Improving Properties Of Bulk Metallic Glasses Via Manipulating Multiple Length-scale Structural Heterogeneities

Bulk metallic glasses(BMGs)have attracted extensive attention due to their high strength,high hardness,and unique physical and chemical properties.However,the high cost,limited glass-forming ability(GFA)and room-temperature brittleness restrict their widespread application.In addition,tailoring the properties of BMGs is far more difficult than that in crystalline counterparts because of the limited knowledge of the structure-property relationship in BMGs.BMGs show intrinsic structural heterogeneity at multiple length scales due to its atomic packing with short-range order(SRO).Recently,structural heterogeneity in BMGs was found to be closely associated with GFA and plasticity.However,very limited work has been conducted in this regard.In this thesis,in order to overcome the aforementioned issues,influences of alloying elements and fabrication processes on structural heterogeneity at multiple length scales and properties of BMGs were systematically investigated.The main findings of this thesis are as follows:Oxygen contamination is one of the major problems for mass production of BMGs with low-grade raw materials under industry conditions.It is still in debate on how oxygen affects the GFA of ZrCu-based BMGs which are the most widely investigated glassy alloys at the moment.We report that,in Al-or Be-containing ZrCu-based BMGs,oxygen would prefer to forming covalent-like bonds with either Al or Be.The directional covalent-like bonds frustrate the dense atomic packing with icosahedral local motifs and broaden the distribution of specific polyhedra,thus destabilizing the undercooled liquid structure.Nevertheless,in the ZrCu-based BMGs with neither Al nor Be,oxygen prefers to forming metallic-like bonds withZr instead.Proper oxygen addition then enhances the atomic packing density and stabilizes the undercooled liquid of this kind of BMGs,thus improving the GFA.To design ZrCu-based BMGs with high oxygen-tolerance,the standard Gibbs free energy of oxide formation for the other constituent elements should be higher than that of ZrO2.Generally,enhancing loose packed atomic motifs known as free volume is beneficial to the plasticity of BMGs.However,we reveal that in the Zr20Cu20Ti20Hf20Ni20 BMG,0.1?0.2 at%addition of small elements,such as oxygen,nitrogen,carbon and boron,could significantly increase the plasticity(over 50%).With proper addition of these small elements,the number of localized regions with a size of 2?10 nm,which have an excessively high packing density,is ascended.These dense-packed regions possess high potential energy and would be easily activated during deformation,leading to the plasticity enhancement.In addition to the increase of free volume,enhancing the formation of excessively dense-packed regions is another effective approach to promoting plastic deformation in BMGs.Nevertheless,the bonding nature between alloying and constitution elements should be taken into account as far as addition of small elements in BMGs to enhance plasticity is considered.For example,addition of 252 at ppm oxygen significantly deteriorates plastic deformation capability of the Fe65Nii7Pii.5C6.5 BMG.Oxygen doping transfers inter-atomic bonds from s-like to more covalent-like p-d hybrid bonds,which is responsible for the decreased plasticity.In contrast to the traditional wisdom,we reveal that addition of elements with positive heat of mixing(PHM)into Zr-Cu-Al BMGs not only facilitates glass formation but also enhances plasticity.It is found that the elements with a smaller PHM value have a larger optimal dose and are more effective for enhancing GFA,which is due to stabilization of the liquid phase with an enhanced packing density and destabilization of the crystalline phase by slowing down the atomic rearrangement.Moreover,addition of the elements with a mediate PHM value is beneficial for plastic deformation because of the enhanced unstable clusters with liquid-like behavior in local atomic structure.Last,a series of BMGs with a gradient distribution of free volume has been prepared by long-term isothermal melting,followed by water quenching with silica tubes.The outer layer in the as-prepared BMG rods with a thickness of 300 ?m shows a higher concentration of free volume and lower hardness than the interior.The plasticity of the Fe74Mo6Pi3C7 BMG is facilitated from 0.2 to 5%.and a significant increase of 5?15 times is achieved in Zr,Ti and Pd-based graded BMGs.The softer surface layer promotes the nucleation of shear bands while the harder interior prohibits their propagation,thus promoting the plastic deformation.