Effect Of Melt Hydrogenation On The Glass Forming Ability And Mechanical Properties Of Zr-based Bulk Metallic Glasses

Bulk metallic glasses （BMGs） have a series of superior mechanical properties,e.g., high yield strengths, large elastic strain limits, good corrosion resistance, andgood soft magnetism and hard magnetism, which is a popular research for currentmaterial world. In this dissertation, hydrogen is added to Zr-based bulk metallicglasses （BMGs） by melt hydrogenation. Melt hydrogenation is a new hydrogenationmethod. In this method, the alloys are melted in the gaseous mixture of hydrogenand argon, and the hydrogen diffuses into alloy melt. The effects of melthydrogenation on the melt purification, glass-forming ability （GFA）, mechanicalproperties and mechanisms are studied.The removal effect of melt hydrogenation on the impurities of oxygen inZr₅₅Cu₃₀Ni₅Al₁₀alloy is studied first, and it is found that this method has significantdeoxidation effect in the process of melt hydrogenation. When the hydrogenpercentage in gaseous mixture and melting time are increased, oxygen content ofthe alloys gradually decreases. A hydrogen percentage of15%and melting durationof400s are effective in deoxidizing of these alloys and gaining the completelyamorphous alloy structure. Based on the change of Gibbs free energy, the Gibbsfree energy of reaction hydrogen with oxygen is lower than oxygen with zirconium.Therefore, hydrogen and oxygen is more prone to react. It will avoid the formationof zirconium oxide or zirconium/oxygen clusters which can deteriorate the GFA ofBMGs due to heterogeneous nucleation.The melt hydrogenation is applied to Zr₅₅Cu₃₀Ni₅Al₁₀master alloys, and it isfound that the volume fraction of crystalline phases CuZr3and AlZr₂graduallydecreases with the increase of hydrogen percentage in the gaseous mixture ofhydrogen and argon, and the crystalline phases completely disappear when thehydrogen percentage increases up to10%, indicating the structure of the sample ismostly amorphous. The GFA of Zr₅₅Cu₃₀Ni₅Al₁₀alloys before and after melthydrogenation is comparatively investigated using wedge shaped samples. Criticalsection size of Zr₅₅Cu₃₀Ni₅Al₁₀alloys gradually increases with the increase ofhydrogen percentage in the gaseous mixture of hydrogen and argon. When thehydrogen percentage increased up to10%, critical section size reaches maximumvalue.After melt hydrogenation, the fragility parameter and critical cooling rate arestudied. In contrast to non-hydrogenated Zr₅₅Cu₃₀Ni₅Al₁₀alloy, hydrogenated alloypossesses smaller fragility parameter and critical cooling rate. In addition, the change of Gibbs free energy difference of Zr₅₅Cu₃₀Ni₅Al₁₀alloy before and aftermelt hydrogenation is compared. It is found that the Gibbs free energy difference ofhydrogenated Zr₅₅Cu₃₀Ni₅Al₁₀alloy is samller than that of non-hydrogenatedZr₅₅Cu₃₀Ni₅Al₁₀alloy. They explain that hydrogenated Zr₅₅Cu₃₀Ni₅Al₁₀alloy has ahigh GFA from thermodynamics and dynamics.The structural change of Zr₅₅Cu₃₀Ni₅Al₁₀alloys after melt hydrogenation isinvestigated based on positron annihilation life time spectroscopy （PALS） analysisand Bernal holes in dense random packing of hard spheres model. Three lifetimecomponents are identified, indicating the presence of three distinct size ranges foropen volume defects in the glass. The components are assigned to annihilation inthe interstitial holes in the densely packed, flow defects and sub-nanometer voids.The increase of the concentration of interstitial holes in the densely packed showthat the efficiently packed regions are increased after melt hydrogenation, lead to adenser random packed structure, which explains the increase of GFA in terms ofstructural points of view.Room temperature mechanical properties of Zr₅₅Cu₃₀Ni₅Al₁₀andZr₅₇Al₁₀Cu_15.4Ni_12.6Nb₅are studied, and it is found that room temperaturecompression plasticity does not decrease, but it is greatly improved. Hydrogen-enhance plasticity is attributed to the formation of stress concentration induced byhydrogen which promotes proliferation and stability of shear band. Thus it canweaken the highly localized shear deformation, leading to larger plastic strain ofamorphous alloy.