Suppression of Shear Banding and Transition to Necking and Homogeneous Flow in Nanoglass Nanopillars

The Modulation of Compositional Heterogeneity for Controlling Shear Banding in Co-P Metallic Nanoglasses

Shear banding is much dependent on the glass-glass interfaces (GGIs) in metallic nanoglasses (NGs). Nevertheless, the current understanding of the glass phase of GGIs is not well established for controlling the shear banding in NGs. In this study, Co-P NGs are investigated by molecular dynamics simulations to reveal the phenomenon of elemental segregation in the GGI regions where the content of Co is dominant. Specifically, Co segregation results in the formation of GGIs, whose atomic structures are comparatively less dense than those present in the interiors of glassy grains. It is suggested that the Co segregation significantly reduces the shear resistance of GGIs. Thus, such compositional heterogeneity influences the mechanical properties of Co-P NGs. Particularly, shear banding is much altered through enhancing the Co segregation in the GGI regions, which leads to improvements in the ductility of Co-P NGs. This study advances knowledge of the formation of the GGI phase in NGs, which could enable GGI engineering in enhancing the mechanical properties of NGs.

Mechanical property dependence on compositional heterogeneity in Co-P metallic nanoglasses

The glass-glass interfaces (GGIs) are in a unique glass phase, while current knowledge on the interfacial phase has not completely established to explain the unprecedented improvements in the ductility of metallic nanoglasses (NGs). In this work, Co-P NGs prepared through the pulse electrodeposition are investigated, whose GGI regions clearly show elemental segregation with chemical composition dominated by element Co. Such compositional heterogeneity is further verified by molecular dynamics (MD) simulation on the formation of GGIs in Co-P NGs and atomic structures of GGIs with Co segregation are found to be less dense than those of glassy grains. More importantly, Co segregation at GGIs is closely related to the improved ductility observed in Co-P NGs, as demonstrated by nanoindentation measurements and MD simulations. This work facilitates the understanding on the relations between compositional heterogeneity and improved ductility as observed in Co-P NGs, and thus opens a new window for controlling the mechanical properties of NGs through GGI engineering.

Phase Engineering of Nanostructural Metallic Materials: Classification, Structures, and Applications

Metallic materials are usually composed of single phase or multiple phases, which refers to homogeneous regions with distinct types of the atom arrangement. The recent studies on nanostructured metallic materials provide a variety of promising approaches to engineer the phases at the nanoscale. Tailoring phase size, phase distribution, and introducing new structures via phase transformation contribute to the precise modification in deformation behaviors and electronic structures of nanostructural metallic materials. Therefore, phase engineering of nanostructured metallic materials is expected to pave an innovative way to develop materials with advanced mechanical and functional properties. In this review, we present a comprehensive overview of the engineering of heterogeneous nanophases and the fundamental understanding of nanophase formation for nanostructured metallic materials, including supra-nano-dual-phase materials, nanoprecipitation- and nanotwin-strengthened materials. We first review the thermodynamics and kinetics principles for the formation of the supra-nano-dual-phase structure, followed by a discussion on the deformation mechanism for structural metallic materials as well as the optimization in the electronic structure for electrocatalysis. Then, we demonstrate the origin, classification, and mechanical and functional properties of the metallic materials with the structural characteristics of dense nanoprecipitations or nanotwins. Finally, we summarize some potential research challenges in this field and provide a short perspective on the scientific implications of phase engineering for the design of next-generation advanced metallic materials.

Excess free volume and structural properties of inert gas condensation synthesized nanoparticles based CuZr nanoglasses

Nanoglass (NG) as a new structure-tunable material has been investigated using both experiments and computational modeling. Experimentally, inert gas condensation (IGC) is commonly employed to prepare metallic glass (MG) nanoparticles that are consolidated using cold compression to generate an NG. In computational modeling, various methods have been used to generate NGs. However, due to the high computational cost involved, heretofore modeling investigations have not followed the experimental synthesis route. In this work, we use molecular dynamics simulations to generate an NG model by consolidating IGC-prepared Cu64Zr36 nanoparticles following a workflow similar to that of experiments. The resulting structure is compared with those of NGs produced following two alternative procedures previously used: direct generation employing Voronoi tessellation and consolidation of spherical nanoparticles carved from an MG sample. We focus on the characterization of the excess free volume and the Voronoi polyhedral statistics in order to identify and quantify contrasting features of the glass-glass interfaces in the three NG samples prepared using distinct methods. Results indicate that glass-glass interfaces in IGC-based NGs are thicker and display higher structural contrast with their parent MG structure. Nanoparticle-based methods display excess free volume exceeding 4%, in agreement with experiments. IGC-prepared nanoparticles, which display Cu segregation to their surfaces, generate the highest glass-glass interface excess free volume levels and the largest relative interface volume with excess free volume higher than 3%. Voronoi polyhedral analysis indicates a sharp drop in the full icosahedral motif fraction in the glass-glass interfaces in nanoparticle-based NG as compared to their parent MG.

