Byung-Gil Yoo

Rate-dependent inhomogeneous-to-homogeneous transition of plastic flows during nanoindentation of bulk metallic glasses: Fact or artifact?

There has been considerable controversy over the “apparent” rate-dependent transition from inhomogeneous-to-homogeneous flow during nanoindentation of bulk metallic glasses (BMGs) at room temperature: whether it arises from the existence of homogeneous-flow regime in BMG deformation map or is an artifact due to the instrumental blurring at high rates. To provide a clue to address this dispute, the authors performed nanoindentation experiments on a Zr-based BMG with two geometrically self-similar indenters. The results are discussed in terms of the discrete plasticity ratio, which is a useful parameter in analyzing the contribution of inhomogeneous plasticity to the total plastic deformation.

A study on the evolution of subsurface deformation in a Zr-based bulk metallic glass during spherical indentation

Due to its several advantages, including a simple testing procedure and the use of a small volume of material, the instrumented indentation technique has been widely performed to elucidate the characteristics of plasticity in bulk metallic glasses (BMGs) which typically show very different deformation behaviour from that of crystalline counterparts. As an effort to shed additional light on the topics, here we examined the inhomogeneous plastic deformation of a Zr–Cu–Ni–Al–Ti BMG by performing both macroscopic instrumented indentation (with a spherical indenter) and nanoindentation (with a three-sided pyramidal Berkovich indenter) on the interface-bonded sample as well as on a bulk sample. Especially, we put emphasis on analysing the evolution of shear-band-ruled deformation generated underneath the indenter during spherical indentation. It was revealed that the inter-band spacing and the shear band density are independent of indentation load and thus stress level. Furthermore, subsequent performance of nanoindentation showed that the subsurface region under the indenter was indeed softened and had quite different deformation characteristics from that of the un-deformed region.

BLUNTNESS MEASUREMENT OF A BERKOVICH INDENTER

Indenter blunting is inevitable in nanoindentations and results in unexpected contact properties. Both relaxation of the indentation size effects and deepening of the substrate effects under a blunted indenter cause a change of the bathtub-shaped hardness with indentation depth in a soft film on hard substrate. Thus an identification of three-dimensional morphology of an indenter apex is necessary for precise measurements of hardness in thin films. We observed an actual Berkovich indenter using an atomic force microscope (AFM). Through a quantitative analysis on the AFM image, data pairs of contact area versus contact depth were obtained; curvature radius of the apex was estimated by searching a sphere well-fitted to the indenter apex morphology. The estimated curvature radius and blunted height were 1043.9±50.9 nm and 44.4 nm, respectively. By comparing with the result from the modified Kicks law, both blunted heights were comparable each other within a 7 nm difference. This confirms validity of the direct observation method with the AFM.