Intragranular micromechanical fields at triple junctions

Sven E Gustafson, Wolfgang Ludwig, Raquel Rodriguez-Lamas, Can Yildirim, Katherine S Shanks, Carsten Detlefs, and Michael D Sangid

New research provides a 3D investigation using multiple synchrotron x-ray techniques to enable targeted zoom-ins onto six individual grains and spatial linking across length scales, characterizing the intragranular micromechanical fields along all triple junctions within select grains.

HEDM image, larger image and description below.

What is the discovery?

Triple junctions are intersections of three or more grains in polycrystalline solids. They are known locations of potential stress concentration and strain localization, and therefore important for understanding failure in structural metals. In a new paper appearing in Acta Materialia, a team from Purdue University, University of Lyon, the ESRF, and CHEXS present a study of intragranular lattice curvatures and elastic strains measured at triple junctions in an Al-Li binary alloy. These measurements are accomplished via a novel zoom-in style combination of different synchrotron x-ray techniques. The research was overseen by Prof M Sangid, from the School of Aeronautics and Astronautics at Purdue. The lead author is former Purdue graduate student and current CHESS postdoc Dr. Sven Gustafson.  By combining several advanced diffraction microscopy techniques (performed at the FAST beamline at CHEXS, as well as the ID06 and ID11 beamlines at the ESRF), the team was able to draw several new conclusions about the behavior of triple junctions during mechanical loading. Triple junctions were shown not only to act differently than the bulk of the grain, but also separate themselves from grain boundaries, with increased lattice curvature and local misorientation. The new paper offers a previously unexplored 3D view of triple junctions and exposes the spatial complexity of intragranular micromechanical fields surrounding them.

Image description in caption.
Figure 1 | Combined Near Field and Far Field High Energy Diffraction Microscopy (NF-HEDM and FF-HEDM), both performed at the FAST beamline. Morphologies of thousands of crystalline grains are colored with their corresponding stress along the loading direction. Al-Li binary alloy sample was studied in-situ at FAST, while undergoing cyclic loading in the RAMS2 load frame. The four panels show the evolution of grain morphology and stress state during loading. (a) Unloaded – Prior to loading, (b) Peak load (65 MPa) – First cycle, (c) Peak load (55 MPa) – 100th cycle, (d) Unloaded – After loading.

 

Why is this important?

Triple junctions are universal spatial features within all polycrystalline metals, with complex geometric compatibility requirements due to the intersection of multiple crystallographic orientations. As such, they are relevant for understanding failure in all structural metals. Triple junctions are known sites of stress/strain localization; however, a grain’s triple junction network, in comparison to its grain boundaries, as well as the entire length of individual triple junctions, had not previously been thoroughly examined due to the sparseness of high-resolution 3D experimental datasets. This research provides a new 3D investigation using multiple synchrotron x-ray techniques to enable targeted zoom-ins onto six individual grains and spatial linking across length scales, characterizing the intragranular micromechanical fields along all triple junctions within select grains. This characterization allowed for voxels close to triple junctions to be statistically compared to voxels close to grain boundaries, and for the unique role of triple junctions to be precisely quantified. The rapidly increasing precision of x-ray diffraction microscopies is providing researchers with a fundamentally new level of detail to investigate heterogenous strain in structural metals.

Image description in caption.
Figure 2 | After loading at FAST, six specific grains of interest (all featuring triple junctions) were identified for further study using dark field x-ray microscopy (DFXM) at the ESRF.

Why did this research need CHEXS?

This research made use of the world-leading capabilities for combined NF-HEDM and FF-HEDM at the FAST beamline. These techniques are enabled by sophisticated x-ray detectors and unique computational infrastructure, as well as the custom-built RAMS2 system for precision sample rotation under cycling loading. This work also exploited unique microscopy capabilities available at the ESRF. Combining and co-analyzing datasets from both synchrotrons was critical to enable this discovery. The work could not have been completed at either synchrotron alone.

Image of an individual grain of metal.
Figure 3 | Reconstruction of grain 4 from dark field x-ray microscopy data taken at ESRF with color indicating non-local curvature.

How was the work funded?

National Science Foundation, Directorate of Engineering, CMMI-1651956

Sandia National Laboratories (under contract 2189722)

The Center for High Energy X-ray Sciences (CHEXS), NSF MPS/BIO/ENG (DMR-1829070)

Sandia National Laboratories is funded by US Dept of Energy, NNSA (DE-NA0003525)

Reference

Revealing 3D intragranular micromechanical fields at triple junctions

SE Gustafson, W Ludwig, R Rodriguez-Lamas, C Yildirim, KS Shanks, C Detlefs, MD Sangid

Acta Materialia 260, 119300 (2023); https://doi.org/10.1016/j.actamat.2023.119300