Compared to bulk metallic glasses, the free- volume content is higher in sputtered metallic glasses due to the high cooling rate of the magnetron sputtering fabrication technique. This excess free volume accommodates larger amounts of local shear during deformation and improves the material’s ductility. However, not much research has been done on understanding the impact of room temperature aging on the size and distribution of free volume in sputtered metallic glass.
In our research, we sputtered five µm- thick Zr-Ni-Al onto a Si substrate and used focus ion-beam (FIB) milling to fabricate micro- and nano- sized pillars. The diameter of the pillars ranged from 300 nm to 1.3 µm with heights ranging from 900 nm to 3.9 µm, respectively. Uniaxial pillar compressions were performed on the as- sputtered Zr-Ni-Al. The elastic modulus and yield strength were measured to be 75.5 ± 12 GPa and 2.49 ± 0.24 GPa, respectively. The as-sputtered Zr-Ni-Al was aged at room temperature in a nitrogen dry box for three years and new pillars were fabricated using FIB milling and then, tested under uniaxial compression. After aging, the elastic modulus and yield strength increased to 82.5 ± 4.1 GPa and 2.73 ± 0.09 GPa, respectively. To provide insight into this increase in mechanical property, we performed dynamic mechanical analysis (DMA) to determine the storage and loss modulus of the as-sputtered and aged Zr-Ni-Al.
For the DMA experiments, new pillars were fabricated on both the as- sputtered and aged Zr-Ni-Al. These pillars were tested at room temperature at frequencies between 3 and 700 Hz. The peak of the loss modulus corresponds to the activation of β relaxations in the material. For the as- sputtered and aged Zr-Ni-Al, this occurred at a frequency of 440Hz and 450 Hz, respectively. The higher frequency of the aged metallic glass indicates that the activation energy of the β relaxations is larger. As expected, room temperature aging redistributed the free volume to a more energetically favorable configuration. Thus, even higher energy is required to relax the aged metallic glass. In addition, the magnitude of the loss modulus increased as a function of pillar diameter. The loss modulus ranged from 20 GPa to 100 GPa for pillar diameters ranging between 300 nm to 1.3 µm, respectively. The total free volume in large pillars is greater than that in small pillars, thus resulting in increased loss modulus. The cooperative shear model (CSM) demonstrated that β relaxations are associated with the transition of shear transformation zones (STZs) in an elastic matrix. Utilizing CSM, the volume and activation energy of the STZs can be calculated. These findings elucidate the effect of room temperature aging on sputtered metallic glass and pave the road for future work on quantifying the activation energy and volume of STZs, the rearrangement of which is the mechanism of deformation in metallic glass.