Microstructure, Thermal Stability And Mechanical Properties Of Cu Based Bulk Metallic Glass

BMGs are considered to have many potential applications as the structural materials, because of their unique physical and mechanical properties. In particular, the Al enhanced the GFA of Cu-Zr alloy system have a better combination of thermal stability, strength, ductility and lower production cost. In this dissertation, tapered sample of Cu₅₀Zr₄₂Al₈ was fabricated by suspend melting under an argon atmosphere using a water-cooled Cu mold. And the microstructure under different cooling rate was investigated. The critical thickness of Cu₅₀Zr₄₂Al₈ bulk glassy alloy is up to 4.8 mm. The mechanical behavior changes distinctly due to the presence of different microstructure. TheФ4mm BMG exhibits high compressive fracture strength of 2260MPa with elastic strain of 2.0% and plastic strain of 0.4%. TheФ5mm BMG composites containing CuZr martensite phase and Cu10Zr7 crystallizing phase with orthorhombic structure, exhibits a combination of high compressive fracture strength (1849MPa) and yielding strength (1670MPa) with elastic strain of 1.6% and plastic strain of 1.9%. The final fracture behavior is controlled by competition of the two crystallizing phases. Thermal crystallization of Cu₅₀Zr₄₂Al₈ BMG was studied. The characteristic and local activation energy were calculated by Kissinger and FWO methods respectively, thus the relationship between crystalline volume fraction and crystallization temperature, crystallization activation energy were obtained. The as annealed BMGs treated by different cooling medium exhibits different microstructure and mechanical behavior, because of their different effective annealing time. And the effects of liquid nitrogen quenching and furnace cooling on crystallization and compression fracture behavior of as annealedФ5mm BMG composites in the supercooled liquid region, were investigated respectively.The best quaternary glass former in Zr-Cu based alloy system, Cu₃₆Zr₄₈Al₈ BMG is a strong liquid with a fragility parameter m of 33, resulting in high glass forming ability and high fracture strength with a distinct plastic strain. Moreover, the Cu₃₆Zr₄₈Al₈ alloy shows a single-event feature on its melting curve, indicating that the alloy is located at a quaternary Zr-Cu-Al-Ag eutectic point. And the high GFA has been considered to result from the high stability of the undercooled melt. In this dissertation, the effect of repetitive melting on the master alloy microstructure before copper mold suction was analyzed. And the effects of overheated level and overheated time on the thermal stability and mechanical properties of Cu₃₆Zr₄₈Al₈ BMG were investigated under the same cooling rate, thus the optimal cast conditions were determined. Furthermore, the effect of structural relaxation on mechanical properties of the different as-deposited states BMGs was studied. The results show that higher overheated temperature enhances the thermal stability of bulk amorphous alloys, corresponding to higher specific-heat capacity and the smaller initial defect concentration. Bulk amorphous alloys exihibit good compressive plasticity at small overheat levels, whereas the compressive fracture strength and microhardness exihibit a significant increase first and then a slightly decrease. The mechanical properties of bulk metallic glasses fabricated by different casting process are determined by the interaction of the free volume and residual stresses. Under the same casting temperature, the combination of thermal stability and mechanical properties decrease with the overheated time prolonged. As the Cu₃₆Zr₄₈Al₈ BMG is located at a Zr2Cu+AlCu2Zr eutectic point, the eutectic with the brittle structure nucleate preferentially in the melt. Thus as the atomic arrangement is gradually ordered and even crystallized, the combination properties of BMG are destroyed. BMGs at different as-deposited states were annealed in the Sub-Sub-T_g and Sub-T_g region respectively. And mechanism of the pronounced mechanical properties changes was also discussed.The effect and mechanism of addition of Fe element in Cu₃₆Zr₄₈Al₈ BMG on the thermal stability and mechanical behavior were investigated. Fe element with smaller atomic size has the positive heat of mixing with Cu and Ag elements. From the phase diagram, the binary Fe-Cu alloy exhibits a small miscibility gap, as well as Fe-Ag is almost immiscible. Rapid cooling accelerates the fltting speed of the liquid-solid interface, and inhibits the long range diffusion of atoms. Thus Ag selectively dissolves in Cu, and BMG displays a distinct phase separation with a Fe-rich phase homogenously distributed in the Cu, Ag-rich phase matrix. And the phase separation was confirmed by XRD and TEM tests. Fe-rich phase has higher atomic packing density than Cu, Ag-rich phase. Therefore, the Fe-rich phase should be the hard phase, and Cu, Ag-rich phase be the soft one in BMG. The formation of a distinct phase-separating glassy structure depends on the ratio of Fe/Cu and Fe/Ag contents in the quinary Cu–Zr–Al–Ag-Fe alloys. The results show that, the addition of Fe delays the glass transition of BMG, and accelerates the crystallization reaction, thus decreasesΔTx and T_rg. When xFe=3, the bulk amorphous alloy shows a single-event feature on its melting curve, indicating that the alloy is almost located at the eutectic point. AndΔT_x, T_rg andγof this alloy are 103, 0.566 and 0.424 respectively. As the two amorphous separating phases possess different modulus and critical shear stresses in the BMG, and take effect on the shear band during compression, thus mechnical properties increase evidently compared with the matrix alloy. When xFe=5, the phase separation increases as well as the nanocrystalline form and dispersion strengthen the amorphous matrix. The compressive fracture strength and plastic strain increase to 2249MPa and 4.9% respectively. With the increasing Fe content, the alloys start to crystallize and precipitate the Cu10Zr₇ and FeZr₃ brittle phases, and exhibit the increase of sensitivity of embrittlement as well as the decrease of mechanical properties. Vickers hardness tests were used to study the effect of addition of Fe on the hardness and shear band features of the BMG.