Ca–Mg–Zn–(Ag) bulk metallic glasses prepared by unidirectional quenching

We fabricated ternary Ca–Mg–Zn and quaternary Ca–Mg–Zn–Ag bulk metallic glasses with diameter of 7 mm by unidirectional quenching into water-cooled Ga–In–Sn liquid alloys. It was suggested that the electromagnetic stirring caused by induction eddy current facilitated the glass formation. Glass forming ability of Ca62.5Mg17.5Zn20−xAgx (x = 0, 1, 3, 5, 7, 9) system was dependent on Ag content. It was found that in the system the in-situ formed crystalline phases enhanced the second-stage crystallization, whereas there seemed no contribution to the third-stage crystallization.

As the electromagnetic stirring is to the grain refinement of conventional crystal materials, we consider that the forceful stirring caused by the induction eddy current facilitates the glass formation here. To confirm the supposition, the comparison experiments designed are as follows.We selected Ca60Mg20Zn20 as the master alloy which has the glass forming critical diameter up to 4 mm. Graphite sleeve was placed between the alumina tube and induction coil to shield the induction magnetic field and obstruct electromagnetic stirring. In this case, the melt would be heated by thermal radiation of graphite. The unidirectional quenching was performed without (sample a) and with graphite sleeve (sample b), respectively. The figure gives XRD patterns of the two samples with diameter of 7 mm. One single broad diffraction peak indicated the amorphous nature of sample (a) prepared with electromagnetic stirring. But XRD pattern of sample (b) prepared without electromagnetic stirring shows numerous sharp peaks correspond to crystalline phases. The inset of figure displays the top and cross-section of the two samples. Sample (a) has a swirl-shaped top and shiny cross-section which is typical of BMGs. But the top of sample (b) is irregular and the cross-section is intensively dim.Figure optionsDownload as PowerPoint slideResearch highlights
► Ca–Mg–Zn–(Ag) BMGs were fabricated with diameters of 7 mm by unidirectional quenching.
► The electromagnetic stirring facilitated the glass formation.
► GFA of Ca62.5Mg17.5Zn20−xAgx (x = 0, 1, 3, 5, 7, 9) was dependent on Ag content.
► The in-situ formed crystalline phases enhanced the second-stage crystallization.