The Deformation Behaviors Of Bluk Metallic Glasses With Various Enthalpy States Under Multi-Axial Stress

Bulk metallic glasses(BMGs)have excellent mechanical properties such as high strength,high hardness and large elastic strain limit.It is an advanced metal material with great application prospect,but the room temperature brittleness seriously restricts its wide application.As a metastable material,the internal energy state of BMG is directly related to its mechanical properties.Recent studies have found that deep cryogenic thermal cycling(DCT)can regulate the structural heterogeneity to improve plasticity at room temperature.However,there is still a lack of sufficient experimental data and mechanism in this research direction,mainly due to the lack of experimental methods and perfect theoretical models for accurate characterization of BMGs microstructure.Nanoindentation technique can be used as a nano-scale structural probe to characterize the flow unit and structural heterogeneity of BMGs due to its high load and displacement resolution.In view of this,four typical BMGs of as-cast Zr₅₅Cu₃₀Al₁₀Ni₅(Ro-Zr₅₅),as-cast Zr₅₇Cu_15.4Ni_12.6Al₁₀Ni₅(Ro-Zr₅₇),H-charged Zr₅₅Cu₃₀Al₁₀Ni₅(H-Zr₅₅)and H-charged Zr₅₇Cu_15.4Ni_12.6Al₁₀Ni₅(H-Zr₅₇)were prepared by plasma-assisted hydrogenation method in this thesis.The energy state of BMGs with different energy states was regulated by DCT.The intermittent plastic deformation behavior and creep behavior of amorphous alloys with different energy states during multiaxial stress compression were studied by nanoindentation technology,thermal analysis,uniaxial compression and scanning electron microscope.The results show that:(1)The BMGs still maintain complete amorphous structure after hydrogenation and DCT.The glass transition temperature(T_g)and crystallization temperature(T_x)change little in the range of 1?3K.(2)There is an upper limit to the structure rejuvenation of as cast BMGs by DTC.For the Ro-Zr₅₅,the maximum value appears at the 50th cycle.The relaxation enthalpy increases from 0.57 k J/mol to 0.6 k J/mol.For Ro-Zr₅₇,the maximum value appears in the 60th cycle.The relaxation enthalpy increases from 0.62 k J/mol to 0.78 k J/mol.For hydrogen BMG after DCT also has induced structure rejuvenation.After 70 cycles,the relaxation enthalpy of the H-Zr₅₅ rose to 1.40 from 1.04KJ/mol.The relaxation enthalpy of H-Zr₅₇ up from 1.07 KJ/mol to 1.53 KJ/mol that close to ribbon amorphous alloy of same composition.(3)The plasticity of Ro-Zr₅₅ increases by DTC that the engineering strain increases from 0.17%to 0.89%.More than 50 cycles,the engineering strain decreased to 0.31%with the increase of cryogenic cycles.The plasticity of Ro-Zr₅₇ enhance with the increase of the number of cycles.The engineering strain increases from 0.33%to 3.72%.The plasticity of H-charged samples increases with structure rejuvenation.The engineering strain of H-Zr₅₅ increases from 0.38%to 4.46%and H-Zr₅₇increases from 2.59%to 6.86%.(4)The dynamic behavior of serration flow(Pop-in)events in the load-displacement(P-h)curve of amorphous alloy was studied by nanoindentation.As the loading rate increases from 0.5m N/s to 10m N/s,the serration flow phenomenon gradually weakens and finally the serration flow phenomenon completely disappears.Statistical analysis shows that when the temperature and loading speed are constant,with the increase of energy,the serrated rheological dynamics changes from chaotic state to self-organizing critical(SOC)state.The SOC cut-off values of BMG under SOC state gradually decrease with the increase of energy.When observing the shear bands around the indentation,it is found that the BMG in the SOC state generates more interactive shear bands.(5)Based on the cooperative shear theory and flow unit model,the influence of energy on the flow unit in BMGs were studied by using the statistical analysis of serration flow during nanoindentation loading and the creep behavior during holding stage.With the increase of energy,the density of flow unit in BMG increases.The plasticity of the material increases with the increase of shear transition zone(STZ).The number and distribution of flow units directly affect the activation of STZ.As the number of flow units increases,the activation of STZ becomes easier.The flow unit with a small range contributes more to the plastic improvement.Under external force,hydrogen atom can assist the formation of flow unit and STZ.