased the strength, viscosity and GFA of the metallic glasses.3. The experimental and AIMD simulation results of the(Zr_0.5Cu_0.4Al_0.1)_100-xNb_x（x=0, 3, 6 at.%） BMGs shows that: After adding a small amount（3 at.%） of niobium into Zr₅₀Cu₄₀Al₁₀ BMG, both the Al-centered and Nb-centered icosahedral（and/or icosahedrallike） stable clusters were formed in alloy. These stable clusters are connected and matched to form a new and more stable heterogeneities structural, leading to the elasticity modulus, yield strength and room-temperature ductility increase. However, adding excess（6 at.%） of niobium into alloy, as a result, some Nb clusters instead of Al clusters, reduced the degree of heterogeneity of structural and decrease the strength and macroscopic ductility of alloy.4. Ternary Zr₆₇Ni_33-xAl_x（x=8, 15, 21 at.%） BMGs have been investigated in detail by the AIMD method. The results show that there exist two kinds of stable clusters, which are Ni-centered Bernal polyhedra（such as <0 3 6>, <0 3 6 1>, <0 2 8> and <0 2 8 1>） and Alcentered icosahedral（and/or icosahedral-like） clusters（such as <0 2 8 2>, <0 3 6 3>, <0 0 12 0> and <0 1 10 2>） in the alloys. The fraction of the former drops and that of the latter rises with increase of Al content. That makes the strength and elastic modulus gradually increase, while the the degree of heterogeneities of structure and room-temperature ductility of the alloy gradually decreased.5. The uniaxial compression of Zr-Cu-Al（-Nb） and Zr-Ni-Al BMGs have been investigated by AIMD method, and the atom-scale serrated flow behavior of stress-strain curve was observed in the plastic flow stage. It is found that the content of 1551 bond-type and icosahedral continuously decreases with the increasing of strain, and there exist certain correlations between the content reduction of 1551 bond-type and the stress drop. Further analysis showed that the clusters with high degree of five-fold symmetry and strong geometric stability（such as icosahedral） transformed to “liquid-like” clusters with weak geometric stability under stresses, and the remarkable decrease of stress is directly attributable to the content decrease of stable clusters in alloy. These results indicate that the high five-fold symmetry structures（icosahedral and/or icosahedral-like stable clusters） play a role as the backbone of resistance to plastic flow in alloy.