•Ti-based BMG composites are prepared by substituting transition metal.•The transition metal is distributed mainly in the glass-matrix.•The plasticity relates highly to the stress concentration factor of dendrite.•The strength of BMG composites relates highly to that of glass-matrix.A series of Ti-based Ti45Zr25Nb6Sn2TM5Be17 (at.%, TM = Cu, Ni, Co and Fe) BMG composites were studied to show the effect of transition metal elements on the microstructure, intrinsic properties and mechanical properties of Ti-based BMG composites. The research shows that the transition metal elements are distributed mainly in the glass-matrix, substitution of which influences significantly the Young's modulus and hardness of glass-matrix. Different from the load-bearing model, the yielding strength of the BMG composites relates highly to not only the yielding strength of the dendrite-phase but also that of the glass-matrix. The plasticity of the BMG composites is highly dependent on the average stress concentration factor of the dendrite-phase. A useful rule for preparing of BMG composite with high strength and excellent plasticity is herein proposed, where tailoring simultaneously the properties of dendrite-phase and glass-matrix is suggested to be a better choice to obtain a good compromise between strength and plasticity.

•The samples are studied by compression and tension tests under different strain rate.•The Ti-based BMG composite has negative strain rate sensitivity.•The thermal softening effect is prominent upon deformation at higher strain rates.•The shear bands extension is prominent upon deformation at lower strain rates.The Ti45Zr25Nb6Sn2Cu5Be17 bulk metallic glass composite was investigated by uniaxial quasi-static compression and tension tests under strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1 at the room temperature. The deformation behavior was characterized by analyzing the lateral and fracture surfaces. With the increase of strain rate, the yielding stress and plasticity decrease significantly, indicating negative strain rate sensitivity. At higher strain rates, the thermal softening effect is more prominent and the yielding stress is lower, while lower strain rates, the shear band toughness of the dendrites is higher than that of the glass matrix. Then the dislocations have sufficient time to be generated, multiplied and aggregated, thereby the shear banding can be retarded and hindered efficiently by dendrites. Furthermore, the primary shear bands are able to block more effectively the secondary ones, which results in more uniform distribution of plastic strain and thereby higher plasticity. The current work shows that the coordination of the shear band toughness of the dendrites and the glass matrix and the stable extension of the shear bands are of primary importance for obtaining excellent mechanical properties.

In-situ formed (Cu0.6Zr0.3Ti0.1)95Nb5 bulk metallic glass (BMG) composite with Nb-rich dendrite randomly dispersed in hard glassy matrix was prepared by casting into a water-cooled copper mold. The dendrite has much smaller hardness and elastic modulus than glassy matrix, and the stress concentration at interface provides a channel for the initiating and branching of shear bands upon loading, thus leading to a high compressive fracture strain of 6.08% and fracture strength about 2200 MPa. Comparing with other Cu-based BMG composite, the fracture strength of present (Cu0.6Zr0.3Ti0.1)95Nb5 composite is not significantly reduced, indicating that the addition of Nb in the current work is an effective and effortless way to fabricate new practical BMG composites with enhanced strength and good plasticity.

•Growth velocities in the undercooled Ni31Si12 intermetallic compound were measured.•Growth orientations in the undercooled Ni31Si12 intermetallic compound were obtained.•Fragmentation of <112¯0>-oriented dendrites was found to be the origin of grain refinement.•Seaweed dendrite if presents should not be an intermediate in grain refinement.The melt of Ni31Si12 intermetallic compound was undercooled by a melt fluxing technique. The recalescence processes were in-situ observed by a high-speed camera and the solidification microstructures were analyzed by electron back-scattering diffraction. A sharp increase of the growth velocity was found at a critical undercooling ΔT*≈128 K and a second β2-Ni3Si intermetallic compound was observed around the primary Ni31Si12 intermetallic compound when the undercooling ΔT>119 K, indicating that the Ni31Si12 intermetallic compound is actually not stoichiometric. With the increase of undercooling, the microstructure changes from coarse striped grains, coarse dendrite to refined equiaxed grains, while the growth orientation transits from <112¯0>, mixed <112¯0>&<11¯00> to <11¯00>. Fragmentation of <112¯0>-oriented dendrites with the reduction of interface energy and solute diffusion as its driving forces, is suggested to be the origin of grain refinement. Since fragmentation happens adequately upon the transition of growth orientation, no experimental evidences can be found for the formation of seaweed dendrite, which if occurs should not be an intermediate for grain refinement. The current work is not only helpful for understanding non-equilibrium solidification but also useful for controlling microstructures of intermetallic compounds.Download high-res image (484KB)Download full-size image

•In situ Ti-based BMG composites are synthesized by arc-melting casting.•The intrinsic properties of dendrite-phase are tailored by adding Sn element.•The work-hardening of dendrite-phase dominates in the plastic deformation stage.•The Sn2 has the largest ductility (εu ≈ 10%), ultimate strength (σu ≈ 1.5 GPa).Bulk metallic glass (BMG) composites have demonstrated enormous potential for improved ductility and toughness over the traditional BMGs by in-situ formed crystalline phase in glass-matrix, which brings about delocalized strain and can inhibit shear bands rapid propagation in glass-matrix. However, an early onset of necking after yielding arises upon tension loading process. Enhancing work-hardening of bulk metallic glass (BMG) composites is therefore vitally important for practical applications. By tailoring the intrinsic properties of a Ti47Zr25Nb6Cu5Be17 BMG composite with an addition of Sn to reduce the shear and the elastic modulus of dendrite-phase, the current work gives full play to strain-induced work-hardening of dendrite phase and makes it succeed the completion with strain-induced work-softening of glass-matrix. Because of the significant enhancement of work-hardening by dislocation deformation in dendrite-phase, and the stabilized plastic flow in glass-matrix, large tensile ductility (tensile strain till necking εu ≈ 10%) and high tensile strength (σu ≈ 1.5 GPa) are attained simultaneously by the Sn2 BMG composite.

The kinetics of triple-junctions (TJs) in eutectic solidification is modeled by the thermodynamic extremum principle (TEP). It consists of two parts. First, TJs as the interaction of interfaces follow the interface kinetics according to which the temperature and concentration at the TJs are determined. This interface part of TJ kinetics is closely related to the eutectic point in the kinetic phase diagram. Second, TJs have their specific kinetics according to which their morphology (e.g., the contact angles in two dimensions) is determined. Using a new solution of solute diffusion in liquid, the TJ kinetics is incorporated into the current lamellar eutectic growth model. The model is applicable to the concentrated alloy systems and can be extended to any kind of eutectics. Simulation results of the rapid solidification of a lamellar Ni5Si2–Ni2Si (γ–δ) eutectic show that both parts of TJ kinetics can play important roles in eutectic solidification and need to be considered to improve the current eutectic theory.

The maximal entropy production principle was applied to model the growth kinetics of a multi-component stoichiometric compound. Compared with the solid-solution phase and the non-stoichiometric compound, the dissipation by the trans-interface diffusion makes the interface slow down by decreasing the effective interface mobility and does not result in solute trapping or disorder trapping. An application to the crystallization of a CuZr stoichiometric compound shows that the transition from the thermodynamic-controlled to the kinetic-controlled growth can be predicted.

•The thermodynamic principles for phase-field modeling are reviewed.•The recent progress in the phase-field modeling of isothermal solidification of binary alloys are reviewed and analyzed theoretically.•The importance to solve directly the additional constraints in the modeling system self-consistently in thermodynamics is highlighted.•The relation between the free energy functional and the grand potential functional are discussed.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.

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.