Effect of grain size on iron-boride nanoglasses

Abstract

Metallic nanoglasses are made of amorphous grains that are separated by lower-density amorphous boundaries, which have been proposed to enhance plasticity through the deflection of cracks and shear bands at interfaces. It has been difficult to experimentally control grain size and interfacial structure to understand their roles in plastic deformation. Here, we fabricate bulk nanoglasses via compaction and sintering of colloidally synthesized amorphous iron-boride nanoparticles. These nanoglasses have amorphous grains with diameters from 116 nm to 576 nm and were tested using nanoindentation and micropillar compressions. The nanoglass with a grain size of 576 nm shows the highest elastic modulus and hardness of 101 GPa and 7.4 GPa, respectively. Transmission electron microscopy reveals that nanocrystals form within the nanoglasses during compaction. Higher nanocrystal density correlates with higher nanoparticle crystallization enthalpy, an increase in plasticity, and a decrease in yield strength. Plastic strain of 5.0%, yield strength of 3.8 GPa, and ultimate compressive strength of 2.7 - 3.8 GPa were achieved. We show that the compaction of colloidal metallic glass nanoparticles results in robust bulk samples, with mechanical properties similar to that of other iron-based bulk metallic glasses.