Fabrication Of High-strength And High-toughness CuZr-based Amorphous Composites By Local Melting

Bulk metallic glasses(BMGs)have received extensive attention due to their good glass-forming ability(GFA),high strength,large elastic limit(about 2%),and good wear resistance.However,BMGs show poor room-temperature plasticity due to their disordered structure,which severely limits their application as structural materials.Recent studies show that CuZr-based bulk metallic glass composites by introducing the shape memory B2 CuZr crystalline phase into the glassy matrix,the room-temperature plasticity can be effectively improved due to the phase transformation induced plasticity effect.However,the ductility of CuZr-based BMG composites is closely related with the size,volume fraction,distribution and particle spacing of B2 CuZr crystals.In this paper,the high-vacuum electric arc welding and the high vacuum medium power electron beam welding machines are adopted to locally and rapidly heat and cool CuZr-based BMGs in order to fabricate ductile BMG composites.By controlling the input parameters,the crystalline volume fraction and spatial distribution of B2 CuZr crystals can be effectively controlled.Firstly,CuZr-based BMG plates were prepared by a high-vacuum arc melting-suction casting furnace.Then by using a special tungsten electrode to locally melt CuZr-based BMG plates,CuZr-based BMG composites were prepared.The formation of B2 CuZr phase can be induced by controlling the input power(317 ± 57 W,422 ± 58 W,604 ± 124 W)and changing the distance between the arc and the sample plate(1-2 mm,2-4 mm).The rapid cooling rate allows the B2 CuZr phase to be kept to room temperature.The size of the B2 CuZr phase can be adjusted by controlling the input currents and the distance between the tungsten electrode and the sample plate.Furthermore,by shifting the position of the tungsten electrode during local melting,the spatial distribution of B2 CuZr crytals can be controlled.When the volume fraction and spatial distribution of B2 CuZr phase are well controlled,the yield strength of the prepared CuZr-based BMG composites can reach 1700 ± 10 MPa,and the corresponding room-temperature compressive plastic strain is larger than 14%.SEM measurements show that during the initial deformation stage,B2 CuZr phase undergoes martensitic transformation and then induces the initiation of multiple shear bands at the interface between B2 CuZr phase and glassy matrix;with further increasing strain/stress,the twinned martensitic and multiple shear bands further develop,thereby leading to a large room-temperature plasticity.Although the arc local melting can artificially control the formation of B2 CuZr phase,but the input power during arc melting is difficult to be precisely controlled and the repeatability is also very poor during local melting,which is not good for fabrication of CuZr-based BMG composites.In order to precisely control the input power during local melting,the high-vacuum medium power electron beam welding machine is used to locally melt CuZr-based BMG plates.During electron beam local melting,three types of melting modes were adopted,i.e.the point scanning,line scanning,and surface scanning.By changing the parameters such as beam sizes,scanning times,scanning speeds,and scanning positions,the volume fraction and spatial distribution of B2 CuZr phase can be controlled precisely.Finally,CuZr-based BMG composites with the formation of B2 CuZr crystals can be explored artificially.(1)When the spot scanning method was adopted,the beam size was changed from 0.6 mA to 0.9 mA and the melting time was controlled to be 2 s.The local melting area can be increased by increasing the beam current and mainly consists of the melting zone,the heat affected zone,and the matrix part.With increasing the beam current,the structure of the melted zone changes from B2 CuZr crystals together with some other intermetallic compounds to a nearly pure B2 CuZr crystals and even a small amount of amorphous phase.The heat affected zone mainly contains B2 CuZr crystals,amorphous phase,and a small amount of other intermetallic compounds.Since this heat affected zone area is induced by heating not by melting,the crystals come from the crystallization products of BMGs during rapid heating and cooling.For the glass matrix region,it is still in an amorphous state due to the influence of the electron beam melting is very small.(2)For the line scanning mode,the beam size was changed from 1.4 mA to 1.9 mA,and the scanning rate was fixed to be 1000 mm/min.With increasing the beam current,the content of crystalline phase in the melting region is increased.In the melting zone,even some amorphous phase forms due to the rapid cooling rate after local melting while the heat-affected zone is dominated by B2 CuZr and amorphous phases as well as minor intermetallic compounds.(3)For surface scanning mode,the beam size was changed from 4.0 mA to 5.5 mA and the scanning rate was 1000 mm/min.The structures of different regions after local melting were similar to those obtained under the line scanning mode.When the beam size was 1.4 mA under the line scanning mode,the tensile yield strength of the sample was about 1450 MPa,and the room-temperature tensile plasticity was about 2%.When the beam size under the surface scanning was 4.0 mA,the tensile yield strength of the sample was about 1730 MPa,and the ductility was about 0.8%.Meanwhile,by increasing beam currents,the content of crystals in the samples increases,leading to the decrease of the tensile yield strength and the increasing of the tensile plasticity.Therefore,a proper control of the beam parameters can effectively control the volume fraction and spatial distribution of B2 CuZr crystals,resulting in the fabrication of ductile and strong BMG composites.