Medium-Range Order And Fragility Of Superheated Melts In Al-Based Alloys

In this paper, the amorphous structures of Al-Ni-Pr alloys with different compositions have been systematically studied by X-ray diffraction (XRD). The results of our experiments show that a distinct prepeak which characterizes medium-range order (MRO) occurs in the structure factors of each amorphous alloy. The position of the prepeak shifts to smaller Q values as the concentration of Pr increases. The structure unit size increases with increasing Pr concentration. The glass forming ability (GFA) of Al-Ni-Pr alloys has been calculated by differential scanning calorimetry (DSC). The results show that the addition of Pr elevates the reduced glass transition temperature, and increases the GFA of amorphous Al-Ni-Pr alloys which is in agreement with the medium-range order. Therefore, the medium-range order is closely related to the glass forming ability in amorphous Al-Ni-Pr alloys.The crystallization behaviour of amorphous Al-Ni-Pr alloys has been investigated by XRD, DSC, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) techniques. It has found that the crystallization behaviour of Al87Ni10Pr3 can be expressed as follows: Fully amorphous→amorphous + fcc-Al→fcc-Al + Al3Ni + AlPr; and Al85Ni10Pr5 has a different process which can be expressed as: Fully amorphous→amorphous + fcc-Al+ Al3Ni + AlPr→fcc-Al + Al3Ni + AlPr. Crystallization processe of Al87Ni10Pr3 amorphous alloy has the typical features of primary crystallization. And the monotonously decreasing isothermal calorimetric signal, characteristic of a grain growth process has been observed in the course of precipitation of fcc-Al inAl87Ni10Pr3.This paper presents the viscosity behaviour of Al-Ni-based glass forming melts.Viscosity measurements on molten alloys were made using an oscillating viscometer. The amorphous Al-Ni-based ribbons were produced by the single roller melt-spinning technique under an argon atmosphere. The parameters of the crystallization in amorphous Al-Ni-based ribbons have been identified by using DSC analysis. Then, from the value of its viscosity and the parameters of its crystallization, a new concept, the fragility of superheated melt M has been calculated. It has been found that this kind of fragility value of liquid above liquidus temperature is closely related to the glass forming ability in Al-Ni-based amorphous alloys. The less the value of M is, the more the glass forming ability. By calculation of the M, the glass forming ability of Al-based amorphous alloys could be predicted prior to the preparation for amorphous solids.