Mechanical Behaviors Of A B2 Phase Reinforced CuZrAlNb Bulk Metallic Glass Composite

B2 phase reinforced bulk metallic glass composites(BMGCs)have attracted much attention due to their high strength and large plasticity.The high strength originates from the amorphous matrix.The large plasticity is attributed to phase transformation induced plasticity(TRIP)effect of the B2 phase and its blocking effect to the propagation of shear bands.Nevertheless,current studies mainly focus on the B2 phase at room temperature.As a component of the composite,the deformation behaviors of amorphous matrix are also of essence for developing the high performance BMGCs.Meanwhile,the deformation mechanism of B2-BMGCs at low and high temperatures still remains mystery,and the relationship between the volume fraction of the B2 phase and shear bands needs to be further studied,which is significant to reveal the deformation nature of B2-BMGCs.Therefore,in this thesis,with in situ tensile tests,synchrotron X-ray diffraction and nanoindentation technologies,the effect of cooling rates during solidification on structural features of amorphous phase and its deformation behaviors in a B2 phase reinforced Cu_47.5Zr₄₈Al₄Nb_0.5 BMGC was systematically studied.Furthermore,the interaction between the B2 phase and shear bands,and the deformation mechanism of the selected BMGC at low and high temperatures were also explored.The incipient plasticity of an amorphous solid represents the onset of shear deformation,which is a stochastic progress closely related to microstructure evolution.It is well known that the cooling rate plays a crucial role in the microstructure evolution of a glass.Nevertheless,how the cooling rate affects the incipient plasticity is still unclear.the incipient plastic deformation of the amorphous phase in the studied BMGC cast with different cooling rates is systematically studied.The incipient plasticity of the glassy matrix of the BMGC is found to occur via a first displacement burst,the occurrence of which is more stochastic as the cooling rate decreases.According to the auto-correlation functions and cooperative shear model,the stochastic behavior is attributed to the effects of the cooling rate on the initial free volume and subsequent activated shear transformation zones(STZs).A higher content of free volume is present in the glassy matrix of the studied samples cooled at a faster rate,which facilitates the operation of STZs.Moreover,the larger incipient burst size is correlated with the higher content of free volume and larger STZs in the glassy matrix.The larger STZs result in more preferable propagation of multiple shear bands,leading to less stochastic deformation and more obvious plastic deformation.The rheological behavior of amorphous phase is important to characterize the intrinsic relationship between structural features and mechanical behaviors.In order to further clarify the relationship between the structure and properties of amorphous matrix,room temperature nanoindentation creep method was used to characterize the effect of cooling rates on the rheological behavior of amorphous matrix.The amorphous phase fabricated via higher cooling rates are found to exhibit more prominent nanoindentation creep,but a smaller shear viscosity.The volume of STZs in the amorphous phase calculated based on a cooperative shear model increases with the cooling rate.The evolution of excess free volume created during nanoindentation creep deformation is clarified.A looser atomic arrangement leads to a larger STZ volume,thus facilitating nanoindentation creep deformation at room temperature.To trace the shear banding evolution and understand the effect of individual shear band on deformation mechanisms of the studied BMGC,tensile tests of the notched BMGC samples have been in situ studied.It is found that all the types of shear bands can be considered as shear bands controlled by stress affected zones(SAZs)at the crystal/matrix interface and those far away from SAZs.The fracture is dependent on shear bands far away from SAZs.Based on the analysis of elastic energy dissipation,the propagation of shear bands far away from SAZs can be delayed by reducing the average elastic energy dissipation.High Young's modulus and volume fraction of the B2 phase are effective for decreasing the average elastic energy stored in shear bands,thus improving the plasticity of BMGCs.Further studies show that the type and sequence of shear bands are closely related to the volume fracture of the B2 phase for unnotched BMGCs during in situ tests.Analysis suggested that there exists a critical volume fraction(?29.63%)of the B2 phase determining the deformation behaviors.Only shear bands controlled by SAZs are produced and the plastic stability is obtained when the B2 phase volume fraction is higher than the critical value.However,shear bands far away from the SAZs appear during deformation when the volume fraction of the B2 phase is below the critical value.Besides,the B2 phase with a large size bears more significant strain than that with a small size,facilitating a fracture mode transition from brittle to ductile.The effect of low temperatures on deformation behaviors of a Cu Zr-based BMGC has been investigated using high energy X-ray diffraction.Tensile strength,work-hardening ability and plastic strain of the studied BMGC show an increasing trend from 298 K to 153 K,which is attributed to a higher degree of martensitic transformation for the B2 phase and a more significant multiplication for shear bands in amorphous matrix at a lower temperature.During high temperature deformation,it is found that there exists a linear relationship between the strength of the studied BMGC and temperatures when the temperature is lower than 1.1 T_g(T_g is glass transition temperature).0.9 T_g is the transition temperature for the deformation of the BMGC from inhomogeneous to homogeneous.From 663 K to 723 K,the deformation of the BMGC is dominated by the amorphous phase.Nevertheless,the deformation of the crystalline phase plays an important role in the deformation of the composite at 763 K.