Effect of phase separation induced by Nb microalloying on the glass formation and crystallization of CuZr-based bulk metallic glasses

The effects of Nb on the glass-forming ability (GFA) and thermal stability of CuZr-based bulk metallic glasses (BMGs) remain poorly understood, particularly given the potential for phase separation in the supercooled liquid due to the positive enthalpy of mixing between Nb and both Cu and Zr. In this work, we systematically investigate the critical casting diameter, crystalline products upon heating and cooling, thermal properties, and structural and chemical evolutions at the primary crystallization stage of Nb-containing Cu–Zr–Al alloys, using a combination of experimental techniques, including X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, atom probe tomography, and synchrotron-based small-angle X-ray scattering. Our results reveal that an appropriate Nb addition (≤1 at.%) does not significantly compromise thermal stability or alter crystalline phases and can even enhance GFA. However, at higher Nb contents (≥3 at.%), the alloy composition enters an Nb-induced miscibility gap. This triggers phase separation, leading to the deterioration of both GFA and thermal stability. A phase separation-mediated crystallization scenario is proposed to explain the altered crystallization pathway, reduced activation energy for crystallization, and the consequent decline in GFA and thermal stability. These findings provide new insights for developing BMGs or BMG composites by strategically inducing phase separation in the supercooled liquid.