Martensitic Transformation And Structural Tuning Of A CuZrAlNb Bulk Metallic Glass Composite

In the present dissertation,the mechanism of martensitic transformation（MT）of the B2 CuZr phase in Cu₄₈Zr_47.2Al₄Nb_0.8 bulk metallic glass composite（BMGC）has been studied using pre-straining and cryogenic treatment.Structural characterization has been utilized including in-situ X-ray diffraction and neutron diffraction during the study.Aging treatment and ion irradiation have been employed to finely tune the structure of Cu₄₈Zr_47.2Al₄Nb_0.8 BMGC.Then,the enhanced plasticity of the studied BMGC has been achieved.The effect of stress state on the phase transformation of Cu₄₈Zr_47.2Al₄Nb_0.8 BMGC during the compressive process has been studied.Structural characterization proves that the yielding behavior of BMGC closely relates to the MT of the B2 CuZr phase embedded in the glassy matrix.After the yielding of BMGC,the content of martensite increases with strain.To finely control the content of the martensitic phase,the aspect ratio（AR）of the BMGC sample is decreased to 0.5,which changes the stress state exerted on the B2 CuZr phase.Small AR induces a higher yielding strain of 1.9 % than that of 0.8 % in the sample with regular AR（2.0）.Decreasing AR exerts hydrostatic pressure on the B2 CuZr phase which inhibits the MT.Molecular dynamics（MD）demonstrates that hydrostatic pressure confines the deformation in the radial direction.Comparing to the sample with regular AR,more strain is required for the phase transformation in the small AR sample from the B2 CuZr phase to the martensitic phase.In the view of thermodynamics,the suppression of hydrostatic pressure to the MT of the B2 CuZr phase is clarified.Different internal stress state of BMGC induces different structural evolution during the cryogenic treatment.In-situ high energy X-ray diffraction（HEXRD）reveals that larger shrinkage of the amorphous phase than the B2 CuZr phase causes enhanced hydrostatic pressure on crystals in the as-cast BMGC during the cooling process.The structure of BMGC remains unchanged in the isothermal process at 100 K.The heating process witnesses the recovery of the shrinkage of phases.The cryogenic treatment induces no structural variation in the as-cast BMGC.The 5 % pre-strained BMGC sample experiences the MT when the temperature decreases to 100 K.Subsequently,the content of the martensitic phase reduces in the isothermal process.During the heating process,MT occurs unusually.Thermodynamically,the complex structural changes in the pre-strained BMGC can be attributed to the effect of temperature and the internal hydrostatic pressure.MD verifies the structural evolution of the B2 CuZr phase in BMGC during the cryogenic treatment.The phase transformation of BMGC during the cooling process has been studied under different tensile stresses.In-situ HEXRD results indicate that a decrease in the testing temperature cannot cause structural change under small tensile stress.When the tensile stress exceeds 750 MPa,the MT of the B2 CuZr phase arises from the cooling process,and the content of the martensitic phase gradually increases as the temperature drops.MD verifies that under enough tensile stress,the martensitic phase could initiate and grow in the B2 CuZr phase in the cooling process.Based on all the experimental results and analyses above,a thermodynamic model of MT of the B2 CuZr phase is established using temperature,uniaxial stress,and hydrostatic pressure as parameters.The structural evolution and the change in the mechanical behavior of BMGC during the aging treatment are studied.With the observation of transmission electron microscopy（TEM）,in the period of room temperature（RT）aging treatment,the gradual precipitation and grow-up of the martensitic phase are found in the B2 CuZr phase.In the amorphous phase of BMGC,structural relaxation occurs during the RT aging process.Nanocrystal precipitates in the glassy matrix after 4-years RT aging treatment.With prolonging the RT aging time,the yielding strength of BMGC decreases.The plastic strain of BMGC reaches the maximum（15.1 %）after 2-years RT aging.The physical aging treatment at 473 K accelerates the precipitation of the martensitic phase in the B2 CuZr phase.The degree of order rapidly increases in the beginning 2 days of physical aging treatment,then grows slowly.The plasticity of BMGC decreases whereas the yielding strength increases after physical aging treatment.Owing to the difference of contraction between the B2 CuZr phase and the glassy matrix,crystals in BMGC undergo hydrostatic pressure at RT after solidification.The hydrostatic pressure hinders the MT of the B2 CuZr phase and stabilizes the B2 lattice.Aging treatment induces a decrease in the hydrostatic pressure and then facilitates the MT.The evolution of the hydrostatic pressure and the structure of the glassy matrix accounts for the changed mechanical behaviors of BMGC after aging treatment.Ion irradiation has been employed to finely tune the structure and mechanical behavior of BMGC.Macroscopically,an increase in the fluence of N+ ion irradiation on BMGC induces the increase in the contents of both the amorphous phase and the martensitic phase,which is characterized by neutron diffraction.Microscopically,the cascade collision caused by the incident N+ ions enriches the concentration of defects in the sample surface,i.e.the amorphization of crystals and the reduction of the degree of order in the glassy matrix.After transferring the kinetic energy of the incident ions to atoms in the surface of BMGC,the temperature of impacted regions firstly rises and then rapidly reduces,causing the disordered structure of the surface.MD verifies the ion irradiation on the surface and confirms the formation of excessive defects.Meanwhile,the B2 CuZr phase inside the BMGC sample undergoes MT during ion irradiation,which can be ascribed to the rising temperature.Increasing ion fluence induces higher tensile plastic strain from 8.0 % to 25.5 %,and lower yielding strength from 900 MPa to 350 MPa.