Role of Free Volume in Strain Softening of As-cast and Annealed Bulk Metallic Glass

Role of Free Volume in Strain Softening of As-cast and Annealed Bulk Metallic Glass
유병길박경원이재철U. Ramamurty장재일
Issue Date
Journal of materials research
VOL 24, NO 4, 1405-1416
Plasticity in amorphous alloys is associated with strain softening, induced by the creation of additional free volume during deformation. In this paper, the role of free volume, which was a priori in the material, on work softening was investigated. For this, an as-cast Zr-based bulk metallic glass (BMG) was systematically annealed below its glass transition temperature, so as to reduce the free volume content. The bonded-interface indentation technique is used to generate extensively deformed and well defined plastic zones. Nanoindentation was utilized to estimate the hardness of the deformed as well as undeformed regions. The results show that the structural relaxation annealing enhances the hardness and that both the subsurface shear band number density and the plastic zone size decrease with annealing time. The serrations in the nanoindentation loaddisplacement curves become smoother with structural relaxation. Regardless of the annealing condition, the nanohardness of the deformed regions is ~12–15% lower, implying that the prior free volume only changes the yield stress (or hardness) but not the relative flow stress (or the extent of strain softening). Statistical distributions of the nanohardness obtained from deformed and undeformed regions have no overlap, suggesting that shear band number density has no influence on the plastic characteristics of the deformed region.
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