Study On Deformation And Fracture Of ZrCuNi(Co)AlNb Bulk Metallic Glasses

Metallic glasses with long range disorder,short range order in atomic arrangement and isotropic mechanical property,is complex multibody interaction system.Owing to its unique structure,metallic glasses showing amazing mechanical properties,such as high strength,high hardness,high wear resistance,high corrosion resistance and excellent magnetic properties.Although,metallic glasses still face five engineering application problems,including unknown amorphous structure,glass transition mechanism,deformation mechanism,poor glass forming ability and nearly zero tensile plasticity.They extremely restrict their application as structure parts.In this paper,Zr base amorphous alloys are taken as the research objects to study glass forming ability,strain hardening,tensile plasticity.The effect of alloy elements on glass forming ability of Zr_63.78Cu_14.72Ni_10-xCo_xAl₁₀Nb_1.5?x=0,1,2,5,8,9,10?amorphous alloys was investigated by adjusting the content of Co and Ni.The strategy that pre-melting Zr-Nb eutectoid alloy to prepare master alloy.The results show that when the Co content is 0-10%,the maximum glass forming ability is greater than 4 mm.Partially replacing Ni with Co in Zr-Cu-Ni-Al-Nb amorphous alloy,the mixture enthalpy of+6 kJ/mol between Co and Cu is formed,and the phase separation of amorphous alloys is induced by positive mixing enthalpy.The atomic ratio of 1-10%Co replaced the Ni in the alloy system,and the phase separation of the alloy system occurred.On the compressive mechanical properties,the amorphous alloys containing Co all showed strain hardening phenomenon,while those without Co did not.The strength increment d?/d?=-1.9,1.7,2.4,1.6,1.4,1.6 and 3 GPa/unit strain for Co content of 0%,1%,2%,5%,8%,9%and10%,respectively.A large number of dimples appeared on the fracture surface and a lot of shear bands appeared on the surface of the samples.They all have good plasticity,and their advantage plasticity is 9.7%,8.2%,9.4%,7.7%,9.2%,7.7%and 9.0%,respectively.the strength reduction of Co contained samples is less than 4%,and the average yield strength is1547,1518,1506,1527,1566,1523 and 1495 MPa for Co content of 0%,1%,2%,5%,8%,9%and 10%,respectively.Two phase separations was found,and the size of primary separated phase was 10-20 nm and distributed on the substrate.The size of the second separated phase was less than 4 nm,distributing near the shear band with width 100-300 nm.The surface ultrasonic impact treatment of Zr_63.78Cu_14.72Ni₈Co₂Al₁₀Nb_1.5 bulk metallic glasses has been carried out to change the internal structure and further change the mechanical properties.The ultrasonic frequency was 20 kHz and the power was 1 kW.The number of times for ultrasonic impact treatment were 0,4,8 and 12 times respectively.Ultrasonic impact treatment time each time was 3-5 s to treat the upper and lower surface of the amorphous sample.It is found that the yield platform is obvious after the ultrasonic impact treatment compared with the untreated cast samples.Moreover,with the increase of ultrasonic treatment times,the plasticity increases firstly and then decreases.When the times of impact treatment are 4,the average plastic strain reaches the maximum value of 1.24%.After ultrasonic impact treatment,phase separations in middle and nanocrystalline on the surface of samples are the main reasons for tensile ductility of amorphous alloys.From the fracture surface,the fractography of the samples without ultrasonic impact treatment is typically brittle and amorphous,and composed of vein patterns and smooth area.After ultrasonic impact treatment,the vein patterns decreased obviously,which is related to the change of the stress state of the sample.A series of amorphous alloy components with GFA greater than 4 mm were obtained by partiallyreplacingNiwithCointheamorphousalloycomponent Zr_63.78Cu_14.72Ni_10-xCo_xAl₁₀Nb_1.5.The mechanical properties of amorphous alloys were enhanced by the component design and the ultrasonic impact treatment,and more than 7.7%of the room temperature compression plasticity and up to 1.24%of the room temperature average tensile plasticity were obtained.A strong work hardening phenomenon found in amorphous alloys lays a foundation for the application of amorphous alloys in the field of structural materials.