Locally fluctuating cooling rate as possible reason for non-crystalline plasticity in metallic glasses

Abstract

The preparation process of a CuZrAl metallic glass is simulated by molecular dynamics. Different temperatures of the initial liquid state and variation of the cooling rate over five decades are considered. Elastic moduli, mass density and frequency of icosahedral clusters follow a power-law scaling with the cooling rate. The ratio of shear to bulk modulus is most sensitive to changes of the cooling rate. Assuming local fluctuations of the cooling rate occurring during the preparation process, regions characterized by comparably low values of shear modulus, mass density and frequency of icosahedral clusters can be proposed as atomistic realizations of flow defects, at which non-crystalline plastic deformation is initiated.

Martensitic Transformation and Plastic Deformation of TiCuNiZr-Based Bulk Metallic Glass Composites

“…T g (K) In order to further confirm the crystalline volume fraction and to illustrate both the distribution and morphology of the B2 crystals in the glassy matrix, OM and SEM measurements were performed on the rods (Figures 2 and 3), respectively. Based on the areas of the crystals in the glassy matrix, the crystalline volume fraction (f c2 in Table 1) showed a similar tendency as the f c1 values determined from calorimetry but exhibited small differences, which were most likely due to the slightly different cooling rates from the top to the bottom of the copper mold-cast rods [40,41]. As shown in Figure 2, the quasi-spherical crystals percolated with each other and only some crystals were isolated.…”

“…In our case, the critical f c for the microstructural transition in the TiCu-based BMG composites is of the same order. As is well-known, it becomes easier for the amorphous phase to form at the surface of rods than in its center due to the gradual decrease of the cooling rate from the surface to the center [40,41]. Hence, when the applied cooling rate is a little lower than the critical cooling rate for full glass formation, crystals preferentially precipitate in the center of the sample while the amorphous phase ( Figure 2) appears mostly at the surface of rods.…”

Evidence for cooling-rate-dependent icosahedral short-range order in a Cu–Zr–Al metallic glass

“…In the present work, we consider a Cu-Zr-Al bulk metallic glass. From molecular dynamics simulations it is known that the fraction of icosahedral clusters, ' ic , increases with decreasing cooling rate R (Cheng et al, 2009;Hao et al, 2010;Hermann et al, 2012). XRD studies (Xu et al, 2010) on the formation of the icosahedral phase in Cu-Zr-Al alloys and its dependence on R support these theoretical results.…”

“…Reference models for the interpretation of the XRD patterns were generated by molecular dynamics simulations as described by Kokotin (2010) and Hermann et al (2012). A model box consists of 5000 atoms and is subjected to periodic boundary conditions.…”

“…Since 1952, the icosahedron has been considered as a fundamental structural motif of the atomic order in liquid metals and in glassy metallic alloys (Frank, 1952;Bernal, 1960;Finney, 1977;Spaepen, 2000). Recent molecular dynamics studies on the structure of bulk metallic glasses have unambiguously shown by means of Voronoi analyses that icosahedral clusters occur in computer-simulated models (Sun & Shen, 2009;Cheng et al, 2009;Fang et al, 2009;Hao et al, 2010;Wang & Wong, 2012;Hermann et al, 2012;Zhang et al, 2012;Sha et al, 2012). In contrast, experimental verification of the existence of icosahedral clusters is difficult and ambiguous.…”