Glass Forming Ability And Fracture Behavior Of Fe-Based Bulk Metallic Glasses

In this dissertation, the effects of yttrium, nickel and cobalt element additions in Fe-based bulk metallic glasses (BMGs) were studied. A new criterion for assessing the glass forming ability (GFA) of bulk amorphous alloys was developed, and the mechanical properties of Fe-based BMGs at room temperature were investigated.The method of using yttrium and nickel elements to substitute for iron and cobalt elements was used to search the optimal composition of the alloys with high GFA in Fe-Cr-Mo-C-B and Fe-Co-Zr-Mo-W-B alloy systems, respectively. Fully amorphous rods of 6 mm in diameter have been fabricated by conventional copper mold casting when the addition of yttrium was 2 at% in Fe45-xCr16Mo16C18B5Yx (x=1, 2, 3) alloy systems. The beneficial effect of yttrium addition in Fe45-xCr16Mo16C18B5Yx (x=1, 2, 3) alloy systems have been analysed. Moreover, the Fe₅₈Co₆Ni₄Zr₁₀Mo₅W₂B₁₅ amorphous rod with a 3 mm diameter can be produced easily under the same fabricating conditions in Fe₅₈Co10-xNixZr₁₀Mo₅W₂B₁₅ (x=3, 4, 5, 6) alloy systems. The relation between the amount of Ni addition and the GFA and thermal stability of the alloys has been examined. The role of nickel addition in Fe₅₈Co₁₀-xNixZr₁₀Mo₅W₂B₁₅ (x=3, 4, 5, 6) alloy systems in improving GFA has been explained from the aspect of competing crystalline phases.Due to insufficient understanding of the mechanism of bulk glass formation, the commonly used approach is to use a parameter, which can exhibit GFA of amorphous alloys, to find bulk amorphous alloys with high GFA. Based on classical theory of nucleation and growth, a new criterion has been proposed. Based on a large volume of data reported to date in literature, the feasibility and validity of the new criterion has been examined using critical size (Dmax) of amorphous samples as a reference, and the results show that the new criterion,δ, is much better in estimating the tendency of GFA of various bulk glassy alloys. Compared with other commonly used criteria such as ?Tx, Trg andγ, the new criterion can better embody the physical nature of BMGs and more accurately predict the GFA of bulk glassy alloys. It is worthy to mention that a serial of Fe- based BMGs have been successfully developed using this new criteion.According to solid solution theory and phase competition theory, the effect of cobalt substitution for iron on GFA in Fe48-xCoxCr15Mo14C15B6Y2 (x=3, 5 7, 9) alloy systems has been investigated. Experiment results show that the additions of cobalt can obviously improve the GFA, especially, of the Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ bulk amorphous alloy, which can be fabricated into a fully amorphous rod with at least 16 mm in diameter using industrial-purity raw materials and copper mold casting. X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution transimission electron microscopy (HRTEM) analysis futher confirm the alloy being a single amorphous phase. At the same time, it is confirmed that the new BMG has the highest glass forming ability among all reported Fe-based glassy alloy systems. The study on structure of the alloy shows that additions of cobalt can lead to a more confusable and stable alloy melt and enhance the suppression of crystalline phases. The values of critical cooling rate Rc and Gibbs free energy difference between the undercooled liquids and the corresponding crystalline solids ?G are relative small, which explains that the Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ bulk amorphous alloy has a high GFA in terms of thermodynamics and dynamics.Fe-based BMGs with a high strength and high hardness are often called amorphous steel, which have extensive applications as structure materials. The fracture behavior of the Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ alloy subjected to various stress states has been studied. The bend experiment results show that the fracture surface is 57o with axial direction, and has brittle fracture characteristics. The characteristics of brittle materials and the nonuniformity of the microstructure for as-cast samples have also been verified by indentation experiment. The values of elastic modulus and hardness of the Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ BMG measured by nanoindentation test are 265.1 GPa and 15.9 GPa, respectively. The compression test shows that the fracture strength, elastic strain and plastic strain are 3.5 GPa, 1.5 % and zero, respectively, implying this new Fe-based BMGs being typical brittle alloy with a high strength. It is noted that the compressive fracture at room temperature for Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ BMG occurs in an explosive mode, which is remarkably different from the fracture features of other bulk glassy alloys. Finally, the brittle fracture process at room temperature and the involved fracture mechanism of the Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ bulk amorphous alloy have been investigated by observing the morphologies of the compressive fracture surfaces.