Kaikai Song

Correlation between internal states and plasticity in bulk metallic glass

We report a close correlation between the internal states and plasticity in a bulk metallic glass (BMG) and discover that the optimization of copper-mold casting current can induce large plasticity stably in an otherwise brittle BMG. It is possible to confirm that larger plasticity corresponds to the internal states with more average free volume (FV) as revealed by lower density, higher enthalpy change, and higher Poisson’s ratio. The enhanced plastic deformation mechanism is interpreted based on the FV model of BMGs, and our results may have some implications for understanding the role of the FV during plastic deformation of BMGs.We report a close correlation between the internal states and plasticity in a bulk metallic glass (BMG) and discover that the optimization of copper-mold casting current can induce large plasticity stably in an otherwise brittle BMG. It is possible to confirm that larger plasticity corresponds to the internal states with more average free volume (FV) as revealed by lower density, higher enthalpy change, and higher Poisson’s ratio. The enhanced plastic deformation mechanism is interpreted based on the FV model of BMGs, and our results may have some implications for understanding the role of the FV during plastic deformation of BMGs.

Correlation between the microstructures and the deformation mechanisms of CuZr-based bulk metallic glass composites

The variation of the transformation-mediated deformation behavior with microstructural changes in CuZr-based bulk metallic glass composites is investigated. With increasing crystalline volume fraction, the deformation mechanism gradually changes from a shear-banding dominated process as evidenced by a chaotic serrated flow behavior, to being governed by a martensitic transformation with a pronounced elastic-plastic stage, resulting in different plastic deformations evolving into a self-organized critical state characterized by the power-law distribution of shear avalanches. This is reflected in the stress-strain curves by a single-to-“double”-to-“triple”-double yielding transition and by different mechanical properties with different serrated flow characteristics, which are interpreted based on the microstructural evolutions and a fundamental energy theorem. Our results can assist in understanding deformation behaviors for high-performance metastable alloys.

Correlation between glass-forming ability, thermal stability, and crystallization kinetics of Cu-Zr-Ag metallic glasses

The glass-forming ability (GFA) of the Cu-Zr-Ag system is evaluated based on the large amount of literature data available and discussed in the frame of a predictive amorphization criterion which combines topological instability and electronic criteria. The correlation between GFA, thermal stability, and crystallization kinetics of (Cu0.5Zr0.5)100−xAgx (x = 0, 2, 6, and 10) metallic glasses is further investigated. The enhancement of the GFA of the alloys and the thermal stability/fragility of the supercooled liquid can be traced back to a large size effect/volume mismatch and electronic effects. However, the apparent activation energy of crystallization decreases with increasing Ag content in the alloys which may be due to a nanoscale microstructural heterogeneity induced by the Ag addition. At a certain Ag content, a small amount of AgZr crystals precipitate together with Cu10Zr7 and CuZr2 and the crystallization mechanism changes from interface-controlled one-dimensional growth to three-dimensional gro...

Pronounced ductility in CuZrAl ternary bulk metallic glass composites with optimized microstructure through melt adjustment

Microstructures and mechanical properties of as-cast Cu47.5Zr47.5Al5 bulk metallic glass composites are optimized by appropriate remelting treatment of master alloys. With increasing remelting time, the alloys exhibit homogenized size and distribution of in situ formed B2 CuZr crystals. Pronounced tensile ductility of ∼13.6% and work-hardening ability are obtained for the composite with optimized microstructure. The effect of remelting treatment is attributed to the suppressed heterogeneous nucleation and growth of the crystalline phase from undercooled liquid, which may originate from the dissolution of oxides and nitrides as well as from the micro-scale homogenization of the melt.

Local microstructure evolution at shear bands in metallic glasses with nanoscale phase separation

At room temperature, plastic flow of metallic glasses (MGs) is sharply localized in shear bands, which are a key feature of the plastic deformation in MGs. Despite their clear importance and decades of study, the conditions for formation of shear bands, their structural evolution and multiplication mechanism are still under debate. In this work, we investigate the local conditions at shear bands in new phase-separated bulk MGs containing glassy nanospheres and exhibiting exceptional plasticity under compression. It is found that the glassy nanospheres within the shear band dissolve through mechanical mixing driven by the sharp strain localization there, while those nearby in the matrix coarsen by Ostwald ripening due to the increased atomic mobility. The experimental evidence demonstrates that there exists an affected zone around the shear band. This zone may arise from low-strain plastic deformation in the matrix between the bands. These results suggest that measured property changes originate not only from the shear bands themselves, but also from the affected zones in the adjacent matrix. This work sheds light on direct visualization of deformation-related effects, in particular increased atomic mobility, in the region around shear bands.

