Study On Mechanical Behavior And Deformation Mechanism Of TiZrCuNiBe Bulk Metallic Glass

Ti-based bulk metallic glasses（BMGs）has attracted wide attention due to its strong amorphous forming ability,large critical size,excellent mechanical properties,physical and chemical properties.Due to the brittleness of BMGs at room temperature,the plastic deformation is extremely difficult.Although a lot of researches have been carried out on the deformation behavior of BMGs,the deformation mechanism is still one of the core theoretical problems,which needs to be further studied systematically.Aiming at the mechanical behavior and deformation mechanism of Ti-based BMGs,Ti₃₃Zr₃₀Cu₉Ni_5.5Be_22.5 BMG was selected as the research object in this paper to systematically studied the mechanical behavior and deformation mechanism of Ti-based BMG in room-temperature,supercooled liquid region and cryogenic cycle treatment state by X-ray diffractometer（XRD）,differential scanning calorimeter（DSC）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,universal material testing machine,dynamic thermal simulation testing machine（Gleeble 3500）,nano-indentation tester etc.At the same time,the effect of introducing oxygen element in the preparation of Ti-based BMGs on its microstructure,thermal stability and mechanical behavior at room temperature was also studied.The room-temperature mechanical behavior of Ti₃₃Zr₃₀Cu₉Ni_5.5Be_22.5 BMG was studied by a serious of compressive tests.The results showed that the specimen size has a significant effect on its mechanical behaviors.As the specimen diameter decreased from 4mm to 2 mm,the compressive plasticity of the BMG specimen increased from 1.0%to2.4%,while the fracture strength decreased from 2070 MPa to 1870 MPa,revealing a significant size effect.On the basis of the free volume model,combined with the analysis of the relationship between the critical cooling rate and the size of the BMG specimens,it can be concluded that the specimen with a smaller size has a larger cooling rate during the formation process,so more free volumes can be formed in the specimen to support the atomic movement.The specimen with smaller diameter,therefore,showed greater plastic deformation ability.In the supercooled liquid region（SLR）,Ti₃₃Zr₃₀Cu₉Ni_5.5Be_22.5 BMG exhibited an obvious viscoplastic flow phenomenon,and its high-temperature mechanical behavior was closely related to the test temperature and strain rate.The experimental results showed that the activation volume of Ti-based BMG in SLR gradually increased with the increase of test temperature.Moreover,the analysis of fracture behavior in SLR indicated that as the temperature increased from 618 K to 638 K,the critical strain rate of brittle fracture of the specimens increased from 3×10^-3s^-1 rises to 6×10^-3s^-1.Additionally,the analysis results of the stress state on the shear surface demonstrated that the cohesive strength of Ti-based BMG increased gradually,accompanied by the gradual increase of fracture angle.The Ti₃₃Zr₃₀Cu₉Ni_5.5Be_22.5 BMG was subjected to deep cryogenic cycle treatment for10,20,30 and 40 times,respectively.It was found that the amorphous structure and thermal stability of the specimens were hardly affected by cryogenic treatment.Notablely,the relaxation enthalpyδH_r of the BMG specimens all increased significantly after treatment,implying that the deep cryogenic cycle treatment led to the rejuvenation of Ti-based BMG.Further,the DCT30 specimen showed the most significant degree of rejuvenation.Mechanical behavior studies indicated that the room-temperature plasticity of the Ti₃₃Zr₃₀Cu₉Ni_5.5Be_22.5 BMG improved greatly after 30 cycles of treatment（from 1.4%of the as-cast to 7.8%of the DCT30）,while maintaining high strength.Meanwhile,The average stress drop and the shear band formation energyΓreached the lowest value,which were about 30%and 21%lower than that of the as-cast specimen,respectively.In the preparation process of Ti₃₃Zr₃₀Cu₉Ni_5.5Be_22.5 BMG,the effect of oxygen introduction on its microstructure and mechanical behavior was complicated.The microstructural analysis exhibited that when the oxygen addition amount does not exceed0.7 at.%,oxygen can be uniformly dissolved in the Ti-based BMG without forming any crystalline phase or oxide.While the oxygen addition amount reached 1.0 at.%,nanocrystals with a size of more than 10 nm were precipitated on the amorphous matrix.With the oxygen addition further increased to 3.0 at.%,the specimen crystallized obviously and precipitated the Be₂Zr type crystallization product.The results of mechanical property analysis showed that the strength and plasticity of the specimens are significantly improved after adding 0.5 at.%and 0.7 at.%oxygen.Notablely,the BMG specimen possessed the maximum strength and plasticity after adding 0.7 at.%oxygen（2084 MPa,3.72%）.With further increase of oxygen addition,the strength and plasticity of the specimens began to decrease.When the oxygen addition amount increased to 3.0at.%,the strength of Ti-based BMG was greatly reduced to 1724 MPa,and its plasticity is completely lost.Here,the deformation mode transformed into a typical brittle fracture.