The Influences Of Isothermal Annealing On The Structure And Mechanical Properties Of Cu-Zr Amorphous Alloys By Molecular Dynamics Simulations

Amorphous alloys exhibit structural characteristics of long-range disorder and short-range order.These specific atomic arrangements make amorphous alloys have novel properties,such as high strength,high hardness,excellent wear resistance,corrosion resistance and soft magnetic.As metastable alloys,the excellent properties of amorphous alloys will change with crystallization,greatly limiting their applications in functional and structural materials.Hence,studying the crystallization of amorphous alloys has important theoretical and practical significance.In this thesis,the annealing process of Cu-Zr binary amorphous alloys was simulated using molecular dynamics simulation to investigate the influences of concentration,isothermal time,pressure and temperature on structure,as well as the mechanical properties of annealed Cu₆₄Zr₃₆alloy.The main results are as following:1.By investigating the structure of Cu₃₆Zr₆₄,Cu₅₀Zr₅₀ and Cu₆₄Zr₃₆ amorphous alloys that were isothermally annealed above glass transition temperature(850K).It was found that the compositional effect plays an important role on the crystallization of Cu Zr amorphous alloys.With the increase of Cu content,the Cu₆₄Zr₃₆ system partially crystallizes,and the ordered structure consists of Cu-centered<0 0 12 0>and Zr-centered<0 0 12 4>clusters.As we all known,icosahedra(<0 0 12 0>)play an important role in glass formation ability and mechanical properties.It can stabilize the supercooled metallic liquids,as well as presenting a barrier to crystallization.However,in our simulations,a number of icosahedra formed during annealing process,indicating that the icosahedra are not the only factor of the glass forming ability.2.The annealing time and pressure have an effect on the fraction of crystal in Cu₆₄Zr₃₆amorphous alloy.The crystal fraction increases with increasing annealing time.In general,the fraction of crystal increases fast at first,then becomes slow for longer treatment.For the effect of pressure on crystallization during annealing,the results indicate that the system contains less crystal cluster under high pressure.With increasing of pressure,a more compacted local structure was obtained,and atomic diffusion was also impeded,eventually resulting in the inhibition of grain growth.3.The Cu₆₄Zr₃₆ amorphous alloy partially crystallizes at 850K and 950K.The degree of crystallization at 950K is higher than that at 850K.However,the structure annealed at 1050K does not present the crystallization behavior.It is not conductive to crystallization as the annealing temperature is too low or too high.In addition,the specimen annealed at 950K contains the Mg Zn₂-type Laves?Mg Cu₂-type Laves and five-fold symmetry local structure.4.Comparing with amorphous Cu₆₄Zr₃₆ alloy,its annealed specimen possesses higher elastic modulus and yield strength during the compression test.The yield strength is relatively more sensitive to strain rate at higher temperature,and the yield strength increases with increasing the strain rate.However,at low temperature,the strain rate sensitivity of yield strength is poor.Under the test condition of temperature10K and strain rate 5?10⁷ s-¹,serration phenomenon in the stress-strain curve was observed obviously.The free volume increases with the temperature rapidly increase and decrease in the shear band.However,the abrupt rise in temperature lags behind the variation of free volume,suggesting that the free volume causes the initialization of shear band and then results in the increase of temperature.In addition,the decrease of temperature in the shear band corresponds to a dissipation of the thermal energy into the surrounding matrix.Therefore,the failure of shear band may not only result from the accumulation of heat.