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Friction and wear are the two most important properties that determine the long-term performances of metallic glasses as much as strength and ductility. With this in mind, a multilayered strategy was proposed to improve the nano-scratching properties of metallic glass thin films and the well-characterized Zr-based metallic glasses were chosen to be the representative model materials in current work. It was found that the metallic glass could exhibit improved wear resistance and reduced friction coefficient if it is made in the form of multilayers. The complexity in the temporal scale of the lateral force signal is investigated, elucidating the inhomogeneous (and homogeneous) shear-banding processes during the nanoscratching process. The statistical and dynamic analyses of the stick-slip behaviors showed a transition from a chaotic to a self-organized critical state in the multilayers, indicating that the hetero-interfaces promote extensive shear band-interface interactions and induce the scratch hardening effect, both of which are responsible to the excellent tribological properties. The current work not only highlights the significant roles of hetero-interfaces in improving the nano-tribological properties but also make the multilayered metallic glass thin films a promising candidate towards structural and functional applications for metallic glasses.

Diffusion-controlled phase transformations are of singular importance in controlling microstructures and mechanical properties but are difficult to model and calculate for Fe-C-X alloys because of the large difference in the diffusivities of the interstitial element C and the substitutional element X. In this work, the thermodynamic extremal principle was applied to propose a modified quasi-sharp-interface model that integrates trans-interface diffusion from the product phase to the interface, trans-interface diffusion from the interface to the parent phase, interface migration and bulk diffusion of C and X. Applications to isothermal and cyclic phase transformations showed that the model allows the arbitrary setting of the initial conditions. For isothermal phase transformations, three different kinds of characteristic phase transformation kinetics (i.e., a gradual transition from non-partition to partition, a sharp transition from non-partition to partition and solely partition) were found, and their dependence on the interface properties, the grain size and the isothermal temperature was discussed. For both isothermal and cyclic phase transformations, the important roles of trans-interface diffusion and interface migration were highlighted.

The present work investigated systematically the glass forming ability, the mechanical properties and the shear deformation of the TiZrHfCuBe high entropy bulk metallic glass (HE-BMG) with minor Sn addition. The results revealed that the glass forming ability and the thermal stability are enhanced by minor Sn addition but within a rather limited composition range. A nanoindentation study of the mechanical response in the (TiZrHfCuBe)1-xSnx (x = 0, 1, 2 and 3 at. %) HE-BMGs showed that the structural heterogeneity caused by Sn addition resulted in higher hardness, larger number of shear bands and smaller size of shear transformation zone, ascribing to the positive enthalpy of mixing between the Sn and Cu/Be elements. A statistical analysis of the serrated flow revealed that the chemical heterogeneities promote a relatively large population of shear deformation units and lead to multiple nucleation of shear bands. The present work might has implications in establishing the link between minor element addition and structure heterogeneity, which is of critical importance for understanding of shear banding behaviors in BMGs.

The thermodynamic extremal principle was applied to propose a model in which trans-interface diffusion from the product phase to the interface, from the interface to the parent phase and interface migration are integrated for diffusion-controlled phase transformations in Fe-C alloys. Compared with the classical models with either a local-equilibrium condition or a constrained carbon equilibrium condition, the current model is able to predict massive transformations in the two-phase region. Application to isothermal phase transformations showed that the phase transformation mode is independent of (dependent on) trans-interface diffusion when the initial composition is close to the T0 line (close to the α/α + γ boundary). Ascribed to the large solute diffusivity of C, the thickness of the spike upon massive transformations could be much larger than the atomic spacing and the diffusion-controlled phase transformations could be faster than the interface-controlled phase transformations. Three stages, i.e., the diffusive transformation, massive transformation and the soft impingement stages, were predicted for phase transformations upon continuous cooling, according to which the critical limit between diffusive and massive transformations was determined to be within the two-phase region, being consistent with the experimental results in ultra-low-carbon Fe-C alloys. The current work could be very useful to control diffusion-controlled phase transformations and modulate the mechanical properties of steels.

The thermodynamic extremal principle was applied to model of rapid solidification of non-stoichiometric intermetallic compounds and the Co-xat.%Si alloys (x = 50, 53, 55) were undercooled to test the model. It is the model with but not the model without solute drag that can be derived self-consistently in thermodynamics. Unique dual sluggish and abrupt growth stages were found in undercooled Co-53 at.%Si and Co-55 at.%Si alloys. The first (second) sluggish stage is solute-controlled (thermal-controlled). The first (second) abrupt growth stage is ascribed to the sharp occurrence of solute trapping (inverted partitioning) and disorder trapping that initiates the transition from solute-controlled (thermal-controlled) to thermal-controlled (kinetic-controlled) growth. Since the predictions by the current (previous) model with (without) solute drag predicted well (deviate drastically from) the experimental results, solute drag was suggested be significant upon rapid solidification. The current work solved such an open problem, i.e. solute drag in solidification, and is helpful for not only understanding the non-equilibrium phenomena that is of theoretical importance but also controlling the non-equilibrium microstructures that is of technological importance.

