Siddhartha Pathak

Studying grain boundary regions in polycrystalline materials using spherical nano-indentation and orientation imaging microscopy

In this article, we report on the application of our spherical nanoindentation data analysis protocols to study the mechanical response of grain boundary regions in as-cast and 30% deformed polycrystalline Fe–3%Si steel. In particular, we demonstrate that it is possible to investigate the role of grain boundaries in the mechanical deformation of polycrystalline samples by systematically studying the changes in the indentation stress–strain curves as a function of the distance from the grain boundary. Such datasets, when combined with the local crystal lattice orientation information obtained using orientation imaging microscopy, open new avenues for characterizing the mechanical behavior of grain boundaries based on their misorientation angle, dislocation density content near the boundary, and their propensity for dislocation source/sink behavior.

Probing nanoscale damage gradients in ion-irradiated metals using spherical nanoindentation

We discuss and demonstrate the application of recently developed spherical nanoindentation stress-strain protocols in characterizing the mechanical behavior of tungsten polycrystalline samples with ion-irradiated surfaces. It is demonstrated that a simple variation of the indenter size (radius) can provide valuable insights into heterogeneous characteristics of the radiation-induced-damage zone. We have also studied the effect of irradiation for the different grain orientations in the same sample.

Strong, Ductile, and Thermally Stable bcc-Mg Nanolaminates

Magnesium has attracted attention worldwide because it is the lightest structural metal. However, a high strength-to-weight ratio remains its only attribute, since an intrinsic lack of strength, ductility and low melting temperature severely restricts practical applications of Mg. Through interface strains, the crystal structure of Mg can be transformed and stabilized from a simple hexagonal (hexagonal close packed hcp) to body center cubic (bcc) crystal structure at ambient pressures. We demonstrate that when introduced into a nanocomposite bcc Mg is far more ductile, 50% stronger, and retains its strength after extended exposure to 200u2009C, which is 0.5 times its homologous temperature. These findings reveal an alternative solution to obtaining lightweight metals critically needed for future energy efficiency and fuel savings.

Quantifying the mechanical effects of He, W and He + W ion irradiation on tungsten with spherical nanoindentation

Recent advances in spherical nanoindentation protocols have proven very useful for capturing the grain-scale mechanical response of different metals. This is achieved by converting the load–displacement response into an effective indentation stress–strain response which reveals latent information such as the elastic–plastic transition or indentation yield strength and work-hardening behavior and subsequently correlating the response with the material structure (e.g., crystal orientation) at the indentation site. Using these protocols, we systematically study and quantify the microscale mechanical effects of He, W, and Hexa0+xa0W ion irradiation on commercially pure, polycrystalline tungsten. The indentation stress–strain response is correlated with the crystal orientation from electron backscatter diffraction, the defect structure from transmission electron microscopy micrographs, and the stopping range of ions in matter calculations of displacement damage and He concentration. He-implanted grains show a much higher indentation yield strength and saturation stress compared to W-ion-irradiated grains for the same displacement damage. There is also good agreement between the dispersed barrier hardening model with a barrier strength of 0.5–0.8 and void models (Bacon–Kochs–Scattergood and Osetsky–Bacon models) with the experimentally observed changes in indentation strength due to the presence of He bubbles. This finding indicates that a high density (~xa09xa0×xa01023xa0m−3) and concentration (~xa01.5 at.%) of small (~xa01xa0nm diameter) He bubbles can be moderate to strong barriers to dislocation slip in tungsten.

Spherical Nanoindentation Stress-Strain Analysis of Ion-Irradiated Tungsten

This paper discusses applications of spherical nanoindentation stress-strain curves in characterizing the local mechanical behavior of materials with modified surfaces. Using ion-irradiated tungsten as a specific example, this paper demonstrates that a simple variation of the indenter size (radius) can identify the depth of the radiation-induced-damage zone, as well as quantify the behavior of the damaged zone itself. Using corresponding local structure information from electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) we look at (a) the elastic response, elasto-plastic transition, and onset of plasticity in ion-irradiated tungsten, zirconium and 304 stainless steel under indentation, and compare their relative mechanical behavior to the unirradiated state, (b) correlating these changes to the different grain orientations as a function of (c) irradiation from different sources (such as He, W, and He+W for tungsten samples).