Influence Of Liquid-liquid Transition In Zr Based Metallic Glass On Glass Forming Ability And Mechanical Properties

Bulk metallic glasses(MGs)are considered as promising new alloy materials due to their high hardness,large elastic limit,excellent corrosion resistance and good soft magnetic properties,etc.,and MGs are expected to be widely used in the fields of national defense,medical equipments,and electronics.However,due to the high cooling rate required in the preparation process,and the room-temperature plasticity does not match the ultra-high strength and toughness,the size of MGs and the brittle fracture in the loading process are the main bottlenecks that limit the wide application of MGs as engineering structural materials.The MGs are prepared from high-temperature melt and inherit the disorder structure of the melt.Therefore,clarifying the relationship between the alloy melt and the the formation of MGs,and exploring the influence of the evolution of liquid structure on the mechanical properties at room temperature of MGs are of great significance for improving the theoretical system related to the field of glass transition and improving the comprehensive mechanical properties of MGs.In this paper,the Zr₆₅Cu_17.5Ni₁₀Al_7.5(at%)MG and its melt are carefully examined.The structure changes of the melt,the crystallization kinetics of MGs and the mechanical properties at room temperature have been systematically investigated by means of high temperature nuclear magnetic resonance(NMR),X-ray diffraction(XRD),differential scanning calorimetry(DSC),scanning electron microscope(SEM),transmission electron microscope(TEM),universal mechanical testing machine,Vickers hardness tester,nanoindentation apparatus,ultrasonic material test system.The research contents are described as follows:(1)The high temperature NMR probe and relevant heating system were designed and processed.Based on the high temperature NMR platform,the structure of Zr₆₅Cu_17.5Ni₁₀Al_7.5 melt was characterized.The liquid-liquid transition was found in the melt at T_LL=1383 K,and the structure change of equilibrium melt was closely related to the dynamics of the liquid.(2)The crystallization kinetics,crystallization products and glass forming ability of the as-cast metallic glasses were studied.The amorphous ribbons acquired by quenching the melt from the temperature above T_LL(denoted as high temperature alloy,HTA)and below T_LL(denoted as low temperature alloy,LTA)were prepared by melt-spinning method.The crystallization process and products of the two ribbons were analyzed by XRD and DSC.The MGs show different fragilities,crystallization activation energy and crystallization process.The HTA exhibits a two-step crystallization process in the continuous heating process.A metastable fcc Zr₂Ni phase is formed around 720 K,and then decomposes into the final stable phase of Zr₂Cu and tetragonal Zr₂Ni.While the LTA directly crystallizes into the final stable phase around 730 K.Further analysis reveals that the metallic glass formed at fast cooling rate shows the characteristic of“self-doped glass”,while the metallic glass formed at slow cooling rate shows the characteristic of“chemically homogeneous glass”.Further analysis reveals that the fcc Zr₂Ni like clusters formed in the high temperature liquid make the HTA shows the characteristic of"self-doped glass",while such clusters are absent in the low temperature liquid,which makes the LTA shows the characteristic of"chemically homogeneous glass".The Zr₂Ni like clusters in the high temperature melt would frustrated the formation of the stable crystallization products(Zr₂Cu and tetragonal Zr₂Ni),which significantly enhances the glass formation ability of the high temperature melt.(3)The comprehensive mechanical behaviors of HTA and LTA was studied.The results show that the yield strength of HTA is 1641.5 MPa and the compression plasticity reaches11.64%at room temperature.The SEM test reveals that the fracture of HTA shows a cold shear behavior.The yield strength of LTA is about 1555.7 MPa,the plasticity is only about0.74%,and the fracture surface shows the characteristic of vein pattern.In the fracture experiment,HTA shows a higher fracture toughness K_Q.Analysis of the fracture surface shows that HTA cracks appear multiple inflexion during fracture,and a large number of slip striations appear,showing a better ability of resistance to crack propagation.Hardness and modulus tests show that HTA has higher surface hardness and modulus,and HTA's higher modulus and Poisson's ratio are matched with its higher strength,notch toughness and plasticity.The excellent strength and toughness of HTA may be attributed to the local heterogeneity resulting from the formation of the fcc Zr₂Ni like clusters in the glass.