The Effect Of Nb Or V Addition On The Microstructure And Properties Of Ti-based Bulk Metallic Glasses

In this thesis, the effect of Nb or V addition on the microstructure, mechanicalproperties and corrosion properties of TiZrNiCuBe alloys has been systematicallystudied.The effect of Nb or V addition on microstructure of Ti-based alloys has beenstudied by using X-ray diffraction (XRD), scanning electron microscope andtransmission electron microscope (TEM). The alloy samples with out Nb, V or with4%Nb or V are totally amorphous whereas4%Nb or V alloy possessed more orderedatomic configuration and thus less free volume. The alloy samples containing8%and12%Nb or V exhibited composite structure with dendritic crystalline phases distributedwithin featureless amorphous matrix.The effect of Nb, V addition on the crystallization activation energy of Ti-basedbulk metallic glasses has been studied by using differential scanning calorimetry(DSC), Compared with Nb or V free alloy, the crystallization activation energy of thealloys containing4%Nb or V almost remain unchanged whereas the activationenergy of the alloys containing8%and12%Nb or V dramatically decreases due tothe precipitation of solid solution phases from the amorphous matrix.The compression tests at room temperature have indicated that, compared with Nbor V free alloy,4%Nb or V alloy exhibited lower plastic strain due to their less freevolume.The alloy containing8%Nb element consists of a composite microstructurewith dendritic phases homogeneously distributed in glassy matrix and then has excellentplastic deformation capacity. For the case of12%Nb alloy, the non-uniform crystalphases with high crystallization fraction cannot suppress the rapid propagation of shearbands and thus result in the limited plasticity. The brittle Cu10Zr7phases in the alloyswith8%or12%V will cause the catastrophic fracture without global plasticityVickers hardness and nanoindentation tests have shown that, the4%Nb or V alloywith higher glass transition temperature Tgexhibits higher hardness. The crystal phyasewith small size in8%Nb alloy can easily interact with shear bands and thensignificantly enhance the hardness. The high crystallization fraction and non-uniformdistribution of crystal phases in12%Nb alloy will lead to the brittle fracture and lowhardness. Brittle Cu10Zr7crystal phase deteriorates the mechanical properties of thealloy containing8%V element. However, the smaller crystal size will induce fine grainstrengthening for12%V alloy and then enhance its hardness.Electrochemical corrosion tests have indicated that, in0.5mol/L H2SO4solution,12%Nb and12%V alloys exhibit the highest corrosion potential, the lowest corrosion current density and the highest polarization resistance.It suggests their best corrosionresistance. This phenomena can be attributed to the pronounced the stability of passivefilm formed at the alloy surface due to the addition of Nb or V. In1mol/L NaOHsolution, the alloy without Nb or V addition exhibits the smallest corrosion currentdensity and highest polarization resistance, demonstrating their better corrosionresistance.