Formation Of Cu-based Bulk Metallic Glass And Properties

In recent years, Cu-based metallic glasses have attracted widely attention since its low price, excellent glass forming ability and good mechanical properties. In this paper, the new bulkier Cu-Zr-Al-Me metallic glasses were synthesized by minor alloying using copper mold casting. And the mechanical properties of these alloys were investigated and found some compositions with high strength, high hardness and good plasticity. In addition, we studied tension and stress relaxation behavior of a new developed La-based BMGBy adding minor rare earth (La, Ce, Nd, and Gd) and transition metal (Y, Ti and Ag) elements into ternary Cu-Zr-Al system, a series of BMGs were successfully prepared, and some centimeter-scale BMGs were found in the compositions of Cu₄₅Zr₄₆Al₇Gd₂, Cu₄₅Zr₄₇Al₇Y₁, Cu₄₅Zr_46.5Al₇Ti_1.5 and Cu_45-xZr₄₈Al₇Ag_x (x=0, 2, 5 and 8 at.%). The present results suggest that this micro-alloying method can enhance largely the glass forming ability (GFA) of the Cu-Zr-Al-Me system.The thermodynamics features of the above BMGs were analyzed using differential scanning calorimeter. We found that best GFA alloy is not certainly eutectic point composition. The parameters, super-cooled liquid region (ΔT=T_x-T_g), reduced glass transition temperature (T_rg=T)g /T_l) and γ value (γ=T_x/(T_g+ T_l)) were estimated and analyzed. The results show, comparing to super-cooled liquid region and reduced glass transition temperature, γ value can evaluate more accurately GFA of amorphous alloys.Mechanical measurements results of above developed BMGs, including plasticity, Vickers hardness, strength and Young's modulus, show that micro-alloying remarkablely affects the mechanical properties of the alloys. Addition of Gd largely decrease the plasticity strain from Cu₄₅Zr₄₈Al₇ with plascity strain 2.4% to totally brittle alloy material as Cu₄₅Zr₄₆Al₇Gd₂. Both Vickers hardness and fracture strength also decrease with increasing Gd content. However, addition of Ti and Ag are very helpful to increase the plasticity of the Cu-Zr-Al-Ti(Ag) systems alloys, plasticity strain of Cu₄₃Zr₄₈Al₇Ag₂ composition is up to 3% and Cu₄₅Zr_46.5Al₇Ti_1.5 compositionis 9.5%. At the same time, Vickers hardness and fracture strength with Young's modulus of the Cu-Zr-Al-Ti(Ag) systems alloys also increase slightly. Recent researches indicate that high plasticity on Cu-Zr-Al system BMGs is due to nanoscale crystallization phase separation during compression. The correlation between crystallization activation energy and BMG plasticity was investigated based on three alloy compositions, Cu₄₅Zr₄₈Al)7, Cu₄₃Zr₄₈Al₇Ag₂ and Cu₄₀Zr_46.5Al₇Ti_1.5 which have approximate component but very different plasticity strain. The results suggest that Cu₄₀Zr_46.5Al₇Ti_1.5 alloy system with largest plasticity strain has the lowest crystallization activation energy. With value of crystallization activation energy increasing, plasicity strain decrease, which also verifies the above mechanism of plasticity forming.Dry sliding wear behaviors of a series of bulk metallic glass of compositions Zr₄₈Cu_45-xAl₇Ag_x(x=0, 2, 5, 8 at.%) are investigated by pin-on-disk method at room temperature. The results show that these BMGs exhibit predominant wear-resisting properties. Value of wear coefficient range in 2.4-5.4× 10^-4 and frictionai coefficient range in 0.152-0.206. Wear resistance is high to 0.96～2.55×10¹³ Pa. Compared with Ag-free Zr₄₈Cu₄₅Al₇ glassy alloy, wear resistances of the Ag-doped compositions are enhanced significantly with the Vickers hardness increasing. And it seems that there is a reverse correlation between plasticity and wear resistance. Further, SEM observations demonstrate that the cracks usually initiate and propagate perpendicularto the frictional direction during the debris formation.Tension and stress relaxation behaviors of a La₆₂Al₁₄Cu_11.7Ag_2.3Ni₅Co₅ bulk metallic glass (BMG) as a function of isothermal annealing time have been investigated. It is found that annealing at 373 K below the glass transition temperature (423 K) of the BMG alloy, causes an increase of special heat difference at glass transition and density of the alloy, indicating a reduction of free volume in the BMGalloy with annealing time. Compared with as-cast sample, fracture strength, Vickers hardness, viscosity, Young's modulus and stress relaxation stability of the annealed BMGs increase with annealing time, which were caused by the reduction of free volume in the annealed samples. Furthermore, a change of fracture morphology from a mixture of smooth and furrow zones in the as-cast sample to mainly furrow zone in the sample annealed for 8 hours was also observed. All samples exhibit brittle behavior during tension tests.