Effect of residual nanocrystals on thermal stability and mechanical properties of metalloid-containing amorphous alloys

Abstract

Herein we present the amorphization, thermal stability, crystallization behavior, and mechanical properties of (Ni60Nb40)100-x-ySixBy (x = 0, 5 and 10 at.%, y = 0, 5 at.%) alloys with six distinct compositions. We investigated the influence of the nanocrystals formed by the addition of Si and B on the thermal stability and mechanical properties of the (Ni60Nb40)100-x-ySixBy (x = 0, 5 and 10 at.%, y = 0, 5 at.%) alloys. The phase and thermal stabilities of the alloys were studied using X-ray diffraction, high-resolution transmission electron microscopy, and differential scanning calorimetry and the mechanical properties were characterized by nanoindentation testing. It was found that single-step crystallization occurred in B-free alloys, whereas double step crystallization occurred in B-containing alloys. B-free (Ni60Nb40)90Si10 alloy exhibited the highest onset temperature of 714 °C, and B-containing (Ni60Nb40)95B5 alloy shows the lowest onset temperature at 642 °C. The microstructure of B-containing amorphous alloys consists of nano-sized metallic crystals with a size of less than 5 nm embedded in an amorphous phase. The existence of residual nanocrystals facilitates heterogeneous nucleation during crystallization of the amorphous phase, weakening the thermal stability of amorphous alloys. Crystallization of the amorphous phase remarkably enhances the mechanical properties of (Ni60Nb40)100-x-ySixBy (x = 0, 5 and 10 at.%, y = 0, 5 at.%) alloys, with a 40–60% increase in hardness and 19–88% increase in elastic modulus. An increase in mechanical properties after crystallization is lower in B-containing alloys because of the lower nano-intermetallic phases formation after crystallization, due to the presence of the residual nanocrystals.