Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 140969

140969 - Grain Interface Functional Design to Create Damage Resistance in Polycrystalline Tantalum 

Our NSF-DMREF project seeks to fundamentally change how we approach the design and manufacture of materials by controlling both defect/feature character and internal stress state to achieve a 30% reduction in accumulated damage. For this research, we aim to provide methods, theories, and computational models for predicting the nucleation and propagation of damage from internal material interfaces/boundaries when materials are deformed. Achieving these goals will involve experimentally validated predictions that provide the physical basis for designing and processing materials and experimental tests that respect the statistical nature of ductile damage and failure.

We focus on quantitatively analyzing and understanding the damage behavior of refractory BCC materials across grain boundaries under certain deformation conditions. Tantalum is the choice BCC metal for this experiment because of its excellent mechanical, chemical, refractory, and thermal properties. Also, high-purity Tantalum does not display deformation twinning or structural phase transformation within moderate pressure loading regimes, which makes the deformation mechanism easier for the physical interpretation of experimental results. To quantitively understand the polycrystalline deformation of Tantalum, our approach is to understand the deformation of all the individual grains present in the polycrystal to establish a reference parameter for studying the near grain boundary regions. This requires obtaining large enough grains to perform nanoindentation experiments without grain boundary interferences.

To achieve this, our first task was to create stress-free grains and large near-grain boundary regions in the sample by annealing in our polycrystalline sample. Therefore, our samples were annealed at 0.6 Tm for 24 hours, followed by electron backscattered diffraction (EBSD) spectroscopy to obtain sufficient grain orientation data. Using the EBSD data, a total of fifteen grain orientations representative of all the crystal orientations present in polycrystalline Tantalum were selected for our experimental analysis. However, since it is challenging to obtain grain orientations representative of the three corners of the stereographic triangles (100, 110, and 111), single crystal tantalum samples were utilized.

Our spherical nanoindentation experiments on the selected grain orientations demonstrate our capabilities to obtain indentation stress-strain curves from the load-displacement data of each grain orientation. The stress-strain curves depict the anisotropic elastic-plastic mechanical behavior of each grain orientation in tantalum. Therefore, we obtained surface maps describing the elastic-plastic anisotropic behavior in Ta. Also, in this experiment, we observed the occurrence of plastic pop-ins leading to the occurrence of dual indentation strain hardening behavior. Using a series of examples, we demonstrate the capabilities of our data analysis procedures in characterizing the local indentation yield strengths in individual grains of deformed polycrystalline metallic samples and relating them to increases in the local slip resistances.

Also, since we have quantitative data on the deformation behavior of individual grains, we need to understand the interaction of deformation with grain boundaries in polycrystalline tantalum. The change in deformation behavior of two adjacent grain orientations as a function of strain. Based on the misorientation angles, the grain that accommodates more dislocations near the grain boundaries may act as the dislocation sink and the other as the source. Since grain boundaries serve as precursors for void nucleation, increased strain rate conditions should activate the propensity as potential void nucleation sites.

Our approach investigates the deformation behavior across grain boundaries through interrupted tensile experiments by interrupting the strain on the samples at five points as a function of strain hardening. This approach allows a) correlating the stored energy differences of individual grains to their Taylor factors as a function of imposed cold work, and b) understanding the role of interfaces such as grain boundaries in the deformation of a multi-phase polycrystalline sample. The changes in pop-in stress ratio and strain hardening stress ratio can be indicative of the grain boundary/interface behavior as a function of distance from the interface, thereby providing valuable information regarding the sink/source or barrier/transmission behavior of the interface.

Presenting Author: Olajesu Olanrewaju Iowa State University

Presenting Author Biography: Olajesu Olanrewaju is a second-year Ph.D. student in the Materials Science and Engineering Department at Iowa State University. His research is a collaborative NSF-DMREF project focused on Grain Interface Functional Design to Create Damage Resistance in Polycrystalline Metallic Materials, with a keen interest in studying grain boundary regions in polycrystalline Tantalum using multi-scale deformation techniques under extreme conditions of temperatures and strain rate to contribute valuable insights into understanding the contribution of grain boundaries to damage nucleation.
Olajesu has demonstrated a strong commitment to interdisciplinary collaborations which includes the University of Wisconsin–Madison and Milwaukee, the University of New Hampshire, and the Airforce Research Laboratory. Their work is an iterative loop according to the DMREF framework to create damage-resistant polycrystalline materials through experiments, simulations and modeling, and uncertainty quantification. Olajesu is eager to share his research findings and engage in meaningful contributions.
Outside Research Olajesu is interested in finding inspiration from diverse experiences and perspectives. They are committed to lifelong learning and are eager to explore new opportunities for growth and development.

Authors:

Olajesu Olanrewaju Iowa State University
Kevin Jacob Iowa State University
Curt Bronkhorst University of Wisconsin - Madison
Nan Chen University of Wisconsin-Madison
Marko Knezevic University of New Hampshire
William Musinski University of Wisconsin, Milwaukee
Sid Pathak Iowa State University