Research On Composition Design And Properties Of Co-based And Y-based Amorphous Alloys

Bulk metallic glasses(BMGs)have attracted increasing interests due to their outstanding properties.The major goal is to design new alloy systems with superior glass forming ability(GFA).Due to lack of complete theory predicting GFA for new metallic glasses,Co-based and Y-based amorphous alloys were fabricated by the element substitution method in this wprk.The conclusions are drawn as below:(1)The effects of Zr on the microstructure,GFA and thermal stability of Co₄₅Fe_25-xZr_xNb₁₀B₂₀(x=0,2,4 and 6at.%)alloys have been studied.Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀alloy shows the most homogeneous microstructure,the best GFA and the highest thermal stability.The apparent activation energies of Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀amorphous alloy from Kissinger equation are 720.3kJ/mol for T_g and 424.5kJ/mol for T_x1, respectively.(2)The addition of Y can improve greatly GFA and thermal stability of Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀alloy.The main reason is that the reaction between yttrium and oxygen is thermodynamically favored compared to the reaction between oxygen and the other elements in the system.Therefore, the stability of the remaining liquid could be enhanced by the alleviation of the harmful effect of oxygen,and the GFA of Co-based alloys can be improved.The apparent activation energies of(Co₄₅Fe₂₁Zr₄Nb₁₀B₂₀)₉₆Y₄ amorphous alloy are 940.7kJ/mol for T_x1and 825.0kJ/mol for T_p1 respectively,which are the highest for all known amorphous alloys.The studied Co-based amorphous alloys not only have high GFA,but also have preferable soft magnetic properties,which are potential functional materials.(3)Based on equiatomic substitution,the additions of Ce,Pr and Nd element(5～20at.%)improve GFA of Y₅₆Al₂₄Co₂₀alloy,GFA of Y_56-xM_xAl₂₄Co₂₀(M=Ce,Pr,Nd;x=5,10,15,20)alloys increase with the increasing of M content.It seems that for the supercooled liquid region (ΔT_x)of Y-based amorphous alloy has a close correlation to bond parameters.It increases with the electronegativity difference(Δx),atomic size parameter(δ)and electron concentration(Δn)of Y-based alloys.An equation was deduced for the first time by a linear regression analysis,i.e.,ΔT_x=-273.5+639.6Δx²+1074.5δ²+363.1Δn^2,3.The regression coefficient (R)for this fit is computed to be 0.956.(4)The effects of Zr and Sc elements on GFA of Y₅₆Al₂₄Co₂₀alloy have been studied.The results show that Sc element improves better the GFA than Zr element.The reason is that the heat of formation of the Sc₂O₃ is about -1908.8 kJ/mol,which is lower than that of the oxides of other constituent elements(e.g.Y₂O₃,-1903.6 kJ/mol;ZrO₂, -1100.5kJ/mol;Al₂O₃,-1653.5kJ/mol;CoO,-237.7 kJ/mol).Therefore, the chemical affinity between scandium and oxygen is higher than that between oxygen and other component elements,and the formation of scandium oxide is thermodynamically favored compared to other oxidation reactions.At the same time,a modified calculable parameterε^*,defined as a negative ratio of mixing entropy to formation enthalpy, is proposed,which can predict the GFA of amorphous alloys prepared by the element substitution.(5)Y₅₆Al₂₄Co₂₀and Y₄₁Sc₁₅Al₂₄Co₂₀bulk cylindrical specimens with diameters of 3 and 5 mm were prepared by the method of copper mold suction casting,respectively.The actual cooling rate for Y₄₁Sc₁₅Al₂₄Co₂₀ BMG is 36K/s according to the offset temperature of fusion and the onset temperature of solidification.On the other hand,the isothermal kinetics was modeled by the Johnson-Mehl-Avrami(JMA)equation,the Avrami exponents were calculated to be larger than 3.This implies that the main crystallization of Y₄₁Sc₁₅Al₂₄Co₂₀BMG is governed by diffusion-controlled three-dimensional growth.(6)The fragility parameter m for Y₄₁Sc₁₅Al₂₄Co₂₀BMG from the Vogel-Fulcher-Tammann(VFT)equation is 24,which is smaller than those values for typical Zr_46.75Ti_8.25Cu_7.4Ni₁₀Be_27.5and Pd₄₀Ni₁₀Cu₃₀P₂₀ BMGs.This confirms that the Y₄₁Sc₁₅Al₂₄Co₂₀alloy can be classified into one of the best metallic glass formers.The Poisson's ratio of Y₄₁Sc₁₅Al₂₄Co₂₀BMG is 0.254,which is the smallest for all known BMGs,which implies it is ideal material for studying the fragility of the BMGs.(7)For Y-based amorphous alloys,the hardness increases with the decrease of the cooling rate.There are multiple shear bands or pile-ups around indentations.The reason is that the increase of free volume in the deformation area of indentation.Based on the free volume mode,the higher cooling rate is,the defect concentration is expected to be higher. Higher defect concentrations enhance the probability that an atom will jump and diffuse,which in turn is expected to increase the flux of atoms and decrease the flow stress,thereby induces the softening of amorphous alloys.