Evolution Of Shear Band, Free Volume And Hardness Of As Rolled Zr-based Bulk Metallic Glasses

Bulk metallic glasses (BMGs) have many attractive properties including high strength, large elastic limit, excellent corrosion resistance. The Achilles heel of these materials, however, is the lack of tensile ductility at room temperature, making them prone to catastrophic failure along a single shear band in tensile loading conditions and limiting their application as structural materials. At temperatures far below their glass transition points plastic deformation of metallic glasses occurs via the formation and evolution of nanometer-sized shear bands. Therefore, mechanical performance of metallic glasses at room temperature is closely correlated with shear band evolution. In general, the more shear bands metallic glasses generate upon deformation, the larger plasticity they exhibit. Although monolithic BMGs fail in an apparently brittle manner with a very limited number of shear bands in tension, they usually exhibit significant plastic deformation in constrained loading conditions including indentation, bending, rolling, drawing and uniaxial compression with low specimen aspect ratio. Thus, it is much convenient for us to investigate the evolution of shear bands, such as shear band spacing, angle, offset, and their distribution, as a function of plastic strain under mechanically constrained conditions. It may provide some insight to the deformation mechanisms of metallic glasses, which remain unclear. Meantime, the variation of free volume as a result of cold-working in metallic glasses is still under debate, and further investigation in detail is needed. Besides, it still does not reach a consensus on the issue whether bulk metallic glasses are strain-softening or strain-hardening. Consequently, the effect of plastic deformation, which does not cause crystallization, on the hardness in monolithic BMGs over a wide range of strain is still mysterious.In the present work, we carried out a series of rolling experiments at room temperature on the Zr64.1.3Cu15.75Ni10.12Al0 and Zr46.5Al7Ti1.5 BMGs over a particularly wide range of strain, especially focusing on the initial stage of plastic deformation with small strain interval. The formation and evolution of shear band and free volume were systematically investigated. The effect of rolling on the hardness of the deformed BMGs is discussed. It is found that primary shear bands reach saturation after 7% in thickness reduction and only primary shear bands exist below 20% in thickness reduction based on statistical analyses on the primary shear band spacing, angle and offset. Meantime, rolling creates more free volume. Deformation-induced residual stress distribution in the heavily-rolled specimen relative to the as-cast specimen has been determined. Such residual stresses strongly influence the hardness measured. It is also revealed that after stress-relief, the hardness does not decrease considerably in the heavily-rolled/annealed specimens here as compared to as-cast/annealed specimens probably due to the low shear band density, further confirming that the hardness of as-rolled specimen is closely correlated to residual stress.