On The Glass-forming Ability And Thermal Stability Of Cu-Zr-Al-(X) Alloys

As a promising structural material for industrial application,Cu-based bulk metallic glasses（BMGs）have attracted considerable concern due to their low costs,non-toxicity,and excellent mechanical properties.In order to develop new Cu-based BMGs with high glass-forming ability（GFA）,a new composition design method was proposed in this work based on the proportional mixing method of binary eutectics.A series of ternary Cu-Zr-Al and quaternary Cu-Zr-Al-Sn bulk glass-forming compositions were successfully developed.The GFA,structural relaxation,and crystallization behavior of these newly developed Cu-based BMGs were systematically studied.To further improve the properties of the Cu-based alloys,the effects of microalloying of sulfur on the GFA,thermal stability,crystallization behavior,and mechanical properties of Cu-Zr-Al alloys were examined.First,a new composition design method based on the pseudo-binary theory was proposed.A series of novel Cu-based BMGs were efficiently developed using this method.The driving force for crystallization（35）G_l-x of each binary eutectic was calculated using CALPHAD.The（35）G_l-x data were used to evaluate the precipitation“desire”of binary eutectics.By constructing a balance of the precipitation tendencies between the competing binary eutectic products in Cu-Zr-Al-（Sn）alloy systems,the initial proportionality coefficients of the binary eutectic units were directly obtained.The critical casting diameter d_c of the corresponding initial alloys reaches 3 mm.The coefficients were adjusted to further improve the GFA of the alloys.By adjusting roughly 5%of the proportionality coefficients,the glass-forming compositions with d_cof 5 and 7 mm were successfully located in Cu-Zr-Al(Cu_47.5Zr_45.1Al_7.4)and Cu-Zr-Al-Sn(Cu_47.3Zr_45.8Al_6.4Sn_0.5)alloy systems,respectively.The calorimetric results show that with the adjustment of the mixing coefficients,the liquidus temperature T_l gradually decreases and the corresponding reduced glass transition temperature T_rg increases,correlating very well with the GFA tendency.In contrast,the width of the supercooled liquid region（35）T_x does not correlate with the GFA of the studied alloy systems.In order to better understand the origin of the increase in GFA upon composition adjustment,the thermophysical parameters,such as specific heat capacity,enthalpy of phase transition,low-and high-temperature viscosities of the newly developed Cu-Zr-Al-（Sn）BMGs were measured using calorimetry,thermomechanical analysis,and a custom-built Couette rheometer.The thermodynamic driving force and dynamic fragility D^*were determined.Results show that the improvement of GFA originates from the decrease in（35）G_l-x and the increase in low-temperature D^*,thus revealing the thermodynamic and kinetic origins of the GFA of the studied alloys.The rheological measurements of the viscosities revealed a kinetically more fragile liquid behavior at high temperatures than at low temperatures.This behavior indicates a fragile-to-strong transition in the undercooled liquid.Furthermore,the low-temperature viscosity measurements revealed that after the initial relaxation from the glass to the supercooled liquid state,there is an anomalous viscosity increase in the supercooled liquid of Cu-based alloys during isothermal annealing or continuous heating.The abnormal viscosity behavior is accompanied by a significant exothermic event.These results hint towards a continuous structural change in the supercooled liquids before crystallization.In addition,severe chemical reactions between the Cu-Zr-Al alloy liquid and the graphite crucible were detected at temperatures above 1350 K.The reactions can also induce abnormal increases in viscosity and heat release behavior.A combination of state-of-the-art experimental techniques,including in-situ high-energy X-ray diffraction（HEXRD）,in-situ transmission electron microscopy（TEM）,spherical aberration-corrected TEM,simultaneous small-and wide-angle X-ray scattering,and atom probe tomography was used to investigate the thermal stability and crystallization behavior of Cu-Zr-Al BMGs.The origins of the anomalous viscosity behavior in the supercooled liquid and the rapid nucleation behavior during the primary crystallization process were revealed.The crystallization sequence of the Cu_47.5 alloy at conventional heating rates was determined to be glass→（Cu,Al）₁₀Zr₇+amorphous matrix→Cu₁₀Zr₇+Cu Zr₂+Al Cu₂Zr（τ₄）.The system can only reach the thermodynamic equilibrium state after the second step of phase transformation,which is induced by long-range atomic diffusion and accompanied by partial dissolution of the initial Cu-rich phase.In addition,the microstructural characterizations on the supercooled liquid phase show that a new amorphous phase with obscure composition variations but enhanced local order has precipitated from the initial homogeneous liquid structure prior to crystallization.Combined with the viscosity results of the new annealed state,it is suggested that the new amorphous phase processes a stronger liquid behavior.To describe the transition in liquid,a model analogous to the order-disorder transition in crystalline solid solutions was proposed.This model resolves the long-standing contradiction between observing a so-called liquid-liquid transition vs.a phase separation in the supercooled liquid during crystallization.Furthermore,the nucleation kinetics of the primary Cu-rich phase was revealed to be rapid,exhibiting characteristics similar to the“explosive”or“avalanche”crystallization reported in previous studies.The intermediate strong liquid phase formed in the supercooled liquid should be the structural origin of the impressive nucleation behavior.Based on the experimental results,a schematic diagram was proposed to generalize the complex multi-step devitrification reactions of Cu-Zr-Al BMGs.This work is expected to provide new insights into the underlying mechanisms of the rapid crystallization behaviors of deeply undercooled metallic liquids in terms of thermodynamics,kinetics and atomic structure.Finally,the effects of minor addition of sulfur（with characteristics of low cost and non-toxic modifications）on the GFA,thermal stability,and phase transformation behavior of Cu-Zr-Al alloys were studied for the first time to develop Cu-based BMGs that are more applicable for industrial applications.It is found that sulfur has positive and negative influences on the thermal stability and GFA of the Cu_47.5 alloy.On the one hand,the higher the sulfur content,the stronger the liquid behavior of the sulfur-containing alloys is.The sluggish liquid kinetics inhibits the formation of the eutectic phases upon heating from the glass and during cooling from the liquid,thus improving the stability of the liquid phase against crystallization and promoting glass formation.On the other hand,sulfur can trigger the precipitation of an Al-richτ₃ phase,which is catastrophic for the GFA and thermal stability because it exhibits an extremely high nucleation rate thus easily precipitates.These two factors compete with each other and jointly determine the properties of the studied Cu-Zr-Al-S alloys.The optimal sulfur content for the Cu_47.5 alloy was determined to be 0.75 at.%.Compared with the base alloy,the sulfur-bearing alloy has improved d_c of 8 mm,enlarged（35）T_x,and comparable mechanical properties,making it a promising structural material for industrial applications.Furthermore,the schematic changing tendency of the TTT diagrams of the sulfur-containing alloys was proposed on the basis of experimental observations.