In-situ Neutron Diffraction Studies On Micro-Mechanical Behaviors Of CuZr-based Bulk Metallic Glass Composites With Deformation-induced Phase Transformation

The in-situ bulk metallic glass composites(BMGCs)with deformation-induced phase transformation have garnered much interest due to their excellent mechanical properties and wide application prospects.The metastable CuZr phase in in-situ CuZr-based BMGCs upon loading can undergo martensitic transformation from B2 to B19',which empowers the BMGCs not only the excellent ductility but also the outstanding work hardening ability,and hence attracts researchers' attention.However,the dynamic characteristics of deformation-induced phase transformation as well as its effect on the deformation of BMGCs are still unclear.In-situ neutron diffraction technique is superior to study the deformation behaviors of materials.Besides,the deep penetration of neutrons can obtain the microstructure evolutions under various environments.Moreover,in-situ neutron diffraction can be used to evaluate the microstresses based on the obtained lattice strain and therefrom acquire the stress distribution.Therefore,in this paper,the in-situ neutron diffraction technique was employed to study the phase transformation and the micro-mechanical behaviors in the pure B2-CuZr phase and in-situ CuZr-based BMGCs with different compositions under the cyclic loading-unloading experiments.The in-situ neutron diffraction was performed on the CuZr alloys with the pure B2-CuZr phase throughout the two cyclic loading-unloading experiments.The metastable B2-CuZr phase can undergo martensitic transformation upon loading and the newly formed martensite phase can reversibly transform to the B2-CuZr phase during unloading.The critical stress for the martensitic transformation is reduced due to the retained martensite and the residual stress obtained from the loading-unloading cycles.The convoluted stress partitioning was determined by the elastic and transformation anisotropy as well as the newly formed martensite.The pseudo-elasticity is accommodated by the reversible martensitic transformation.The martensitic transformation and plastic deformation of the B2-CuZr phase in the Cu_46.5Zr_46.5Al₇ BMGC under the compressive loading-unloading experiments are constrained due to the loading sharing of the amorphous matrix.The deformation can be divided into four stages based on the work hardening rate variations.The first stage is the elastic deformation stage.In the second stage,the work hardening rate decreases rapidly because of the initiation of the shear bands(SBs)in the amorphous matrix which results in the softening of the BMGC.In the third stage,the decrease of work hardening rate is inhibited because the plastic deformation strengthens the parent B2-CuZr phase,which facilitates the applied load to transfer from the softened amorphous phase to the strengthened B2-CuZr phase.In the last stage,the decrease of work hardening rate is further retarded because the formation of the large amount of martensite and the further plastic deformation of B2-CuZr phase enhance the work hardening of the crystalline phases and hence counteract the softening of amorphous matrix.The micro-mechanical behaviors in the different deformation stages of Cu46.sZr46.5Al7 BMGC are associated well with the work hardening rate variations.The Cu_46.5Zr_45.5Al₇Nb₁ and Cu46.5Zr45Al7Nb1Ta0.5 BMGCs were fabricated by substituting Zr with Nb or Ta in the Cu46.5Zr46.5A17 BMGC.The volume fraction of B2-CuZr phase in the Cu46.5Zr45.5Al7Nb1 BMGC increases from 40%in the Cu_46.5Zr_46.5Al₇ BMGC to 52%but decreases to 10%in the Cu46.5Zr45-Al7Nb1Ta0.5 BMGC,while the plastic strain decrease from 3.8%to 1.4%and 1.32%,respectively.The decrease of plastic strain can be ascribed to the fact that the severe aggregation of B2-CuZr particles deteriorates the plastic deformation ability in the Cu46.5Zr45.5Al7Nb1 BMGC,while the little B2-CuZr phase in the Cu46.5Zr45Al7Nb1Ta0.5 BMGC fails to suppress the propagation of SBs.However,the similar plastic strain are obtained for both BMGCs.This can be attributed to the addition of Ta,which enhances the uniform size and homogeneous distribution of the B2-CuZr particles.Therefore,the contact area between the amorphous and B2-CuZr phase is improved,which boosts the cooperative deformation and improves the plasticity.