Ni₇₇₋ₓ₋ᵧMoₓCrᵧNb₃P₁₄B₆ bulk metallic glasses reported by Ma et al. (2016) combine high corrosion resistance and 2.5–3.4 GPa compressive strength with almost no compressive plastic strain (0–0.3%), even though the empirical elastic-constant ductility criteria for metallic glasses — high Poisson's ratio and low shear-to-bulk modulus ratio — would predict ductile behavior for these Ni-based alloys. The unresolved question is which atomic-scale structural features suppress shear-band-mediated plasticity in these Ni-based metalloid glasses. Later work by the same collaboration produced ductile Ni–Co–Nb–P–B glasses and implicated cluster-level atomic packing, but the brittleness mechanism of the specific Mo–Cr-containing alloys remains open.

Background

A bulk metallic glass (BMG) is an alloy frozen in a disordered, non-crystalline state with diameters of at least a millimeter; it deforms at room temperature mainly through shear bands, narrow zones of localized sliding whose stability and multiplication determine whether the glass shows macroscopic plasticity or fails brittly. Because single-parameter models cannot predict this behavior, an empirical criterion due to Lewandowski, Wang, and Greer (2005) correlates intrinsic plasticity with elastic constants: metallic glasses with Poisson's ratio ν\nu above roughly 0.31–0.32 (equivalently a low ratio of shear modulus to bulk modulus, G/BG/B) tend to be plastic in compression, while glasses below the threshold tend to be brittle. The criterion has been widely used as a screening tool for ductile glass compositions. Ni-based BMGs alloyed with the metalloids phosphorus and boron are attractive as corrosion-resistant structural materials, and Ma, Zhen, Guo, Li, Chang, and Sun (2016) prepared a Pd-free family (palladium-based glasses being the classic ductile, corrosion-resistant but costly reference alloys) Ni₇₇₋ₓ₋ᵧMoₓCrᵧNb₃P₁₄B₆ — eight compositions with (x, y) = (7, 0), (8, 0), (9, 0), (5, 3), (5, 5), (5, 8), (8, 3), (8, 5), all in at.%, i.e. 64–70 at.% Ni — by B₂O₃–CaO fluxing (a melt-purification treatment that removes oxide inclusions and other heterogeneous nucleation sites) plus J-quenching (a rapid melt-quench casting route), with critical diameters of 1.0–1.5 mm, glass-transition temperatures of 688–703 K, and outstanding corrosion resistance. Mechanically, however, every composition fractured with essentially zero compressive plastic strain. This contradicts the elastic-constant expectation: the alloys are Ni-based with Ni as the majority element (64–70 at.%), and the authors argue from literature values for pure Ni (ν\nu = 0.31, G/BG/B = 0.42, i.e. at the empirical ductility cutoff) and from the reported transformation of brittle Fe₈₀₋ₓNᵢₓP₁₄B₆ glasses into ductile ones upon Ni substitution that these glasses should deform plastically. Two caveats frame the puzzle: the Poisson's ratio and G/BG/B of these particular glasses were not measured but estimated from pure-Ni values, and the terminology of the underlying question refers to short-range order (the packing of the first few neighbor shells, described by coordination polyhedra and their vertex-, edge-, and face-sharing connections), free volume (excess local space relative to densely packed structure), and medium-range order (the arrangement of interconnected clusters over several nanometers). The paper concludes that atomic-level structural arrangement, beyond composition-weighted elastic moduli, controls the deformation mode, leaving the specific structural cause unidentified.

Problem Statement

For the eight Ni₇₇₋ₓ₋ᵧMoₓCrᵧNb₃P₁₄B₆ bulk metallic glasses of Ma et al. (2016) — (x, y) = (7, 0), (8, 0), (9, 0), (5, 3), (5, 5), (5, 8), (8, 3), (8, 5), all in at.%, i.e. 64–70 at.% Ni — which compressively fracture with 0–0.3% plastic strain: identify the atomic-scale structural features that suppress shear-band-mediated plasticity in these specific alloys, and explain why they fracture brittly even though the empirical elastic-constant ductility criteria, as estimated (not measured) from the Ni-rich composition, pure-Ni literature values (ν\nu = 0.31, G/BG/B = 0.42) and the reported Fe₈₀₋ₓNᵢₓP₁₄B₆ Ni-substitution analogy, would suggest ductile behavior. Candidate structural features — such as short-range-order cluster types and their connectivity, free-volume distribution, or medium-range configurational motifs — are illustrative elaborations rather than a mandated list. Since the elastic constants of these glasses were never measured, establishing whether the elastic indicators are in fact favorable is part of resolving the paradox rather than an assumed premise. An adequate answer must connect the proposed structural features to the measured plasticity of these specific alloys (for example through structural characterization or atomistic simulation of the actual compositions, correlated with their measured stress–strain behavior).

References

  1. Xu Ma, Ni Zhen, Junjiang Guo, Qiang Li, Chuntao Chang, Yanfei Sun, 'Preparation of Ni-based bulk metallic glasses with high corrosion resistance', Journal of Non-Crystalline Solids 443 (2016) 91-96, DOI 10.1016/j.jnoncrysol.2016.04.020, https://doi.org/10.1016/j.jnoncrysol.2016.04.020
  2. J. J. Lewandowski, W. H. Wang, A. L. Greer, 'Intrinsic plasticity or brittleness of metallic glasses', Philosophical Magazine Letters 85 (2005) 77-87, DOI 10.1080/09500830500080474, https://doi.org/10.1080/09500830500080474
  3. Yuan-yun Zhao, A. Inoue, Chuntao Chang, Jian Liu, B. Shen, Xin-min Wang, Run-Wei Li, 'Composition Effect on Intrinsic Plasticity or Brittleness in Metallic Glasses', Scientific Reports 4 (2014) 5733, DOI 10.1038/srep05733, https://doi.org/10.1038/srep05733
  4. Xu Ma, Qiang Li, Liang Chang, Chuntao Chang, Hongxiang Li, Yanfei Sun, 'Enhancement on GFA and mechanical properties of Ni-based bulk metallic glasses through Fe addition', Intermetallics 90 (2017) 58-62, DOI 10.1016/j.intermet.2017.06.012, https://doi.org/10.1016/j.intermet.2017.06.012
  5. Zhuanhui Yue, Qiang Li, Wenbiao Zhang, Mingcan Li, Haiming Duan, Chuntao Chang, Hongxiang Li, 'Effect of mutual substitution of Fe and Ni elements on the plasticity of Fe/Ni-based amorphous alloys: Ab initio molecular dynamics simulations', Journal of Non-Crystalline Solids 514 (2019) 46-51, DOI 10.1016/j.jnoncrysol.2019.03.040, https://doi.org/10.1016/j.jnoncrysol.2019.03.040
  6. Weiping Zeng, Yanhua Chen, Qiang Li, He Li, Baoxia Mu, Jiankang Ye, Chuntao Chang, 'Ductile Ni-based bulk metallic glasses at room temperature', Journal of Materials Research and Technology 26 (2023) 2432-2442, DOI 10.1016/j.jmrt.2023.08.062, https://doi.org/10.1016/j.jmrt.2023.08.062