Role of Sc on the glass forming ability and mechanical properties of Al-Y-Ni-Sc bulk metallic glass produced with different cooling rates

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 725; pp. 119 - 126
• Main Authors: Chen, Chih-Yuan, Hsu, Wei-Sheng, Chen, Shih-Fan
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 16.05.2018
• Subjects: Cooling rate; Crystallization; DSC; Metallic glasses; Thermal analysis; XRD; Metallic glasses; Thermal analysis; XRD; DSC; Cooling rate; Crystallization
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2018.04.003

In the present study, the role of Sc in the glass forming ability (GFA) and mechanical properties of (Al87Y8Ni5)99Sc1 alloy ribbon produced by single roller melt-spinning with various rotation speeds has been investigated in an attempt to understand the influence of cooling rate in the transition element of the Al-TM-RE (TM: transition metal, RE: rare earth metal) alloy system. The crystallinities of the two alloy ribbons formed at various cooling rates were evaluated by X-ray diffraction (XRD) in order to examine whether the amorphous structure, α-Al particles and precipitation compounds could be formed at any cooling rate. Results revealed α-Al in the Al87Y8Ni5 alloy, but both phases (i.e., α-Al and Al3Sc) were indexed for (Al87Y8Ni5)99Sc1 alloy at a low cooling rate (1000–2000 rpm). Moreover, larger full width at half maximum (FWHM) of characteristic peaks (corresponding to α-Al) were observed for both alloy ribbons, indicating a lower degree of structural relaxation as the cooling rate was increased. Therefore, better short-range ordering of the atomic configuration can be achieved when the alloy ribbons are synthesized at a lower cooling rate, which allows sufficient time for the atoms to reach their local ordered equilibrium positions. On the other hand, the maximum hardness was obtained for both non-scandium and scandium-doped alloys produced at the intermediate cooling rate (2000 rpm) due to the random and homogeneous distribution of numerous tiny precipitates in the amorphous matrix. In addition, compared to Al87Y8Ni5 alloy ribbon, the occurrence of glass transition temperature (Tg), higher crystallization temperature (Tx, Tp1, Tp2), and smaller amount of crystallization fraction in the (Al87Y8Ni5)99Sc1 during continuous heating proves the advantageous effects, namely higher glass forming ability and better thermal stability, of the scandium addition. The mechanism underlying this advantage should be associated with the enhancement of the viscosity of the alloy melt, which results from the increased packing density of the alloy melt when scandium is added. The shear-banded structures that characteristically appear around the indents were observed to be completely absent around indents made at a low cooling rate, revealing that inhomogeneous deformation is not mediated by shear bands below a certain cooling rate. In addition, observation of the fracture surfaces revealed that the fracture surfaces of both alloy ribbons produced at high and low cooling rates transformed from vein patterns to isotropic dimple structures, revealing a change in fracture mode from brittle to ductile as the cooling rate decreased.