Bulging And Bending Behaviors Of A Zr-based Bulk Metallic Glass

In this thesis,Zr50.7Cu28Ni9Al12.3 bulk metallic glass(BMG)was selected as a model materials.Combined with the practical issues in BMGs' formation and application,high temperature gas free blow forming and room temperature 3-point bending tests by single-edge notched method were carried out to systematically study the deformation process of the Zr-based BMG in complex stress/strain states.The influence of three factors,including temperature,gas pressure and time,on the blow forming process of the BMG was studied.The height and strain of the formed BMG samples were studied by the optical microscopy(OM)observations.The obtained results demonstrated that the height and strain of the formed BMG sheet increased with the increase of the applied temperature,time and gas pressure.Meanwhile,the improvement in the plastic flow intensifies the inhomogeneity of sample thickness.For the case of 2 MPa gas pressure,the BMG samples tended to fail during blow forming process.The structural evolution of BMG during blow forming process was studied by means of differential scanning calorimeter(DSC)and transmission electron microscope(TEM).The results suggested that the crystallization degree of the BMG increased as increasing the temperature and time.Great difference in crystallization volume fractions existed in different parts of the formed BMG,which can be attributed to their different strain state.The strain in the parts closer to the center of the formed BMG samples becomes higher.TEM observations of the microstructure of formed BMG samples indicates that the increase in the bulging temperature causes the increasing size of the crystalline phases.When the bulging temperature reached 485?,crystalline phases with needle-like shape separated from the glassy matrix.This phenomenon deteriorated the mechanical performances including strength and plasticity.The strain field near the notch of the three-point bending sample with different U-type notch size was studied by digital image correlation(DIC).The load-deformation curves showed that,the Zr-based BMG during three-point bending tests exhibited no clear yielding phenomenon.The maximum load and displacement of three-point bending samples decreased as the notch width decreased due to the strain concentration.The strain field results near notch through DIC demonstrated that both linear and shear strain concentrated around near the notch area and the values of the strain gradually increases with the ongoing tests.A smaller notch width will lead to a greater strain concentration.The shear band morphologies were observed by scanning electron microscopy(SEM).A smaller notch width will cause a smaller shear band size and number.The shear band formation and propagation can be attributed to the fact that the local shear strain exceeded the elastic limit.The fracture surfaces of the bent samples consisted of two zones,including crack propagation zone and rapid fracture zone.With decreasing the notch width,the crack propagation zone and the size of dimple patterns in rapid fracture zone became smaller,suggesting the decrease in the toughness of the sample.