Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99588

99588 - Grain Interface Functional Design to Create Damage Resistance in Polycrystalline Metallic Materials 

In this poster we will present initial results from our recently started 2022 NSF DMREF project. Our NSF DMREF project seeks to fundamentally change how we approach the design and manufacture of materials by control of both defect/feature character and internal stress state to achieve a 30% reduction in accumulated damage. It is not possible today to predict when, where, or how metallic materials will fail during dynamic loading conditions of interest to the Air Force. We know only qualitatively that material defects or structural features are attributed to damage nucleation. However, the stress conditions necessary to nucleate damage at specific defect or feature sites remains largely unquantified. For example, the conditions for pore nucleation at inclusions, phase or grain boundaries may be very different. Moreover, polycrystalline metals are aggregates of anisotropic grains which interact strongly during deformation to create very heterogeneous internal stress fields. This local stress drives damage nucleation at defects or structural features, not the mean stress which is measured experimentally and used in nearly all macro-scale damage models.

In this work, we focus on one defect/feature type – grain boundaries – and quantifying the conditions for pore nucleation under extreme loading conditions (strain rates up to 10^4/s). We utilize tantalum as our model BCC refractory material, which has been demonstrated to predominantly nucleate and grow damage from grain and twin-grain boundary junctions. This material also displays important non-Schmid asymmetry in the motion of screw dislocations, similar to other refractory metals for extreme environment applications. Achieving the proposal objective will involve new and innovative modeling, statistical analysis, uncertainty reduction, and computational simulations, which would be validated and verified through material processing including accumulated roll bonding, characterization, and demonstration at multiple length scales, along with continuous iterative feedback between these tasks.

To achieve our research objectives, our initial efforts have focused on nanoindentation experiments conducted on single crystal tantalum to develop the proper surface preparation procedures and produce data to compare against simulations of indentation using our local and strain-gradient single crystal models for Ta. In this poster we discuss the capabilities of spherical nanoindentation stress-strain curves, extracted from the measured load-displacement dataset, in characterizing the local mechanical behavior within individual grains of Ta and near grain boundaries of polycrystalline Ta samples. Since nanoindentation length scales are smaller than the typical grain sizes in polycrystalline samples, this technique is an ideal tool for detailed characterization of the microscale heterogeneities present in these materials and their evolution during various metal shaping/working operations. Using a series of examples, we demonstrate the capabilities of our data analyses procedures in a) characterizing the local indentation yield strengths in individual grains of deformed polycrystalline metallic samples and relating them to increases in the local slip resistances, b) correlating the stored energy differences of individual grains to their Taylor factors as a function of imposed cold work, and c) understanding the role of interfaces such as grain boundaries in the deformation of a multi-phase polycrystalline sample.

We also discuss our first series of rolling experiments on Ta conducted at Ames National Laboratory which were used to produce processed material samples for metallographic examination and compare against polycrystal simulations of rolling which are underway. Rolling experiments were performed in four rolling passes with sample material collected after each pass with the final pass achieving -1.57 strain.

Presenting Author: Manish Kumar Iowa State University Ames

Presenting Author Biography: Mr. Manish Kumar is a Ph.D. student in Materials Science and Engineering at Iowa State University Ames.

Authors:

Manish Kumar Iowa State University Ames
Curt A. Bronkhorst University of Wisconsin - Madison
Nan Chen University of Wisconsin-Madison
Marko Knezevic University of New Hampshire
William D. Musinsky Air Force Research Laboratory
Manny Gonzales Air Force Research Laboratory
Sid Pathak Iowa State University Ames