Effect Of Microaddition On Glass-formation And Properties In Cu-based Bulk Amorphous Alloys

In this dissertation, the effect of micro-addition of foreign elements on glass-formation, corrosion behavior, electrocatalytic behavior for hydrogen evolution reaction, compressive mechanical property, and electrical resistance at high temperature of the as-cast Cu47Zr11Ti34Ni8 bulk metallic glass was investigated profoundly using X-ray diffraction (XRD), optical microscopy (OM), differential scanning calorimetry (DSC), and differential thermal analysis (DTA), potentiodynamic polarization, weight-loss method, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) coupled with energy dispersive X-ray detector (EDX), mechanical testing system(MTS) and resistance measurement system.The alloying ingots with compositions of (Cu47Zr11Ti34Ni8)100-xMx (M=Cr,Mo,W; x=0~5 at%) were prepared by arc-melting the mixture of Cu, Zr, Ti, Ni, and M metals. Sample rods with a diameter of 3 mm or 4 mm were fabricated by casting the master ingots into copper mold. XRD patterns and OM observations demonstrate that a small amount of foreign element does not change the monolithic amorphous structure of the obtained alloys. Bulk metallic glass matrix composites containing crystalline precipitates will be obtained when the content of foreign element is beyond limited range. The glass-forming ability of the alloys obtained and morphologies of the precipitates in the amorphous matrix varies with the content of foreign elements.DSC and DTA thermal analysis show that the onset temperature for glass transition (Tg) and the first crystallization (Tx) were enhanced while the melting temperature (Tm) and liquidus temperature (Tl) were decreased with a micro-addition of foreign element in (Cu47Zr11Ti34Ni8)100-xMx (M=Cr,Mo,W; x=0~5 at%) alloys, leading to the glass transition temperature (Trg=Tg/Tl or Tg/Tm) reduced and parameter (γ= Tx/(Tg+Tl)) increased. Mo-bearing alloy has the lowest Tm and Tl and the largest Trg andγ. For (Cu47Zr11Ti34Ni8)100-xMox (x=0~5 at%) bulk metallic glasses (BMGs).both Tg and Tx1 were increased while both Tm and Tl were decreased with increasing of Mo contents for the single amorphous BMGs, resulting in the enhancement of both Trg andγ. In addition, the addition of appropriate amount of Mo changes the melting process of the base BMG (i.e. Cu47Zr11Ti34Ni8 BMG) from two-step melting process to single one. The results of thermal analyses present that the thermal stability and glass-formation ability were increased with the micro-addition of foreign element, especially with the addition of Mo.The effect of the addition of Cr, Mo or W on corrosion resistance of the base BMG was investigated by means of electrochemical polarization and weight-loss measurements. Electrochemical measurements on various Cu-based BMGs were conducted in 1 mol/L H2SO4 and 1 mol/L NaOH aqueous solutions, respectively, open to air at room temperature. It revealed that the BMGs with a small amount of Cr, Mo or W exhibited a superior corrosion resistance in the two electrolytes as compared with the base alloy (i.e. Cu47Zr11Ti34Ni8 BMG). Mo-bearing BMG exhibited the greatest corrosion resistance, as indicated by the lowest passive current density and corrosion rate, the highest pitting potential and the lowest passive potential among these BMGs. Moreover, it is found that the corrosion resistance of Cu-based BMGs was increased with increasing Mo content for (Cu47Zr11Ti34Ni8)100-xMox (x=0-3 at%) BMGs when x=0-2. However, the corrosion resistance of the sample with 3 at% Mo or higher, which is a composite containing a few of crystallites in amorphous matrix, seemed not very satisfactory, as indicated by relatively high passive current densities and corrosion rate in both electrolytes. XPS results revealed that the improvement of corrosion resistance of Cu-based BMG containing appropriate amount of Mo originated from the enrichment of ZrO2 and TiO2, but depletion in Cu- or Ni- oxides in the passive films formed during electrochemical polarization. Secondly, the galvanostatic-step measurement was performed to investigate the kinetics of the formation of passive films on the BMG surfaces. It is demonstrated that the addition of an appropriate amount of Mo can effectively improve the stability and uniformity of the passive film. Finally, EIS results implied that the micro-addition of Mo increased the surface activity and promoted the generation of positive defects (i.e., oxygen vacancies), but suppressed the formation of negative defects at the interfaces between metal/passive film (M/F). As a result, the addition of Mo could speed up the formation of the passive film of Zr-, and Ti-oxides, and stabilize simultaneously the oxides film. Base on point defect model (PDM), a qualitatively kinetic model is established to explain tentatively the effect of micro-addition of Mo on the improvement of the corrosion resistance of the Cu-based bulk metallic glasses.The results of the catalytic studies indicates that the hydrogen over-potential for single amorphous alloys decreased gradually with the increase of Mo content and the electrocatalytic activity for hydrogen evolution reaction (HER) is improved by means of a microadulteration of Mo. According to the Arrhenius plots of the HER of the electrodes, the apparent energy of activation is then calculated from the temperature dependence of the exchange current densities. It can be seen that the apparent energy of activation decreased for the adulteration of Mo in comparison with the base alloy. SEM and EDX results imply that an increase of the electrocatalytic activity of the alloy with addition of Mo is due to the copper peeling off the surface which leading to Zr and Ti enrichment and increase of porosity on the surface of compared with the base alloy. Therefore, Tafel reaction route or Heyrovsky reaction step was promoted.In addition, quasi-static compression test reveals that both plasticity and strength of the base alloy are remarkably enhanced by the addition of appropriate amount of Mo. The Cu-based BMG containing 2 at% Mo exhibits the large plastic strain of 2.31 and higher fracture strength of about 2.21 GPa. The Cu-based BMG matrix composite containing 3 at% Mo exhibits the highest fracture strength of 2.24 GPa and largest plastic strain of about 3.82. It is believed that the ductile BMG (for example, (Cu47Zr11Ti34Ni8)98Mo2) may follow a different deformation mechanism from the (Cu47Zr11Ti34Ni8)97Mo3 BMG composite as far as the improvement in general plasticity is concerned. However, (Cu47Zr11Ti34Ni8)95Mo5 BMG composite exhibits brittle fracture behavior due to the existence of high volume fraction of large quenched-in crystalline particles.The electric resistance (ER) of the (Cu47Zr11Ti34Ni8)100-xMox (x=0, 2 at %) BMGs obtained were studied via the testing-system self-produce under Ar atmosphere. It was found that the temperature coefficient of resistance (TCR) is negative (-1.64×10-4) for Mo-free Cu-based BMG while the TCR is positive (1.86×10-4) for Mo-bearing. The phenomenon is interpreted based on the modified Ziman-Faber theory. Moreover, there is the maximization of ER for Mo-bearing BMG but not for the Mo-free BMG, which is originated from the different phase separatation due to the microalloying of Mo. In addition, it was found that the plots of resistivity vs. temperature exhibited four distinctive reduction-stages for two BMGs. The drops of ER are related to the change of the microstructure or the formation of crystalline phases in amorphous microstructure with temperature increasing for two BMGs.