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The work suggests that formation of domains with significantly different orientations is due to particles detaching and subsequently reattaching as the microstructure coarsens. The solid volume fraction of the coarsened NPG sample is ~33%, near the limit of bicontinuity in which bicontinuous structures detach into independent particles as they coarsen. A model for coarsening in bicontinuous structures that coarsen by surface diffusion (the dominant coarsening mechanism in nanoporous metals) is constructed through phase-field simulations of a CG structure and used to predict particle detachment rates. A study of the interfacial mean curvature of the CG and NPG structures are compared to provide further evidence of particle detachment. A coarsened NPG structure is used as the initial condition in a phase-field simulation. The simulation shows that particle detachment occurs during coarsening. Using these simulations, a model is developed to predict that up to 5% of the NPG sample may have detached as it was coarsened from its dealloyed state.

This is further confirmed through EBSD measurements. Figure 85 shows a 3D reconstruction of 2D inverse pole figure measurements from a coarsened NPG sample (colour relates to local orientation). The reconstruction demonstrates the presence of a particle-like domain, shown in green, with an orientation far-off the average orientation of its environment (misorientation of around 35°).

PRINCIPAL PUBLICATION AND AUTHORS

Grain boundary formation through particle detachment during coarsening of nanoporous metals, K.L.M. Elder (a,b), W.B. Andrews (c), M. Ziehmer (d), N. Mameka (d), C. Kirchlechner (e), A. Davydok (d), J.S. Micha (f,g), A.F. Chadwick (a), E.T. Lilleodden (h), K. Thornton (c), P.W. Voorhees (a), Proc. Natl. Acad. Sci. USA. 118, 30 (2021); https:/doi.org/10.1073/pnas.2104132118 (a) Northwestern University, Evanston (USA) (b) Lawrence Livermore National Laboratory, California (USA) (c) University of Michigan, Ann Arbor (USA) (d) Helmholtz-Zentrum Hereon, Geesthacht (Germany) (e) Karlsruhe Institute of Technology, Karlsruhe (Germany) (f) Université Grenoble Alpes, Saint-Martin-d Hères (France) (g) ESRF (h) Fraunhofer Institute for Microstructure of Materials and Systems IMWS, Halle (Saale) (Germany)

REFERENCES

[1] I. McCue et al., Annu. Rev. Mater. Res. 46, 263-286 (2016). [2] O. Ultrich et al., Rev. Sci. Instrum. 82, 033908 (2011). [3] A. Davydok et al., Mater. Des. 108, 68-75 (2016).

Fig. 84: Histogram of the uncorrelated misorientation

angles for (a) the as-dealloyed and (b) the coarsened NPG

sample.

Fig. 85: A 3D reconstruction of 2D (001) inverse pole figure maps from EBSD measurements (inset shows standard EBSD

orientation colour key).

This work provides insights into crystallographic changes that occur during coarsening by finding that an in-grain orientation spread develops during coarsening, appearing as low- and high-angle grain boundaries in the structure. Particle detachment and reattachment are identified as a mechanism that leads to the development of a polycrystalline structure. Thus, particle detachment must be understood to have full control of the nanocrystalline-nanoporous structure and improve material properties.