The correlations among the fragility of supercooled liquids, the fragility of superheated melts, and the glass-forming ability for marginal metallic glasses

In an individual alloy system for marginal metallic glasses (MGs), the fragility of supercooled liquids (m) is inversely proportional to the fragility of superheated melts (M), which depends on the differently inherent characteristic temperatures. The more stable the superheated melt, the more fragile the supercooled liquid becomes and the better the glass-forming ability (GFA). The strong supercooled liquids correspond to the bad GFA resulting from heterogeneous nucleation in liquids. For the whole Al-based MGs, m has general linear relations with the GFA and M, respectively.In an individual alloy system for marginal metallic glasses (MGs), the fragility of supercooled liquids (m) is inversely proportional to the fragility of superheated melts (M), which depends on the differently inherent characteristic temperatures. The more stable the superheated melt, the more fragile the supercooled liquid becomes and the better the glass-forming ability (GFA). The strong supercooled liquids correspond to the bad GFA resulting from heterogeneous nucleation in liquids. For the whole Al-based MGs, m has general linear relations with the GFA and M, respectively.

Localized crystallization in shear bands of a metallic glass.

Stress-induced viscous flow is the characteristic of atomic movements during plastic deformation of metallic glasses in the absence of substantial temperature increase, which suggests that stress state plays an important role in mechanically induced crystallization in a metallic glass. However, it is poorly understood. Here, we report on the stress-induced localized crystallization in individual shear bands of Zr60Al15Ni25 metallic glass subjected to cold rolling. We find that crystallization in individual shear bands preferentially occurs in the regions neighboring the amorphous matrix, where the materials are subjected to compressive stresses demonstrated by our finite element simulations. Our results provide direct evidence that the mechanically induced crystallization kinetics is closely related with the stress state. The crystallization kinetics under compressive and tensile stresses are interpreted within the frameworks of potential energy landscape and classical nucleation theory, which reduces the role of stress state in mechanically induced crystallization in a metallic glass.

Correlation between structural heterogeneity and plastic deformation for phase separating FeCu metallic glasses.

Unlike crystalline metals, the plastic deformation of metallic glasses (MGs) involves a competition between disordering and structural relaxation ordering, which is not well understood, yet. Molecular dynamics (MD) simulations were performed to investigate the evolutions of strain localizations, short-range order (SRO) as well as the free volume in the glass during compressive deformation of Fe50Cu50 MGs with different degrees of phase separation. Our findings indicate that the free volume in the phase separating MGs decreases while the shear strain localizations increase with increasing degree of phase separation. Cu-centered clusters show higher potential energies and Voronoi volumes, and bear larger local shear strains. On the other hand, Fe-centered pentagon-rich clusters in Cu-rich regions seem to play an important role to resist the shear transformation. The dilatation or annihilation of Voronoi volumes is due to the competition between ordering via structural relaxation and shear stress-induced deformation. The present study could provide a better understanding of the relationship between the structural inhomogeneity and the deformation of MGs.

Local melting to design strong and plastically deformable bulk metallic glass composites

Recently, CuZr-based bulk metallic glass (BMG) composites reinforced by the TRIP (transformation-induced plasticity) effect have been explored in attempt to accomplish an optimal of trade-off between strength and ductility. However, the design of such BMG composites with advanced mechanical properties still remains a big challenge for materials engineering. In this work, we proposed a technique of instantaneously and locally arc-melting BMG plate to artificially induce the precipitation of B2 crystals in the glassy matrix and then to tune mechanical properties. Through adjusting local melting process parameters (i.e. input powers, local melting positions, and distances between the electrode and amorphous plate), the size, volume fraction, and distribution of B2 crystals were well tailored and the corresponding formation mechanism was clearly clarified. The resultant BMG composites exhibit large compressive plasticity and high strength together with obvious work-hardening ability. This compelling approach could be of great significance for the steady development of metastable CuZr-based alloys with excellent mechanical properties.

Effect of pre-existing shear bands on mechanical properties and serration behaviors in bulk metallic glasses

Pre-existing (multiple) shear bands were introduced into the ductile Zr56Co28Al16 and Zr65 Ni10 Cu15 Al10 bulk metallic glasses (BMGs) through the lateral-deformation, respectively. It was found that the pre-exiting shear bands can further enhance the compressive plasticity of ductile BMGs. According to the serration analysis on the plastic deformation of the as-cast and the pre-deformed samples, the serration events in the stress-strain curves during deformation display a self-organized critical ( SOC) behavior. Compared with the as-cast BMGs, a larger power-law scaling exponent calculated based on serrated flow behaviors becomes larger for the pre-deformed BMGs, implying that the shear banding stability of BMGs is effectively enhanced. This should be caused by the pronounced interactions of shear bands during plastic deformation for the pre-deformed BMGs. However, by introducing a large amount of multiple shear bands into the glassy matrix, it also becomes easier for shear bands to propagate along the pre-existing shear bands, leading to a lower cut-off elastic energy density for the pre-deformed BMGs. More multiple shear bands with stronger interactions for the pre-deformed BMGs could provide a larger chance to activate the shear-band cracking but less local elastic energies are remained for the subsequent crack-linking.