Ranjana Saha

Bio: Ranjana Saha is an academic researcher from Stanford University. The author has contributed to research in topics: Nanoindentation & Indentation. The author has an hindex of 7, co-authored 8 publications receiving 2147 citations.

Deformation Mechanisms of the Zr40Ti14Ni10Cu12Be24 Bulk Metallic Glass

Abstract: We have studied the mechanical behavior of Zr 40 Ti 14 Ni 10 Cu 12 Be 24 through uniaxial compression and nanoindentation experiments. Quantitative measurements of the serrated plastic flow observed during uniaxial compression are reported. These data are used to predict temperature increases in single shear bands due to local adiabatic heating caused by the work done on the sample as shear propagates progressively across the sample. Since the predicted temperature increases are insufficient to reach the glass transition temperature, it is unlikely that localized heating is the primary cause of flow localization. Instead, localization of shear is more likely caused by changes in viscosity associated with increased free volume in the shear bands. The orientation of the shear bands in compression tests and an indentation size effect for the onset of plastic flow in nanoindentation both point to increased free volume as the cause of localization.

Indentation of a soft metal film on a hard substrate: Strain gradient hardening effects

Abstract: A new type of nanoindentation experiment showing the effect of a strain gradient on the flow strength of a crystalline material is conducted and analyzed. We show that by indenting a soft metal film (Al) on a hard substrate (glass) with a sharp diamond indenter a strong gradient of plastic strain is created. The true hardness of the film is observed to increase with increasing depth of indentation when the indenter tip approaches the hard substrate, in sharp contrast to the falling hardness with increasing depth in bulk materials. We associate this rise in hardness with the strong gradient of plastic strain created between the indenter and the hard substrate. We use the mechanism-based strain gradient (MSG) plasticity theory to model the observed indentation behavior. The modeling shows that the MSG plasticity theory is capable of describing not only the decreasing hardness with increasing depth of indentation at shallow indentations, as observed in bulk materials, but also the rise in hardness that occurs when the indenter tip approaches the film/substrate interface.

Soft films on hard substrates — nanoindentation of tungsten films on sapphire substrates

Abstract: We have studied the indentation properties of sputter deposited W thin films on single crystal sapphire substrates in an effort to understand the effects of pile-up on the indentation properties of soft films on hard substrates Hardness and elastic modulus of these films were determined by nanoindentation using a Nano XP™ The hardness and elastic modulus were first determined using the Oliver and Pharr method to estimate the contact areas during indentation This method does not account for the effects of pile-up Because the film and substrate have very similar elastic properties, it was possible to determine the true contact areas directly from the measured contact stiffness and from that determine the true properties of the film This method also provides a quantitative estimate of the pile-up height, which compares favorably with direct AFM measurement of the pile-up height

Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology

Abstract: The method we introduced in 1992 for measuring hardness and elastic modulus by instrumented indentation techniques has widely been adopted and used in the characterization of small-scale mechanical behavior. Since its original development, the method has undergone numerous refinements and changes brought about by improvements to testing equipment and techniques as well as from advances in our understanding of the mechanics of elastic–plastic contact. Here, we review our current understanding of the mechanics governing elastic–plastic indentation as they pertain to load and depth-sensing indentation testing of monolithic materials and provide an update of how we now implement the method to make the most accurate mechanical property measurements. The limitations of the method are also discussed.

Super Plastic Bulk Metallic Glasses at Room Temperature

Abstract: In contrast to the poor plasticity that is usually observed in bulk metallic glasses, super plasticity is achieved at room temperature in ZrCuNiAl synthesized through the appropriate choice of its composition by controlling elastic moduli. Microstructures analysis indicates that the super plastic bulk metallic glasses are composed of hard regions surrounded by soft regions, which enable the glasses to undergo true strain of more than 160%. This finding is suggestive of a solution to the problem of brittleness in, and has implications for understanding the deformation mechanism of, metallic glasses.