Constitutive Relatoinship And Micro Filling Behavior Of Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 Alloy In Supercooled Region

Micro forming plays an important role in promoting the development of products miniaturization due to its higher processing efficiency and material utilization,lower manufacturing cost,better component performance and dimensional accuracy compared with other micromanufacturing technologies.However,the application of traditional metallic materials in plastic micro forming field is limited because of the significant “size effect” during deformation process.However,the bulk metallic glasses（BMGs）have provided a new feasible idea to solve the “size effect” problem during plastic micro forming process as a result of their excellent superplasticity in supercool liquid region（SLR）and unique mechanical and physical properties.At present,the applications of BMG in plastic mirco forming process have not been thoroughly investigated.Thus,the researches on the deformation behavior and crystallization mechanism of BMG in SLR have great theoretical significance and practical value.In this dissertation,the high temperature isothermal compression tests have been performed to investigate the effects of experimental temperature and strain rate on deformation behaviors for Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5（Vit1）BMG in SLR.The experiment data indicates that the increasing temperature and decreasing strain rate result in a transion from non-Newtonian flow to Newtonian flow,the stress overshoot phenomenon on true stress-true strain urves disappare,meanwhile,the flow stress also decreases.A new phenomenological Maxwell-Extreme model,which is used to describe the deformation behavior of Vit1 BMG in SLR,has been built by introducing the effects of experimental temperature and strain rate on elastic behavior and stress overshoot.The comparison results show that the Maxwell-Extreme constitutive model can express the repationship between true stress and true strain under different experimental temperatures and strain rates,as well as effective strain distribution more accurately.It is proved that the Maxwell-Extreme model can be applied to the finite element simulation of the plastic deformation process for Vit1 BMG in SLR.The influence of heating rate on crystallization behavior for Vit1 BMG has been studied according to the results of DSC test data.Experimental results reveal that the first exothermic events on DSC curves split into two peaks at the heating rate of 5K/min and 10K/min,and their correspounding Avrami indexs are also relatively lower.Moreover,the crystallization processes are controlled by atomic diffusion,and the crystallization mechanism for Vit1 BMG has been changed with increasing heating rate.The growth dimensionality reduces from 2 to 1 as in the initial stage of crystallization process,and then it ultimately tends to be identical.By comparing the thermal analysis results and microstructure of the annealed and deformed samples,The Vit1 BMG undergoing plastic deformation has higher characteristic temperatures and closer atomic arrangement because the ordering process induced by high temperature deformation becomes the dominant factor in microstructural evolution.But the influence of high temperature deformation on microstructure gradually reduced with the temperature increasing.For investigating the effects of temperature,imprint load and cavity width on the filling performance of Vit1 BMG and researching the flow mechanism of BMG in micro channels,the imprint experiments have been performed.The Vit1 BMG flows into the micro channels by laminar flow under external loading.During the whole filling process,there is a velocity gradient along the width direction of micro channel because of the friction force between BMG fluid and cavity lateral,resulting in a siginificant radian and incidental tensile stresse on at the front of BMG fluid.So the oxygen-rich surface have cracked due to poorer mobility and lower surface bond strength,subsequently,the BMG in the center of fluid push the surface material to the cavity lateral of micro channel and become new flow front.The relationships between the filling height and imprint load,experimental temperature and the cavity width have been established under variable load condition.The filling performance of Vit1 BMG increases with the increase of experimental temperature,but some crystallization phases with complex composition will be precipitated from amorphous matrix and reduce the filling performance of Vit1 BMG in SLR when the experimental temperature exceeds 703 K.The research related to forming process of Vit1 BMG micro impeller in SLR has been performed based on the experimental results above.FE simulation results exhibit that the tops of micro blades are first formed in the closed extrusion-the initial stage of forming process,and then,the bottom of micro cavity begin to be filled when the top die contact the floating die and the flow front will become straight.Temperature,loading rate and loading method have significant ifulence on the flow behavior of BMG in micro cavity.The forming experiment results of Vit1 BMG micro impeller indicate that a micro impeller with better filling quality can be manufactured under the conditions of forming temperature 683 K,loading rate 2.46mm/min and forming load 3500 N using the cylindrical blank with the size of Φ2×4.1mm.The formed micro blade width is 507.75μm,Blade surface roughness is Ra0.25μm.In summary,the integral forming with composite mould method is suitable for manufacturing the micro component with larger ratio of length to width.