Research On Crystallization Behavior And Properties Of Zr_57.5Cu_27.3Al_8.5Ni_6.7 Bulk Metallic Glass

Non-isothermal crystallization transformation kinetics and isothermal crystallization kinetics of prepared Zr 57.5Cu27.3Al8.5Ni6.7 bulk metallic glass(BMG) were investigated by differential scanning calorimetry(DSC). In isochronal mode, the values of activation energy, Eg and Ex, calculated by Kisinger method are 409.70 k J/mol(± 60.07 k J/mol), 335.53 k J/mol(± 39.94 k J/mol), respectively. In isothermal mode, the S-shape relation curves of crystalline volume fractio n and temperature suggest the typical transition of nucleation and growth. The Avrami exponent n that ranges from 3.0 to 4, indicating that crystallization mechanism in the Zr57.5Cu27.3Al8.5Ni6.7 BMG is mainly interface-controlled. The crystal growth way changes from two-dimensional growth with decreased nucleation rate to three-dimensional growth during isothermal crystallization process. In isothermal mode, the mean value of activation energy is 434.81 k J/mol, which is higher than that in isochronal mode.After annealing at the temperature lower than Tg for 10 min, Zr57.5Cu27.3Al8.5Ni6.7 BMG exhibited a certain extent ordering. When annealing at the temperatures above Tg, crystal phase emerged on the amorphous matrix, the precipitation phase are Cu10Zr7, Ni Zr2, Al3Zr2 and Zr Cu phase. After 693 K isothermal annealing treatment, sample showed a significant yield phenomenon, the plastic deformation of 46.98%, the yield strength of up to 1862 MPa. After isothermal annealing treatment at 733 K, the fracture strength of alloy was decreased. After annealing at 693 K, the fracture morphology of sample appeared herringbone pattern. After annealing at 733 K, the fracture appearance of sample appears cleavage steps similar to the crystalline materials. It shows that the the fracture morphology characteristics of Zr57.5Cu27.3Al8.5Ni6.7 BMG gradually turns to crystalline materials after annealing treatment.It is found that the Zr57.5Cu27.3Al8.5Ni6.7 bulk metallic glass has good fatigue strength of 680 ± 10 MPa, high fati gue ratio of about 0.38 and low slope of the S-N curve. The Zr57.5Cu27.3Al8.5Ni6.7 BMG has excellent toughness since its high bending strength of 3199 ± 11.7 MPa. The calculated fracture toughness is 42 – 49 MPa ?m. The cyclic stress resulted in the precipitation of Cu-rich phase. The Cu-rich phase appeared in fracture surface can preven t fatigue crack propagation by crack bridging. The macroscopic fracture-appearance of each sample presents ?V? shape after fatigue failure, as a result of the high-density shear bands and the multiple branched shear bands in the deformation region.Zr57.5Cu27.3Al8.5Ni6.7 BMG shows higher corrosion resistance ability and pitting potential than Zr57.5Cu34Al8.5 BMG in 3.5% Na Cl solution. The corrosion potential and corrosion current density of Zr57.5Cu27.3Al8.5Ni6.7 BMG are-417 m V and 2.739 m A/m2, respectively. The high corrosion resistance is mainly due to the formation of passivation layer on the BMG surface during the anode polarization process. But pitting corrosion occurred easily for BMG in Na Cl solution, because a great quantity of Cl- was absorbed by the defective residues in the preparation process. Both BMG samples showed similar polarization behavior in PBS solution. They all showed a wide region before pitting corrosion, the corrosion potential and low corrosion current density were simila, but Zr 57.5Cu27.3Al8.5Ni6.7 BMG showed higher pitting potential.