Elastic Microstructures in Bulk Metallic Glasses—Are They Real?

In the past decades, numerous experimental reports have revealed signatures of an elastic microstructure in metallic glasses, some of which with length scales much beyond the expected short- or medium-range order inferred from atomistic simulations. In the latter, extended network formation of interconnected clusters has been reported that give insights into potentially larger emerging structural length scales (JALCOM 821, 153209, 2020). Experimentally, a compatible signature of spatially resolved elastic properties can indeed be revealed for metallic glasses (Phys. Rev. Lett. 106, 12, 2011), directly highlighting structural heterogeneities at the nanometer scale. This reflects the strong interplay of topology and modulus, which may further be affected by internal stresses or density fluctuations occurring during the glass formation. When extending the probed sample area, surprisingly, fluctuations of the elastic modulus are revealed that persist on much larger length scales of the order of 100 nm (Adv. Func. Mat. 28, 1800388, 2018), but their origin continues to remain unclear.<br/><br/>In this talk we discuss the possible origins of such large length-scale heterogeneities. Based on our recent work, we are able to exclude chemical fluctuations as for example caused by chemical spinodal decomposition as the origin of this length scale (Materials & Design 229, 111929, 2023). We consequently hypothesized that either geometrical cooling constraints during casting may induce internal stress fluctuations at the revealed length scale, or that extended density fluctuations are the origin of the elastic microstructure. We now test this hypothesis via a detailed analysis of how thermal treatment and thus physical aging affects the spatially resolved elastic fluctuations and by leveraging nano-beam diffraction to interrogate the structural fluctuations across the elastic microstructure. Clear signatures of systematic annealing-induced fluctuation changes across different metallic glass casts substantiate the existence of a hierarchical elastic microstructure in metallic glasses and therefore the notion of length scales much beyond short- and medium range order distances.