Zr-based Bulk Metallic Glass-Metal Mesh Sandwich Structure Fabricated By Thermoplastic Forming

The bulk metallic glass（BMG）exhibits brittle fracture characteristics at room temperature,which greatly restricts its practical application as engineering materials.The development of amorphous matrix composites is a key way to solve the brittleness of bulk metallic glass at room temperature and realize its engineering application.At present,two important methods for preparing amorphous matrix composites are endogenous and external toughening second phases.For endogenous method,although the interface between the precipitated phase and the amorphous phase is better,the method has limited size of the composite material due to the glass forming ability.The existing second-phase toughening method can effectively control the volume ratio of the second phase,but the distribution of second phase is discontinuous and non-uniform,so that the amorphous base composites prepared by the second phase toughening method are not with tensile plasticity.In this work,we selected the continuous,uniformly distributed and dimensionally adjustable ductile metal mesh,and then the bulk metallic glass and metal mesh were hot-pressed in the supercooled liquid region by using the superplasticity of the amorphous alloy to obtain the sandwich composite structure samples.Firstly,we got the Zr₃₅Ti₃₀Be_26.75Cu_8.25.25 bulk metallic glass,which was prepared by copper mold casting.And then thermodynamics characterization was carried out to obtain the thermoplastic forming time and temperature window.Secondly,the metal meshes of different size and BMGs were hot-embossed to prepare composite samples.The microstructures of the interface between the BMGs and the metal mesh were analyzed by scanning electron microscopy（SEM）.And the results showed that the interface was closely bonded with no obvious voids,atoms diffused in the interface.EDX shows that the atoms have proliferated at both interfaces.The mechanical properties show that the prepared sandwich structure composites have doubled the fracture toughness of pure BMGs.The in situ observation and fracture morphology analysis showed that the toughening of the amorphous matrix composites was mainly due to the fact that the metal mesh effectively prevented the expansion of the crack in the amorphous matrix and promoted the initiation of multiple shear bands.On this basis,we selected the best stainless steel metal mesh（hole size:100 mesh）and Zr-based BMGs to hot-press multi-layer composite materials,and then carried out the fracture toughness and tensile test,but the results did not meet the expected.The analysis shows that this is mainly due to the difficulty of forming multilayer samples,and the pressing is not tight.In this regard,we tried to use ultrasonic vibration,to form the multi-layer metal mesh samples.By changing the amplitude of ultrasonic vibration,find the most effective vibration forming parameters.The existing experiments show that the vibration can easily lead to crystallization of amorphous alloy,the reason may be due to ultrasonic vibration exacerbated the atomic motion,causing a temperature rise.Therefore,it is necessary to carry out a series of experiments to study the effects of ultrasound on the thermoplastic forming of amorphous alloy and its mechanism.In conclusion,the amorphous alloy-metal mesh sandwich composite structure was prepared by hot-press forming.The results show that the fracture toughness of the sandwich structure samples is about twice that of Zr-based BMGs.But the multi-layer structure composite is still facing difficulties in forming,the existence of gaps between BMGS and metal mesh and other challenges.Ultrasonic vibration can not only reduce the viscosity of amorphous alloy in supercooled liquid region,reduce the forming stress,but also reduce the interface friction,it is considered the most effective method to obtain the multi-layer composite structure.How to avoid crystallization caused by vibration,we need to do more experiments to explore.