Deformation Behavior And Work Hardening Mechanisms Of Dendrite Reinforced Ti-based Metallic Glass Matrix Composites

The unique atomic arrangement structure of bulk metallic glasses（BMGs）gives it many mechanical properties that traditional crystalline alloys cannot match,such as high strength,high hardness and large elastic limit.It is precisely because of this unique atomic arrangement that BMGs undergo plastic deformation in a non-uniform plastic deformation mode at room temperature,and the plastic deformation is limited to a shear band with a width of more than ten nanometers.BMGs have extremely low plasticity at room temperature.It usually fails by brittle fracture.In order to effectively inhibit the expansion of shear bands and promote the interaction of multiple shear bands,in recent years,people have successfully developed in-situ dendritic toughened metallic glass matrix composites（MGMCs）,which significantly improve the room temperature plasticity of BMGs.Among them,Ti-based MGMCs shave attracted much attention due to their excellent mechanical properties,low density,easy processing and high glass forming ability.Based on this,this article aims to explore the toughening and work hardening mechanism of in-situ dendritic toughened Ti-based MGMCs,using XRD,DSC,DIC,SEM,TEM,nanoindentation,room temperature stretching and other methods.First,with yielding as the core and room temperature cold rolling as the deformation method,the deformation behavior and work hardening mechanism of Ti₄₈Zr₂₀Nb₁₂Cu₅Be₁₅ MGMCs were systematically studied.Then,with work hardening as the core,through the addition ofβ-phase stabilizing elements Nb,Ta,Mo,the work hardening and softening deformation behavior of Ti₄₁Zr₃₂Ni₆Ta₅Be₁₄X₂（X=Ta,Nb,Mo）in-situ dendritic toughened MGMCs were systematically studied to clarify its hardening mechanism.This provides practical and theoretical basis for the engineering application of MGMCs.The main conclusions are as follows:（1）The deformation behavior and work hardening mechanism of Ti₄₈Zr₂₀Nb₁₂Cu₅Be₁₅MGMCs under different cold rolling quantities were systematically studied.The DIC test system was used to track and analyze the strain evolution during the tensile deformation of the MGMCs in real time.At the same time,it is used as a non-contact optical extensometer to determine the yield strength.Studies have found that proper cold rolling can not only increase the yield strength of MGMCs,but also effectively suppress the shrinkage rate of MGMCs,and the plastic instability is significantly alleviated.With the help of the in-situ stretching platform,it is discovered that the deformed dendritic phase reduces the ability inhibiting the rapid expansion of the shear band due to plastic strain,and the glass matrix will be locally softened earlier.（2）Based on the correlation between the two-phase yield strength and plastic deformation of cold-rolled MGMCs,a model for predicting the yield strength of cold-rolled MGMCs is proposed.First,for the amorphous phase,the relationship between the yield strength and hardness of the non-work hardening material（H=3σ_y）is simultaneously considered based on the cooperative shear model,and the yield strength of the cold-rolled amorphous phase is obtained.Then,for the dendrite phase,considering the classic crystal strengthening mechanism and the contribution of cold working dendrite strain hardening to the yield strength of the dendrite phase,the yield strength of the cold rolled dendrite phase is obtained.Finally,the yield strength of the cold-rolled MGMCs is obtained through the mixing rule.Wih the help of this model,it is found that the theoretically calculated yield strength of as-cast,20%,40%and 60%cold-rolled MGMCs are very close to the experimental values,and the error range is less than5%.（3）Taking Ti₄₃Zr₃₂Ni₆Ta₅Be₁₄ MGMCs（abbreviated as Ta0）as the research object,through the addition ofβ-phase stabilizing elements Nb,Ta,Mo,the deformation behavior of Ti₄₁Zr₃₂Ni₆Ta₅Be₁₄X₂（X=Ta、Nb、Mo）MGMCs was explored systematically.The study finds that the four MGMCs all have excellent tensile properties（a yield strength of more than 1 GPa and an elongation of more than 4%）,but exhibit different tensile deformation behaviors.The Mo2 MGMCs immediately shows strain softening without strain hardening after yielding.In contrast,the Ta0 MGMCs display strain hardening behavior after yielding,and no necking occurs before fracture.Although Ta2 and Nb2 MGMCs exhibit a certain degree of strain hardening after yielding,the overall plastic strain still shows strain softening.The excellent work hardening ability of MGMCs depends on whether the work hardening of the dendritic phase can compensate for the shear softening in the glass matrix.Effectively suppressing the occurrence of strain localization is an effective way to improve the uniform tensile plasticity of MGMCs.Through TEM observation,it is found that the excellent work hardening ability of the MGMCs in this paper comes from the dislocation strengthening contribution of the dendritic phase.The addition of differentβ-phase stabilizer elements can change the hardening ability ofβ-dendritic phase in the same alloy system.At the same time,the larger STZ content in the glass matrix contributes to the plastic deformation of the amorphous phase.The enhancement of the stress transfer ability of the two-phase interface can promote the interaction of multiple shear bands and thus enhance the overall plasticity of MGMCs.