Influence of composition and process on Zr-based amorphous alloy with large supercooled liquid region

Abstract

The successful preparation of new amorphous alloys with a lower cooling rate and a very large region of the supercooled liquid provides the possibility for preparing bulk amorphous alloys not only directly from slowly cooling liquid state but also from warm-compacting the amorphous alloy powders or shaping the amorphous alloy ingot [1–4]. Inoue proposed [5] that the reason why the new multicomponent Zror Ti-based alloy systems have such an enhanced glass-forming ability is presumably due to the combination of the following points: 1) multicomponent alloy systems consisting of more than three elements; 2) significantly different atomic size ratios above about 13%; 3) optimum negative heats of mixing among the constituent elements. However, recent experiments have shown that [6] when adding a fifth component such as Ti, Hf, V, Nb, Fe, Co, Pd or Ag to a Zr-Al-Cu-Ni alloy, the1Tx of the matrix alloy always decreases regardless of the characteristics of the added component. More recently, Seidel et al. [7] and Sagel et al. [8] produced amorphous alloys by mechanically alloying elemental powder mixtures of Zr-Al-Cu-Ni and Zr-AlCu-Ni-Co, respectively. The characteristics of the so obtained amorphous alloys were quite similar for both techniques, mechanical alloying and liquid quenching. According to Inoue’s hypotheses [5], a proper matching of different elements with different atomic size will be helpful for the stability of the amorphous alloy. A recent report [9] confirmed that addition of a small amount of boron enhanced tha stability of Zr-Cu-Al amorphous alloy. We chose Zr-Al-Ni-Cu as a base alloy which has the largest supercooled liquid region, and substituted parts of Ni with smaller boron atoms. The formation of amorphous Zr-Al-Ni-Cu-B alloys by mechanical milling and arc-melting techniques as well as the influence of elemental B on Tg and Tx were investigated. Elemental powders of 99.9% purity, particle size <200 μm with the composition Zr60Al10Ni9Cu18B3 were mechanically milled under a high purity Ar atmosphere in a Spex-8000 vibratory ball mill with a ball to powder of 6 : 1 weight ratio. Milled powders were characterized by X-ray diffraction (XRD) and differential scanning calorimeter (DSC) at different milling time. Bulk samples were prepared by arc-melting followed by sucking the liquid into a copper tube with diameter of 3 mm and a length of 40 mm. XRD was performed in an Enraf Nonius goniometer with Cu Kα radiation. Thermal analysis was carried out with a Perkin Elmer DSC-7 in Al pans under purified Ar atmosphere. Temperature and enthalpy calibration were carried out with indium and tin standards. Fig. 1 shows the XRD patterns of Zr60Al10Ni9Cu18B3 ball milled for different times. It can be seen from Fig. 1 that with continued milling the diffraction peaks of the crystalline phases diffuse gradually, after 20 h of milling only a diffuse amorphous-like peak can be seen. DSC analysis (Fig. 2) shows that there is a sharp exothermal peak around 750 K for 20 h milling sample which corresponds to the transformation from amorphous to crystalline. It should be noted that there exists already an obvious exothermal peak in the DSC spectrum after 5 h milling, the difference is only the size of the peak’s area, which indicates that at a very early stage of milling, an amorphous phase has already formed, and then it grows. It is a well known fact that milling induced amorphization is a completely different approach to the glass formation during liquid quenching. The former requires usually an anomalous diffusion in the crystalline state of one component through another component, while the latter requires to delay the diffusion between the different components in the undercooled liquid. Indeed, for the new generation of Zr-based alloys with their complex compositions close to eutectica, the equilibrium products have a rather complex structure, therefore long range diffusion is needed for the nucleation