Controlling The Distribution Of Structural Inhomogeneities And Its Effect On Mechainical Properties Of CuZrAl Metallic Glasses

In this thesis,the effect of the cooling rate and pre-strain on the structural inhomogeneity of Cu Zr Al amorphous alloys has systematically been studied by X-ray diffraction(XRD),transmission electron microscopy(TEM)and differential scanning calorimetry(DSC).Numerical simulation has been also used to clarify the effect of structural inhomogeneity on the mechanical properties of Cu Zr Al alloys.The Cu₅₀Zr₄₅Al₅ribbon and the bulk glass with a diameter of 2 mm were fabricated by melt spinning and copper mold casting method,respectively.The effect of the cooling rate on the distribution of chemical composition elements was explored.With the increase of the cooling rate,the distribution of Cu and Zr elements remained almost unchanged,whereas the distribution of Al became more uneven.The local order degree of the ribbon and the bulk glass with a diameter of 2 mm are calculated to be 1.5%and 7.8%,respectively,indicating that the structure of the bulk metallic glass(BMG)with a diameter of 2 mm is more ordered.The above results are in good agreement with the simulation results.It can be seen from the DSC curves that as the cooling rate increases,the content of free volume will increase.LAMPPS was used to simulate the structural transformation and shear deformation behaviors of BMGs with cooling rates of 10¹⁰ K/s and 10⁸ K/s,respectively,during the tensile process.The BMG with a higher cooling rate of 10¹⁰ K/s exhibits low yield stress and more uneven shear modulus distribution,faciliating the formation of more shear bands.It can be attributed to more free volume content within this alloy,which can promote its plastic deformation ability.In order to reveal the influence of pre-strain on the structural inhomogeneity of BMGs,pre-deformed BMGs with pre-strains of 10%,29%and 44%are obtained by compressing the as-cast sample.The results of DSC and TEM show that with the increase of pre-strain,the free volume in the BMG firstly increases and then decreases,and the evoltion of the degree of order shows an opposite trend.The results of nanoindentation measurements suggest that the hardness distribution will become more inhomogeneous with the increase of pre-straining.By calculating the shear transition zone(STZ)volume and activation energy of the as-cast and pre-straining samples,it has been verified that the change in the mechanical performance of BMGs is caused by structural inhomogeneities.The structural inhomogeneity caused by severe plastic deformation can significantly decrease the energy barrier of STZ activation,and result in an enhanced STZ events during the pre-compression,resulting in the transition from brittleness to ductility.The above results show that the cooling rate and pre-strain can be utilized to finely control the structural inhomogeneity and thus the mechanical properties of the amorphous alloy.