Effect Of Cryogenic Treatment And Annealing On Microstructure And Mechanical Properties Of Cu-Zr-Ag-Al Bulk Metallic Glass

Metallic glass,which exhibit unique disordered structural periodicity,possesses many unique physical,mechanical and chemical properties.It draws more attention for its prosperous application.Cu-based BMG with high strength and good corrosion resistance are promising structural materials,so some researcher pay more attention to it.BMG showed brittle fracture during the deformation process,which restrict the application of it.Therefore,more and more people attach importance to the improving deformation ability,by annealing and cryogenic treatment.Although many current studies are toward researching the microstructure of BMGs or bulk metallic glass matrix composites(BMGCs)during annealing,little is known about microstructure and mechanical properties of BMGs and BMGCs after cryogenic temperatures.The objective of the present work is to clarify the microstructure and properties of the Cu45Zr45 Ag7Al3 bulk specimens after CT and annealing.And we pay more attention to;the fatigue after CT.(1)Cu45Zr45Ag7Al3 BMG was successfully prepared by casting into a copper mold in argon atmosphere.Crystallization kinetics of Cu45Zr45Ag7Al3 bulk metallic glass in non-isothermal and isothermal modes were investigated by differential scanning calorimetry(DSC).Under continuous heating conditions,the activation energies of the glass transition and crystallization at the onset and first peak crystallization temperatures are determined by Kissinger plots and are found to be 376.8,307.2 and 339.5 kJ/mol,respectively.However,under isothermal conditions the average activation energy for crystallization is about 413.7 kJ/mol obtained by using the Arrhenius equation,which is much larger than that deduced from Kissinger plots.The local Avrami exponent n is mostly in the range 1.82-3.51,implying that the crystallization is mainly governed by diffusion-controlled,three-dimensional growth with nearly constant nucleation rate.The samples showed the same half-moon vein patterns and droplet density as the as-cast amorphous after isothermally annealed at 741 K,746K and 751K for 10 min.After isothermally annealing at 756K,the samples fracture presented three types,a few of half-moon vein patterns accompany with the present of fish scale patterns,the surface of shear zone had low density vein patterns exist cracks,that is similar to the quasi-cleavage fracture morphology of the crystalline materials;jagged morphology,reflecting the typical characteristics of brittle fracture.After isothermally annealing at 741K,the sample just get a small amount of crystalline phase.There are a few of crystalline phase(Cu10Zr7)in the sample after annealing at 746K;After annealing at 751K,AlCu2Zr and Cu10Zr7 phase have been precipitated;at 751K;After 756K isothermally annealing,the main crystalline phase are AlCu2Zr,Cu10Zr7 and Cu8Zr3 phase.(2)Through studying the different cryogenic time of Cu45Zr45Ag7Al3 amorphous alloys,we found that the thermal stability of the amorphous alloy reduced gradually and the plasticity increased with time decreased.After CT 192h,the maximum plastic deformation reaches 0.50%.After CT 240 hours,the ductility decrease and the sample almost exhibited no plastic deformation.The strength and hardness increased continuously and reached maximum after CT 192 hours.However,the tensile strength decrease as CT time increased.This is due to that the pressure reduced the critical radius and made several rich portion changed into a stable grain that can be grown in the cryogenic process.Meanwhile,the pressure caused the "transport of atoms",which would result in crystallization of the amorphous matrix.The amorphous alloy transformed from full amorphous to partial crystallization structure,precipitated AlCu2Zr and Cu5Zr phases.And Cu5Zr performed as reinforcing phase,and AlCu2Zr was brittle phase.(3)In four-point bending fatigue test conditions,bending fatigue limit of Cu45Zr45Ag7Al3 amorphous alloy with different time of CT were 386MPa,487MPa,355MPa,313MPa and 224MPa,respectively.And the ratios of fatigue limit and fracture strength were 0.26,0.31,0.20,0.16,and 0.12,respectively.Fatigue fracture was divided into four areas:the crack initiation zone,crack propagation zone,fast fracture zone and the molten zone.However,the crack growth area of sample with 24h CT increased significantly compared to the as-cast sample.And there were no fatigue strip generate,this is mainly because nano crystalline phase precipitated after cryogenic treatment.Meanwhile,the threshold of amorphous alloys was improved with the extension of the CT,and the longer of cryogenic time,the higher of its threshold.Grain growth rate of sample with 24h CT was lower than other samples with different cryogenic time.The crack of the sample with 192 hours CT will propagate along the main shear board during the fatigue process.And because of the role of stress,the free volume content was increased.Then the density of free volume will increase,and shear board impediment to shear deformation decreases.With further deformation,the main clipboard extended to AlCu2Zr and Cu5Zr between amorphous matrix,and the stress in the tip of the clipboard results in Cu5Zr phase transition into B2 CuZr phase.(4)In uniaxial pressure-pressure fatigue test conditions,the fatigue limit of Cu45Zr45Ag7Al3 amorphous alloy was higher than that of three-point and four-point bending fatigue test.Under the same load above the fatigue limit,the fatigue lives of both were different.Fatigue fracture of the sample under three-point bending fatigue test was the same as four-point bending fatigue test.It consists of four main areas:fatigue crack initiation,crack propagation,fast fracture and melting zone.And there were fatigue strip.The fatigue mechanism was that the formation of shear bands and accumulation of free volume result in a number of gaps,thus contributing fatigue crack initiation in the region,and crack propagation was due to the passivation and re-sharpening phenomenon.Uniaxial pressure-pressure fatigue fracture surface was the same as that of uniaxial compression test.The sample was broken to a tilt angle,and the sample surface was not smooth,there are some visible surface cracks and no fatigue strips,which is due to the formation of surface damage layer during fatigue process.With the increase in the load cycle,the surface damage layer keep extending to the internal of specimen,making the effective loading area of sample gradually decreases,thus resulting in the decrease of the critical shear fracture stress,and making the sample finally shear fracture.