Double-stage glass transition in a metallic glass

Glass-transition region is still a somewhat dark area in the field of materials science. In order to shed more light on this phenomenon, the glass-transition behavior was studied in a Zr(55)Cu(30)Al(10)Ni(5) glassy alloy by monitoring its specific-heat capacity measured using a differential scanning calorimeter during heating in a step-scan mode. This reduces the influence of kinetic effects which shield the transition. It is suggested that two types of relaxation (likely related to the diffusivities of different alloying elements Cu and Ni on the one hand as well as Zr and Al on the other hand) take place and compete in the glass-transition region

On Long-Term Stability of Metallic Glasses

Thermal stability of different types of metallic glasses and partially crystalline alloys stored for at least 15 years at ambient conditions was tested in the present work by differential scanning calorimetry in comparison with that of the original alloys tested in the as-cast state in the earlier works. The structure of the naturally aged alloys was also studied by X-ray diffractometry. The structure of a couple of selected alloys was also tested by transmission electron microscopy. Most of the alloys retained their initial structure and showed only a moderate decrease in the crystallization temperature. Only those alloys which showed visible surface oxidation (Cu-Zr-system based) were partly transformed into a crystalline state forming micron-scale Cu particles in air at ambient conditions

Crystallization and Embrittlement Behavior of a Zr 55 Al 10 Ni 5 Cu 30

Metallic glasses as non-equilibrium materials crystallize at a critical temperature, accompanying their embrittlement. The size of glassy alloys is dependent on their glass-forming ability. The development of an appropriate joining technique is important for the extension of industrial applications. In the present study, we examine crystallization behavior of Zr 55 Al 10 Ni 5 Cu 30 glassy alloy upon annealing. In the beginning of the crystallization process, a clustering, a crystal nucleation and the crystal growth processes take place sequentially. The phase transition behavior was examined by differential scanning calorimetry (DSC). The kinetics of the crystallization process can be analyzed by the changes in the volume fraction of the crystalline phase which scales with the exothermic heat release and density measurement. The crystallization process in the Zr 55 Al 10 Ni 5 Cu 30 alloy is found to be diffusion-controlled. The values of the Avrami exponent are between 2.3 and 2.8. Therefore, the velocity of nucleation and crystallization processes should scale with the diffusion velocity of the constituent elements in the metallic glass. The diffusion rate can be estimated using the diffusion coefficients of the constituent elements, as a function of time and temperature. The slope of the calculated iso-precipitation line of CuZr 2 phase on the fixed TTT diagram is well fitted with the lines representing ductile and brittle behavior of the sample. The results indicate that the most harmful crystalline phase, which causes embrittlement is CuZr 2 . The influence of Si contamination on the crystallization behavior is also studied in the present work