Structure And Properties Of Ce-Based Metallic Glasses

In recent decades,metallic glass?MG?has become a research hotspot both in the field of metal materials science and condensed matter physics due to its disordered microstructure and excellent physical,chemical properties.For materials sicence,a large number of MG systems have been developed with excellent soft magnetics,mechanics,corrosion resistance and a wide range of market application prospects.For condensed matter physics,MG with relatively simple atomic structure could be regarded as a good model system to study a series of fundamental questions in nonequilibrium condensed matter physics,such as glass structure model,structural relaxation,glass transition,melt viscosity and supercooled liquid behavior,etc.Ce-based MG has attracted much attention among many MG systems due to its low glass transition temperature?T_g?,polyamorphic transition under high pressure,high Fermi hehavior at low temperature,etc.Glass-forming ability?GFA?and the structural origin of high GFA are the core issues in the research of MG.Structural heterogeneity and the microstructure characterization of MG have always been the difficult problems that hinder the further development of MG research.In addition,ultrastable MG with exceptionally high stability is one of the cutting-edge research hotspot in recent years.The present thesis focuses on the above three fundamental scientific questions on the basis of Ce-based MG,and the main results are as follows.?1?Ce-based MGs belong to the extremely strong glass-forming system,whose fragility m values lie in a strong liquid range of 18-33.Fragility of Ce₇₀Ga₁₀Cu₂₀ is about 18,which is the lowest value reported so far in MGs.Quantitatively analyzing the effect of m and reduced glass transition temperature(T_rg)on GFA,it was found that the relationship between T_rg and nucleation frequency is approximately linear,while nucleation frequency is probably exponentially changed by m.And,the influence of T_rg on GFA would be submerged by that of m when m is below³0,indicating that m plays a dominant role in the determination of GFA for the strong glass-forming system.?2?Positron lifetime spectroscopy yields two components of positron lifetimes?₁¹30 ps,?₂²60 ps for the Ce₇₀Ga_xCu_30-x MGs.?₂ is corresponding to the vacancy-sized free volume slightly smaller than that of Ce monovacancy,which is widely observed in many kinds of MGs as dominant atomic site.Such a local atomic structure could originate from the conventional random packing concept of glassy alloys,being an intrinsic structural component of the glassy matrix in MG.?₁ is corresponding to a kind of densely-packed local structure,which is even denser packed than the face-centered cubic Ce.First-princlples computer simulation results suggest that this anomalous packing structure is associated with the transition from localized Ce-4f ¹ to delocalized Ce-4f ⁰.The polyamorphism transition with Ce atom volume collapse is attributed to Ga chemical potential.?3?It is unambiguously shown that Ce₇₀Ga₆Cu₂₄ MG exhibits volume expansion together with the density decrease upon crystallization in sharp contrast to other amorphous materials.The in-situ and ex-situ X-ray spectra show that the crystallization steps of Ce₇₀Ga₆Cu₂₄ MG are:Ce₅Ga₃?5Ce+3Ga,CeCu?Ce+Cu,Ce₅Ga₃+CeCu+3Ce?3Ce₃Ga+Cu.Positron lifetime spectroscopy yields two kinds of positron lifetimes for Ce₇₀Ga₆Cu₂₄ crystal.Considering that the positron lifetime?_1,crystal observed for the crystalline Ce₇₀Ga₆Cu₂₄ is close to that of Ga,positrons could annihilate partially with Ga electrons inside the Ce₃Ga crystallite.Annihilation site of dominant?₂ is located at the interfaces among the Ce₃Ga and Cu crystallites.The free volume size at the interfaces among the Ce₃Ga and Cu crystallites for the crystalline Ce₇₀Ga₆Cu₂₄ is larger than those in the amorphous matrix of MG.The formation of interface structurally looser than the matrix of MG could decrease the relative density yielding the volume expansion.Furthermore,the disappearance of densely-packed local structure upon crystallization could contribute to the decrease in the relative density together with volume expansion as well.?4?Ce₇₀Al₁₀Cu₂₀ MG with simple atomic structure and extremely low T_g,providing an ideal system to investigate the prolonged aging effect on non-crystalline materials.After hyper-aging on Ce₇₀Al₁₀Cu₂₀ MG at room temperature for more than ten years,whose T_g and T_f generate a significant enhancement and decrease,respectively,suggesting its kinetic and thermodynamic stability have been significantly enhanced after hyper-aging.And,the variation amplitude could match with those of the vapor-deposited ultra-stable organic or metallic glasses and the hyper-aged amber.It is indicated that ultrastable bulk metallic glass could be obtained by conventional aging for long enough time.Remarkable increase in density,Young's modulus and shear modulus were found in the hyper-aged Ce₇₀Al₁₀Cu₂₀.Results of high energy synchrotron X-ray radiation show that radius of the first shell slightly shrinks approximately 0.2%after hyper-aging.Decrease in positron lifetime for the hyper-aged Ce₇₀Al₁₀Cu₂₀ is the consequence of the shrinkage of vacancy-sized free volume.Hyper-aging only leads to the shrinkage of free volume but maintaining the chemical surrounding environment with solvent Ce atoms.