Preparation And High Temperature Plastic Deformation Behavior Of Zr-based Amorphous Alloys

The excellent properties of bulk amorphous alloys have shown its prospect as an important material to support future high technology.However,in order to be widely used in engineering field,they must be overcome the problem of forming.Because they rarely exceed 2%plastic strain at room temperature,they are difficult to be processed at room temperature.Bulk amorphous alloys have viscous flow characteristics in supercooled liquid region(SLR),so they can carry superplastic forming or casting out in this temperature range to get amorphous alloy parts with complex shape.In addition,the development of bulk amorphous alloys with large plasticity and good thermal stability has always been a research hotspot in materials.Therefore,independent development and design of bulk amorphous alloy components,exploring the influencing factors and deformation mechanism of its deformation behavior at room and high temperature,establishing thermal processing map to get the best processing conditions of the alloy,which is helpful to guide the thermoplastic forming of micro amorphous alloy parts,and lay the foundation for the industrialization of amorphous alloys.In this thesis,based on different proportion mixing,binary eutectic mixing method designed four new compositions of Zr-based bulk amorphous alloys.Some sub-deep eutectic or deep eutectic binary eutectic points were selected as basic units,i.e.,Zr₄₄Cu₅₆,Zr₇₆Ni₂₄and Zr₅₁Al₄₉binary eutectic.Then,the four kinds of alloy bars with?3 mm×90 mm were prepared by copper mold suction casting.XRD,DSC,Vickers microhardness test,compression test and SEM were investigated the microstructure,thermal characteristics and mechanical properties of the designed alloys at room temperature.And high-temperature compression test discussed the deformation behavior of Zr-based bulk amorphous alloys in SLR,analyzed its high-temperature deformation mechanism and got the best processing conditions of the alloy in SLR.The results show that:(1)The four Zr-Cu-Ni-Al alloys designed are all completely amorphous,and their supercooled liquid region(?T_x)exceeds 65 K,especially Zr_55.7Cu_22.4Ni_7.2Al_14.7(Zr55.7)bulk amorphous alloy has the widest?T_x,reaching 82 K.The thermal stability of these four alloys from low to high are Zr_54.4Cu_26.13Ni_6.4Al_13.07(Zr54.4)?Zr₅₇Cu_18.67Ni₈Al_16.33(Zr57)?Zr_56.36Cu_20.53Ni_7.6Al_15.51(Zr56.36)and Zr_55.7Cu_22.4Ni_7.2Al_14.7,respectively.The room temperature plasticity from low to high are Zr57,Zr56.36,Zr54.4 and Zr55.7 bulk amorphous alloys,respectively,especially the plastic strain of Zr55.7 alloy reaches 5.5%,and the compressive strength reaches 1885 MPa.The stronger the multiplication and interaction of shear bands,the higher the plasticity and strength of the alloys.(2)The plastic deformation behavior of Zr-based bulk amorphous alloys at room temperature is highly sensitive to aspect ratio and strain rate.With increasing aspect ratio,the plastic strain and strength decrease,when the aspect ratio is 1:1,the plasticity strain of Zr55.7 reaches 9.9%,and the compressive strength reaches 2036 MPa.With decreasing strain rate,the plastic strain,yield strength,compressive strength and fracture strength increase,where the plasticity strain increases from 0.4%to 8.3%,and the compressive strength increases from 1684 MPa to 2000 MPa.With decreasing aspect ratio and strain rate,the number and density of shear bands and vein-like patterns on the fracture side enlarged.(3)The mechanism of serrated flow was analyzed and discussed from the view of the initiation,propagation and interaction of multiple shear bands.With decreasing strain rate,the uniformity of serrated flow in the first stage is insensitive to the change of strain rate,while the uniformity of serrated flow in the second stage changes.The interaction of multiple shear bands is the main reason for the transition of serrated flow from near uniformity to non-uniformity in the second stage.(4)The high-temperature plastic deformation behavior of Zr55.7 bulk amorphous alloy in SLR is very sensitive to strain rate and test temperature.With decreasing strain rate and increasing test temperature,the microhardness increases,while the steady flow stress and peak stress decrease.The viscosity range of the alloy samples is 7.48×10⁹?2.61×10¹¹Pa·s,and the viscosity decreases monotonously with increasing test temperature and strain rate.In addition,there is a critical transition strain rate for the change of Newtonian flow,and the critical transition strain rate moves to a higher strain rate with increasing test temperature.(5)With the strain rate decreases and the test temperature increases,the appearances of Zr-based bulk amorphous alloys after high temperature compression of the sample changed from"S"-shaped truncated cone to drum shape,and then from drum shape to true truncated cone,i.e.,the deformation of the sample changes from non-uniform deformation to uniform deformation,and then from uniform deformation to near-uniformity deformation.The optimum processing conditions of Zr55.7 alloy in SLR are 728?748 K at low strain rates and 748?758 K at high strain rates.