The Structure Designed And Mechanical Properties Of TiZr-based In-situ Metallic Glass Composites

Bulk metallic glass composites with deformation-induced phase transformation have attracted extensive attention due to their excellent plastic deformation and work-hardening properties.In the process of deformation,the coupling of deformation-induced phase transformation behavior and shear bands initiation and propagation is the reason for improving the plasticity and work hardening ability of metallic glass composites.Therefore,it is a new and effective method to further optimize the mechanical properties of metallic glass composites by controlling the deformation-induced phase transition behavior.In this thesis,the effect of oxygen which acts as an a stable element in Ti alloys has been studied systematically on the morphology,microstructure and phase compositions by the composites of(Ti45.7Zr33Ni3Cu5.8Be12.5)1-0.01xOx(X=0,0.73,1.45,2.16,2.87,3.57,4.26)(referred to as O0?O4.26,collectively referred to as ZTM-O alloys)?-Ti(Zr)and amorphous matrix.Furthermore,the micro-mechanical behavior and phase transformation during loading have been studied by in-situ high-energy X-ray diffraction(HE-XRD).The mechanisms of strengthening and toughening in TiZr-based metallic glass composites with oxygen addition were illustrated carefully corresponding experimental results.With the increase of oxygen content,the microstructures of ZTM-O were changed from the composites of BCC? phase and amorphous matrix to the three-phase coexistence of ? phase,the dispersed ? phase and amorphous matrix.It has a significant mature of ? phase when the O content of ZTM-O alloy increases from O0 to O0,73,and the size of ? phase decreases from 44.03 microns to 17.23 microns.The nanoindentation results showed that the hardness of the dendritic phase increased significantly from 2.48 GPa to 8.143 GPa with the increase of O content,while the hardness of the amorphous phase was stable relatively.The results of mechanical properties show that the addition of oxygen with appropriate compositions could improve the yield strength and fracture strength effectively,and greatly improving the plasticity.O2.87 alloy have the best comprehensive mechanical properties.Its yield strength is 1721 MPa,fracture strength 2169 MPa and plastic strain 12.56%,which are higher than O0 alloy with 29%,20%,and 114%,respectively.The micro-stress analysis of the dendrites shows that the micro yielding of the dendrites is early than the macro yielding in O0.73 alloy.While in O2.87 alloy,both the micro yielding of dendrites and the macro yielding occur simultaneously.The result of in-situ HE-XRD shows that the addition of O could effectively modulate the structural evolution of ZTM-O alloys during deformation.In O0?O1.45 alloys,the martensitic transformation is induced from ? to ?".With the increase of O content,the critical stress of phase transformation increases greatly,from 750 MPa to 1650 MPa,and the amount of phase transformation decreases obviously.TEM results show in fractured O0?O2.87 alloys,the micro-morphology of martensite phases changed obviously:from coarse plate-like a"martensite with a width of 200-300 nm through the whole dendrite to fine martensite with a width of 10-20 nm near the boundary of dendrites.It was found that cooling rate and O content codetermine the size and morphology of dendrites in in-situ metallic glass composites.On the one hand,the diffusion process during crystal growing was controlled by cooling rate.On the other hand,the driving force of crystal growing was controlled by oxygen content.It has a significant influence on mechanical properties,especially for plasticity,on the sizes of dendrites.ZTM-O alloys have the optimal plastic deformation ability when the dendrites sizes are between 26 and 35 microns.