Investigation Of The Effect Of High Pressure On Stability And Mechanical Properties Of Metallic Glass And ? Relaxation Behavior Of LaGa Metallic Glass

Relaxation is a fundamental issue in glassy materials,and a hot topic in condensed matter physics.In this thesis,we first study the ?-relaxation of metallic glasses(MGs).In contrast to most MGs that exhibit weak ?-relaxation peak in their dynamic mechanical spectra,the ternary LaGa-based MGs we fabricated here show a distinct ?-relaxation peak.By adjusting the composition ratio and replacing the La/Ni/Co by Ga in LaGa-based MG systems,it is found that the peak of ?-relaxation becomes stronger with the increase of Ga.We study the stress relaxation behavior of these MGs with different content of Ga,and show the dynamic heterogeneity and evolution of flow units in the MGs.The Ga atoms prefer to be acted as liquid-like atoms or flow units in LaGa-based MGs because of its low melting point.We explain the effect of Ga on the ?-relaxation behavior of MGs through flow units model.The LaGa-based MGs with pronounced slow ?-relaxation could provide a model system to investigate some underlying issues of the relaxation and plastic mechanism of MGs.High pressure is one of effective ways to change structure and properties of materials.We report the formation of an ultrastable Pd40.16Ni9.64Cu30.12P20.08 MG with bulk size under high pressure at room temperature.The ultrastable MG shows remarkably enhanced kinetic stability with substantially increased glass transition temperature Tg and crystallization temperature Tx.The ultrastable MG also shows outstanding high density and hardness,and excellent thermal stability.The unique stability can be further reinforced by higher pressure and maintained even heating above Tg.We also find that the density and hardness of MGs increase with pressure.This result demonstrates that the high pressure is a unique and effective method to produce bulk MGs with high thermal and kinetic stability and excellent properties,which could advance the MG design and the understanding of the fundamental issues in MGs.The boson peak has been the basis and important subject in glassy materials.We investigate the effects of high pressure and physical aging on the boson peak and thermal expansion of a typical MG.Specifically,a monotonic decrease of linear thermal expansion coefficient(TEC)and boson peak intensity is found during physical aging,and both boson peak intensity and linear TEC coincidently experience an incipient decrease and then a subsequent increase under high pressure.The boson peak intensities show a clear linear relationship with the linear TECs both under high pressure and during physical aging.The results indicate a direct link between the boson peak and anharmonic vibration,which might provide a clearer picture for the profound understanding of the nature of boson peak.In order to explore new zero thermal expansion(ZTE)and negative thermal expansion(NTE)material with excellent thermal properties at room temperature and high temperature,we study the effect of pressure on the thermal expansion property in crystal materials from MG.We report the formation of positive thermal expansion(PTE),near NTE and ZTE materials.By changing the pressure applied on MG,the linear TEC of Pd40.16Ni9.64Cu30.12P20.08 material crystallized after pressure processing can controlled from about 1.49 x 10-5 /K to 2.93 x 10-5 /K,even ZTE material can be obtained.The average linear TECs of near ZTE and NTE Pd40.16Ni9.64Cu30.12P20.08 are 0.67 x 10-5 /K and-2.39 x 10-5 /K in the range from room temperature to 553 K,respectively.The linear TECs of crystal gradually decrease with the increase of pressure.It is due to the reduction of flow unit and microstructure homogenization of MG with the increase of pressure,so that the content of crystal phase I with rich Cu without Pd reduces,while the content of crystal phase II with Pd and Cu increases.This result demonstrates that the high pressure is a unique and effective method to produce new ZTE and NTE material with excellent thermal properties at room temperature and high temperature,which could advance the ZTE and NTE material design and application,and the understanding of the fundamental issues in MGs,ZTE and NTE materials.