arXiv subjects

Ujjval Bansal

Publications and source records attributed to Ujjval Bansal.

4 recordsLinked to original sources

arXiv2026

Influence of dislocation density on the tribological response in oxides: case study on SrTiO3

Most ceramics suffer from brittle surface damage and cracking when small particles slide across the surface under low load. Microscratching tests are a useful technique to mimic this loading scenario while retaining the material's deformation history; however, the impact of pre-seeded dislocations, which can significantly facilitate plastic deformation, has not been explored in ceramics. Here, the influence of mechanically seeded dislocations on the microscratching response of oxides is investigated using a model perovskite, SrTiO3. First, various dislocation densities over four orders of magnitude are introduced via room-temperature cyclic Brinell indenter scratching. Subsequently, load-ramped microscratching tests are performed within the pristine and dislocation-seeded regions using a nominally 30 μm spherical diamond tip. On the reference pristine surface, we observe a clear transition from elastic to elasto-plastic deformation, followed by median/radial cracking at higher loads. In contrast, pre-seeded dislocations, accompanied by residual compressive stresses, suppress elastic deformation and lead to median/radial crack shortening and subsequent transition to a partial cone crack. The subsurface cracks are characterized by 3D Nano-CT. The changes in crack geometry, with and without dislocations, were described using the Lawn-Evans-Marshall and Lawn-Wiederhorn-Roberts models. These findings provide direct evidence of dislocation-regulated near-surface damage tolerance, with general applicability to other plastically deformable oxides.

cond-mat.mtrl-sci↗

arXiv2026

Experimental and computational diffusion analysis in Ni-X binary and Ni-Al-X (X = Cr, Mo, Ta, W, Re) ternary systems

An extensive diffusion analysis is presented for binary Ni-X and ternary Ni-Al-X (X = Cr, Mo, Ta, W, Re) systems, which play a crucial role in microstructural evolution and phase stability in Ni-Al-based superalloys. Specifically, we highlight changes in the diffusion coefficients of X in the presence of Al and compare diffusional interactions across systems considered. First-principles calculations, combined with activation energies derived from temperature-dependent experiments, reveal consistent trends in Ni-X systems, with variations in activation energies largely attributed to differences in migration energies. In ternary systems, diffusion coefficients estimated from intersecting diffusion profiles show that the main interdiffusion coefficient of X is comparable to its binary counterpart, with similar activation energies. However, cross-diffusion coefficients are shown to significantly influence fluxes, either enhancing or reducing diffusion lengths depending on the relative directions of diffusing elements. For Ni-Al-Re, a single-profile method is employed to overcome uncertainties in estimating composition gradients at the near-end-member intersecting composition. The diffusion coefficients obtained correlate well with the nature of diffusion paths when represented on Gibbs triangles. To extend these findings, a physics-informed neural network (PINN) optimization method is applied to extract composition-dependent diffusion coefficients across the full composition range. The analysis demonstrates the necessity of incorporating experimentally estimated diffusion coefficients as equality constraints, without which optimization reliability is compromised. Overall, the results establish a robust framework for diffusion studies in Ni-Al-X systems, highlighting the critical role of cross-diffusion effects and constraint-enhanced numerical methods.

cond-mat.mtrl-sci↗

arXiv2024

Columnar grain boundaries are the weakest link in hard coatings: Insights from micro-cantilever testing with bridge notches

The effect of columnar grain boundaries on the fracture toughness was investigated using micro-cantilever fracture testing with a bridge notch, and a unique hard coating consisting of two distinct microstructures: one with columnar grains and another with an epitaxial layer. The bridge-failure sequence qualitatively demonstrated the lower fracture toughness at the columnar-grained structure. Quantitatively, the load drops measured at bridge-failure also revealed a significant decrease in fracture toughness due to grain boundaries. Specifically, the fracture toughness decreased by around 30%, from 4.1 +/- 0.4 MPa m1/2 for epitaxial microstructure to 3.0 +/- 0.3 MPa m1/2 for columnar-grained structure. The fracture toughness of columnar-grained structure is 3.0 +/- 0.2 MPa m1/2 perpendicular to the growth direction higher than 2.7 +/- 0.1 MPa m1/2 along it. These findings suggest that future optimization of hard coatings should focus on grain boundary toughening, and the present toolbox proposes suitable techniques for such microstructure optimization.

physics.app-ph↗

arXiv2024

Data-efficient 4D-STEM in SEM: Beyond 2D Materials to Metallic Materials

Four-dimensional scanning transmission electron microscopy (4D-STEM) is a powerful tool that allows for the simultaneous acquisition of spatial and diffraction information, driven by recent advancements in direct electron detector technology. Although 4D-STEM has been predominantly developed for and used in conventional TEM and STEM, efforts are being made to implement the technique in scanning electron microscopy (SEM). In this paper, we push the boundaries of 4D-STEM in SEM and extend its capabilities in three key aspects: (1) faster acquisition rate with reduced data size, (2) higher angular resolution, and (3) application to various materials including conventional alloys and focused ion beam (FIB) lamella. Specifically, operating the MiniPIX Timepix3 detector in the event-driven mode significantly improves the acquisition rate by a factor of a few tenths compared to conventional frame-based mode, thereby opening up possibilities for integrating 4D-STEM into various in situ SEM testing. Furthermore, with a novel stage-detector geometry, a camera length of 160 mm is achieved which improves the angular resolution amplifying its utility, for example, magnetic or electric field imaging. Lastly, we successfully imaged a nanostructured platinum-copper thin film with a grain size of 16 nm and a thickness of 20 nm, and identified annealing twins in FIB-prepared polycrystalline copper using virtual darkfield imaging and orientation mapping. This work demonstrates the potential of synergetic combination of 4D-STEM with in situ experiments, and broadening its applications across a wide range of materials.

cond-mat.mtrl-sci↗