Modeling of non-equilibrium solidification in multi-component alloys is of singular importance in microstructure control, which however owing to the complex systems with complex additional constraints is still an open problem. In this work, the thermodynamic extremal principle was applied to solve the complex additional constraints self-consistently in thermodynamics. Consequently, short-range solute redistribution and long-range solute diffusion that share the same mobility are integrated naturally into the solute diffusion equations, thus avoiding the introduction of additional kinetic coefficients (e.g. interface permeability) to describe solute redistribution. Application to the non-equilibrium solidification of Al-Si-Cu alloys shows that anomalous solute trapping and anomalous solute profiles within the diffuse interface could occur, thus highlighting the important effect of the interaction among the component elements on the interface kinetics. The current phase-field model might be preferred for simulations not only because of its simplest form of evolution equations but also its feasibility to increase the simulation efficiency by the “thin interface limit” analysis.

The thermodynamic extremal principle was applied to model of diffusion-controlled phase-transformations in multi-component substitutional alloys, in which dissipations by interface migration and trans-interface diffusion were integrated for a sharp interface. In the modeling, a new concept of trans-interface diffusion in two-steps, i.e. from the product phase to the interface and from the interface to the parent phase, was introduced, ascribing to which the model follows the Onsager's reciprocal relation. In contrast to the work of Svoboda et al. (2004) that considers only the interfacial dissipation by interface migration, non-equal jumps of chemical potentials across the interface are herein allowed. Applications to the Fe Cr Ni and Fe Ni alloys showed that the model is able to describe accurately not only the kinetic processes of massive transformation and diffusive transformation but also the critical limit between them. Since the transformation direction is not a priori condition for model calculations, the model is of potential value in applications to the cases in which the migrating direction changes, e.g. cycle phase-transformations etc.

A eutectic dendrite growth model in which the interface, solute diffusion in the liquid, and triple-junction (TJ) are under non-equilibrium conditions is proposed for undercooled binary alloys. This model, compared with previous work, is applicable to concentrated alloys. Application of the model to the undercooled Ag–Cu eutectic alloy obtained a good agreement between model predictions and experimental results. An upper limit on the eutectic growth velocity is predicted above which migration of TJ is not kinetically possible and a transition from co-operative eutectic growth to single-phase growth occurs for concentrated alloys and dilute alloys with linear liquidus and solidus, respectively.

The effective mobility approach is compared with the kinetic energy approach in terms of sharp interface modeling and phase-field modelling of non-equilibrium solute diffusion upon rapid solidification of binary alloys. The two approaches are equivalent for modelling of long range solute diffusion in bulk phases, but only the effective mobility approach can introduce the non-equilibrium solute diffusion effect to short range solute diffusion at a sharp interface or within a diffuse interface. Addition of the kinetic energy terms results in an unreasonable non-bilinear expression of the flux and thermodynamic driving force in the free energy production of interface migration or phase field propagation, whereas the effective mobility approach allows the thermodynamic extremal principle workable.

In recent years, the non-equilibrium solidification technology is developing very fast while the designing of solidification processes still relies on classical theories and empirical relations, which can no longer satisfy the increasing need for controlling of solidification conditions. The classical solidification theories are based on several major assumptions, such as linear liquidus/solidus, ideal solution, local equilibrium and they usually focus on model alloys instead of real industrial concentrated multi-component alloys. The discrepancies between classical theories and practical conditions demand us to abandon these classical assumptions so that the solidification theories can be applied to industrial processes. The current paper reviewed the recent development of non-equilibrium solidification theories for single-phase solid solution alloys. For dilute binary alloys, the microscopic interface kinetics, interface stability, dendrite growth and the micro-macro overall transformation kinetics were reviewed and the significances of nonlinear liquidus/solidus were illustrated. For concentrated binary and multi-component alloys, the interface kinetics, interface stability and dendrite growth were reviewed with an emphasis on the importance of interaction between the components. The newly-developed non-equilibrium theories can give more accurate description of complicated solidification processes, thus promote the application of the solidification theories to industrial processes.

Phase-field modeling is not only a powerful simulation tool to predict the microstructure evolution but also a useful theoretical method to study the interface kinetics. In this paper, the thermodynamic principles for and the recent progress in the phase-field modeling of isothermal solidification of binary alloys are reviewed. Different phase-field models with or without the condition of equal concentrations or equal diffusion potentials are reformulated from the entropy/free energy functional or the thermodynamic extremal principle. Their physics behind, problems and relation to the sharp interface models are analyzed. The importance to solve directly the additional constraints in the modeling system self-consistently in thermodynamics is highlighted.

An effective mobility for non-equilibrium solute diffusion is introduced to develop a hyperbolic phase-field model of rapid solidification in which long-range solute diffusion and short-range solute-redistribution are under local non-equilibrium conditions. At equilibrium, the model provides decoupling of bulk and interface properties. Far from equilibrium, the model predicts a transition from diffusion-limited growth to diffusionless solidification at an interface velocity that is equal to the solute diffusion speed in liquid. At this critical velocity, the solute drag effect disappears abruptly, being consistent with the previous local non-equilibrium model for the sharp interface. A comparison with other phase-field models is made and an agreement between the present model predictions and the experimental results of rapid solidification of Si-9at.%As alloy is obtained.

The Fe 83 B 17 eutectic alloy was undercooled by an electromagnetic levitator ranging up to an undercooling 217 K and the recalescence fronts were in-situ recorded by a high-speed camera. The measured growth velocity as a function of undercooling was well predicted by the current eutectic dendrite model for concentrated alloys with a non-linear phase-diagram but not the previous models for dilute alloys with a linear phase-diagram.