Nucleation instability in supercooled Cu–Zr–Al glass-forming liquids

Abstract

Special role in computer simulations of supercooled liquid and glasses is played by few general models representing certain classes of real glass-forming systems. Recently, it was shown that one of the most widely used model glassformers -Kob-Andersen binary Lennard-Jones mixture -crystalizes in quite lengthy molecular dynamics simulations and, moreover, it is in fact a very poor glassformer at large system sizes. Thus, our understanding of crystallization stability of model glassformers is far from complete due to the fact that relatively small system sizes and short timescales have been considered so far. Here we address this issue for two embedded atom models intensively used last years in numerical studies of Cu-Zr-(Al) bulk metallic glasses. We consider Cu64.5Zr35.5 and Cu46Zr46Al8 alloys as those having high glass-forming ability. Exploring their structural evolution at continuous cooling and isothermal annealing, we observe that both systems nucleate in sufficiently lengthy simulations, though Cu46Zr46Al8 demonstrate order of magnitude higher critical nucleation time. Moreover, Cu64.5Zr35.5 is actually unstable to crystallization for large system sizes (N > 20, 000). Both systems crystallize with the formation of tetrahedrally close packed Laves phases of different types. We reveal that structure of both systems in liquid and glassy state contains comparable amount of polytetrahedral clusters. We argue that nucleation instability of simulated Cu64.5Zr35.5 alloy is due to the fact that its composition is very close to that for stable Cu2Zr compound with C15 Laves phase structure. arXiv:1809.00198v1 [cond-mat.mtrl-sci] 1 Sep 2018

Microstructure evolution during near- Tg annealing and its effect on shear banding in model alloys

“…In the present work, we reexamined near-T g annealing in much larger samples (containing more than 3 × 10 5 atoms) of the typical glass-forming systems Al 90 Sm 10 and Cu 64.5 Zr 35.5 , for the purpose of studying subsequent deformation behaviors under shear loading. During the elongated annealing, Al 90 Sm 10 maintains the glassy structure, while the Cu 64.5 Zr 35.5 alloy develops nanocrystalline Laves phases, which is consistent with several previous studies [19,20]. Meanwhile, Cu 64.5 Zr 35.5 displays higher shear strength but lower ductility than Al 90 Sm 10 .…”

Crystallization Instability in Glass-Forming Mixtures

“…We thus infer that the mechanism for nucleation, at least for these compositions, is the same as that for crystallization in KA. Again, like KA, other mechanisms are also possible in which the crystal may be mixed [41,42]. However, we argue that we have presented a general crystallization mechanism in mixtures, which occurs in the absence of faster, specific, crystallization pathways.…”

“…Noting that small size disparities will permit rapid crystallization, and that for certain size ratios binary crystals form an additional route to crystallization as noted in Sec. VII [14,42], we expect that increasing the size ratio will inhibit crystallization. We leave the prospect of a detailed analysis of the role of size disparity for the future.…”

High-pressure annealing driven nanocrystal formation in Zr50Cu40Al10 metallic glass and strength increase

“…Based on the EDS analyses, Cu 10 Zr 7 (58.8 at.% Cu and 41.2 at.% Zr) 18 , Cu 2 Zr Laves phase (66.7 at.% Cu and 33.3 at.% Zr) 19 , and AlCu 2 Zr (25 at.% Al, 50 at.% Cu, and 25 at.% Zr) 20 may be good candidates for the Cu-rich nanocrystal (~33 at.% Zr, 59 at.% Cu, and 8 at.% Al) observed in this study in terms of composition, although the exact crystal phase of the Cu-rich nanocrystal in this study is not clear. However, it is important to note that the primary phase of crystallization in Zr-Cu-Al MGs has been known to be a Zr-rich phase, such as Zr 2 Cu, ZrCu, and the τ 3 phase [20][21][22][23] , rather than Cu-rich phases.…”