Role of niobium and oxygen concentration on glass forming ability and crystallization behavior of Zr-Ni-Al-Cu-Nb bulk metallic glasses with low copper concentration

Arc melting and suction casting was used to prepare (Zr57Ni20Al15Cu8)(100 - x)Nb-x bulk metallic glass alloys with Nb concentration ranging from 1.9 to 4.3 at.%. The glass forming ability and crystallization behavior of these alloys were investigated-using X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). By varying the Nb concentration, a high glass forming range between 2 and 4 at.% Nb was observed in XRD and SEM. It was also revealed that oxygen has a detrimental effect on the GFA within this high glass forming range and the severity was closely linked to the state in-which oxygen was present. Since thermal activation is an intrinsic property of crystallization, the activation energy for crystallization was computed from Kissinger, Ozawa and Cheng's method's based on non-isothermal DSC data. A comparison of activation energy models revealed that Kissinger and Ozawa show similar trends while Cheng's method depicted varied results for samples containing pre-existing crystals. For amorphous samples, Kissinger and Ozawa showed the highest value of activation energy (316 kJ/mol and 329 kJ/mol, respectively) at 2.7 at%Nb, which is consistent with the supercooled liquid region and resulting in higher glass forming ability at this composition. (C) 2016 Elsevier B.V. All rights reserved.