Nanoglass-based balloon expandable stents

Here, a prototypical metallic nanoglass is proposed as a new alloy for balloon expandable stents. Traditionally, the stainless steel SS 316L alloy has been used as a preferred material for this application due to its proper combination of mechanical properties, corrosion resistance, and biocompatibility. Recently, metallic glasses (MGs) have been considered as promising materials for biodevice applications. MGs often display outstanding mechanical properties superior to those of conventional metallic alloys and overcome some of the weaknesses of SS 316L, such as radiopacity, stainless steel allergy, and thrombosis-induced restenosis. However, commonly used monolithic MGs, which have an amorphous homogeneous microstructure, suffer from lack of ductility that is necessary for deployment of balloon expandable stents. In contrast, nanoglasses, that is, amorphous alloys with heterogeneous microstructure, exhibit enhanced ductility which makes them promising materials for balloon expandable stents. We evaluate the feasibility of a prototypical Zr₆₄ Cu₃₆ nanoglass with a grain size of 5 nm for balloon expandable stents by performing finite element method modeling of the stent deployment process in a coronary artery. We consider the BX-Velocity stent design and the nanoglass mechanical properties calculated from atomistic simulations. The results suggest that nanoglasses are suitable materials for balloon expandable stent applications. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 108B:73-79, 2020.

Nanoscale Structural Evolution and Anomalous Mechanical Response of Nanoglasses by Cryogenic Thermal Cycling

One of the central themes in the amorphous materials research is to understand the nanoscale structural responses to mechanical and thermal agitations, the decoding of which is expected to provide new insights into the complex amorphous structural-property relationship. For common metallic glasses, their inherent atomic structural inhomogeneities can be rejuvenated and amplified by cryogenic thermal cycling, thus can be decoded from their responses to mechanical and thermal agitations. Here, we reported an anomalous mechanical response of a new kind of metallic glass (nanoglass) with nanoscale interface structures to cryogenic thermal cycling. As compared to those metallic glasses by liquid quenching, the Sc₇₅Fe₂₅ (at. %) nanoglass exhibits a decrease in the Young's modulus but a significant increase in the yield strength after cryogenic cycling treatments. The abnormal mechanical property change can be attributed to the complex atomic rearrangements at the short- and medium- range orders due to the intrinsic nonuniformity of the nanoglass architecture. The present work gives a new route for designing high-performance metallic glassy materials by manipulating their atomic structures and helps for understanding the complex atomic structure-property relationship in amorphous materials.

Balancing strength, hardness and ductility of Cu64Zr36 nanoglasses via embedded nanocrystals

Superplasticity can be achieved in nanoglasses but at the expense of strength, and such a loss can be mitigated via embedding stronger nanocrystals, i.e., forming nanoglass/nanocrystal composites. As an illustrative case, we investigate plastic deformation of Cu₆₄Zr₃₆ nanoglass/nanocrystalline Cu composites during uniaxial tension and nanoindentation tests with molecular dynamics simulations. With an increasing fraction of nanocrystalline grains, the tensile strength of the composite is enhanced, while its ductility decreases. The dominant interface type changes from a glass-glass interface to glass-crystal interface to grain boundary, corresponding to a failure mode transition from superplastic flow to shear banding to brittle intercrystal fracture, respectively. Accordingly, the indentation hardness increases continuously and strain localization beneath the indenter is more and more severe. For an appropriate fraction of nanocrystalline grains, a good balance among strength, hardness and ductility can be realized, which is useful for the synthesis of novel nanograined glass/crystalline composites with high strength, high hardness and superior ductility.

Compromising high strength and ductility in nanoglass–metallic glass nanolaminates

Large-scale molecular-dynamics simulations are used to investigate the mechanical properties of 50 nm diameter Cu₆₄Zr₃₆ nanolaminate nanopillars constructed from 5 nm thick layers of metallic glass (MG) or MG and 5 nm grain sized nanoglass.