Preparation And Mechanical Properties Of Zr/Ti-Based Metallic Glasses And Its Composites Thick Strips

In this thesis,a new type of twin-roll casting equipment was used to successfully fabricate thick strips of Zr/Ti-based metallic glasses(MGs)and their composites(MGCs).The preparation process was explored,and the strip-forming rules were summarized.The microstructure and mechanical properties of the prepared strips were investigated.The finite element analysis of the twin-roll casting process of Vit.1 MG strips was simulated by using the fluent software.The steady-state temperature field and temperature distribution along different directions were clarified.The process parameters for strip-forming during twin-roll casting was experimentally obtained.The strips of Vit.1,Zr Cu Al Ni,ZT-3,BT-30,BT-48,BT-60 MGs or MGC strips with a thickness from 300 ?m to 600 ?m have been successfully fabricated.The bending tests of Vit.1 MG strip was conducted,and it was found that the yield strength is about 1800 Mpa and the total strain could reach 4.1 %.The average stored elastic energy under bending is as high as 5.6 × 106 J/m3.The Vit.1 ribbons with a thickness of 40 ?m and 110 ?m and the strips with a thickness of 320 ?m and 490 ?m were produced by single-roll melt spinning and twinroll casting,respectively.According to DSC analysis,nanoindentation measurement and TEM analysis,it was found that the 320 ?m strip,as compared with the 110 ?m strip,exhibits a higher relaxation enthalpy,a smaller Q1 value,a higher content of free volume,but possesses a lower cooling rate.The molecular dynamics simulations of twin-roll casting and single-roll melt spinning were carried out.Molecular dynamics simulations reveal that the extrusion force and rheology during twin-roll casting alter the evolution of connection types of clusters,and the population of 2-atom and 4-atom connections are prone to increase by consuming the number of 1-atom and 3-atom connections,which causes the rejuvenation under a lower cooling rate during twin-roll casting.The microstructural difference between the center and the edge of the MGC strips were investigated.It was found that ?-Ti grains at the center of the ZT-M or BT-60 strips were an order of magnitude larger than that of the edge,which meant the cooling rate of the center of the strip was lower than the edge.Only a small fraction of ?-Ti grains was found in the BT-30 strips,and the diffraction spots split,which is attributed to the rolling force and deformation.In the ZT-M strip,a large fraction of fine ?-Ti grains were found,and no crystallization of the glassy matrix occurred and no big grains formed.Furthermore,the dendrite is not a whole crystal any more,it contains several fragments with different crystalline orientations.This microstructure results in the poor mechanical properties of the ZT-M strips.