Using broadband nanoindentation creep (BNC), we measure room temperature hardness-strain rate properties of Zr50Cu45Al5 metallic glass ribbons and ingots. We estimate the shear stress and strain rate inside the shear bands. We observe (and propose) the following:
1. Aging the glass near Tg increases flow stress but has a secondary effect on the slope of the stress-strain rate curve. This is indication that a back stress exists, which contributes to the measured hardness. We propose that the magnitude of this flow stress is related to the stress required to initiate new shear bands. The result also suggests that deformation kinetics inside the shear band are independent of prior thermal history (consistent with loss of memory of the aged structure because of the large strains encountered inside shear bands).
2. Individual stress-strain rate curves generated from BNC depend on deformation history, particularly initial rate of loading (the same thing is observed in amorphous polymers). We propose that structure inside the shear band varies depending on strain rate.
3. The high strain rate portions of shear stress-strain rates data are dominated by transients, which (we propose) are caused by partial relaxation of the internal stresses related to shear band initiation.
4. At strain strain rates below about 10-3/s there is a downward inflection in flow stress. This feature suggests the onset of a new mechanism at low strain rates.
5. We observe a number of subtle size effects including one in the amount of creep during hold at constant hold. Otherwise, there is not a measureable size effect in either the hardness or the modulus for loads between between 0.1 and 10 mN.
Lastly, the stress-strain rate data generated from BNC, while broadly consistent with data obtained from other experimental methods, seem inconsistent with molecular dynamics simulations extrapolated to laboratory strain rates. We speculate on the origins of the discrepancy.
This work was supported by NSF Grant NSF CMMI-1232731. M.J. Kramer, I. Kalay, and Y.E. Kalay of Ames Laboratory kindly provided the materials.