Jinfu Li

Structural evolution of undercooled Ni-Cu alloys

The structural evolution of the whole Ni–Cu system with undercooling was systematically investigated. For pure Ni and Cu only one grain refinement was found when they were undercooled higher than the critical value ΔT*. For the alloys, however, another grain refinement could occur in a certain range of the lower undercooling. Based on current dendrite growth theory, a thermodynamic concept, dimensionless superheating, was developed to evaluate the tendency of the dendrite remelting. With undercooling increasing, the dimensionless superheating of the alloy increased first and then decreased, which suggested that the dendrite remelting should only be responsible for the grain refinement at low undercoolings although the effect of the remelting during the recalescence on the eventual structural morphology deserved special attention. The decrease of the grain size above ΔT* was attributed to the stress that originated from the impediment of the static liquid to the solidification contraction or expansion of the rapidly growing dendrite skeleton, and led to the distortion and break-up of the dendrites. The recrystallization happening in cooling could make the grain size decrease further. The measurement of the refined grain size indicated that it was a strong function of the composition.

Solidification structure formation in undercooled Fe-Ni alloy

Abstract The Fe alloy melts containing 7.5, 15, 22.5 and 30 at% Ni were bulk undercooled to investigate the structure evolution. When the undercooling of the four melts is lower than the critical value 110, 125, 175 and 325 K, respectively, only the stable face-centered cubic phase crystallizes. In this case a grain refinement caused by solid superheating is observed in all the alloys, but another grain refinement induced by recrystallization can merely occur in the Fe–30 at%Ni alloy undercooled by 190–325 K. Alternate crystallization of the metastable body-centered cubic phase occurs above the critical undercooling. It is indicated that the subsequent heterogeneous nucleation of the stable phase in the metastable solid and remaining liquid coexisting system is influenced not only by the morphology and surface area of the metastable solid, but also by the effective undercooling of the remaining liquid. On the basis of the experimental results and the theoretical analyses, a structure evolution map for bulk Fe–Ni system is constructed.

Free-volume evolution and its temperature dependence during rolling of Cu60Zr20Ti20 bulk metallic glass

The free-volume evolution during rolling Cu60Zr20Ti20 bulk metallic glass at room and cryogenic temperatures has been investigated by differential scanning calorimetry. When the specimen is rolled at cryogenic temperature, the free-volume content increases as the rolling proceeds first, and then saturates accompanied by the occurrence of phase separation as the thickness reduction exceeds 89%. If the rolling is performed at room temperature, although the free-volume content also rises in the earlier stage, it tends to decrease rather than saturate when the thickness reduction exceeds 87%, accompanied by partial crystallization. Phase separation does not change the annihilation rate of free volume, while the appearance of crystal/amorphous boundaries can enhance the annihilation.

CaLi-based bulk metallic glasses with multiple superior properties

We describe a class of metallic glasses that consists of low-cost components and exhibiting multiple superior properties such as exceptionally low glass transition temperature (∼35–60 °C), ultralow elastic moduli (∼23 GPa) comparable to that of human bones, high elasticity and strength, ultralow density (<2 g/cm3), exceptional thermodynamic and kinetic stability in supercooled liquid state, strong liquid fragility, ultrahigh specific strength and lower electrical resistivity, and polymerlike thermoplastic formability near room temperature. Such metallic glasses could have potential applications and facilitate studies of the nature of glasses.

Mode of dendrite growth in undercooled alloy melts

Abstract The mode of dendrite growth in the undercooled Ni–50at%Cu alloy was investigated. At lower undercoolings, the dendrite growth is mainly controlled by solute diffusion, and the formed dendritic morphologies are similar to those of the conventional as-cast equiaxed crystals, except that here the branches are much denser. At higher undercoolings, however, the severe solutal trapping that results from high dendrite growth velocity weakens the effect of solute diffusion on the dendrite growth. In this case, the dendrites branch in the bunching form. The dendrite spacings were measured, and the results were interpreted with the current dendrite growth theories.

Mechanism of anomalous eutectic formation in the solidification of undercooled Ni-Sn eutectic alloy

Anomalous eutectics in the solidification structure of undercooled Ni–18.7 at.% Sn eutectic alloy were examined by optical metallography and electron backscattered diffraction. It was revealed that α–Ni particulates are, in principle, randomly distributed in the anomalous eutectics in the undercooling range investigated. Another eutectic phase, β–Ni 3 Sn, is well orientated at low undercoolings but gradually becomes inconsistent in orientation as undercooling increases, accompanied by an increasing number of grain boundaries in it. As the solidification structure changes from a mixture of anomalous eutectics plus lamellar eutectics to full anomalous eutectics beyond a critical undercooling of 130 K, however, misorientation in the β–Ni 3 Sn phase disappears completely from the measurement area. Partial remelting of the primary solid was proposed to be the origin of the anomalous eutectic formation, and quantitative analyses were performed.

Evaluation of the optimum solute concentration for good glass forming ability in multicomponent metallic glasses

Abstract Previous work suggests that the size factor may be the most crucial one on the glass forming ability (GFA). But the alloys in a system have significant deference for GFA. A criterion λ n is defined in the present work to comprehensively evaluate concentration dependence of GFA of alloys in a system. It is found that the values of λ n of bulk glass formers with greatest GFA in Zr-, Pd-, Mg-, Nd- and Fe-based systems are nearly constant of 0.18. Based on the mathematical description of regular polytopes and the dense random packing of hard sphere (DRPHS) model, a cluster model is suggested. This model provides the physical meaningful origin of the λ n from the view of free volume. The glass structure of bulk glass formers with λ n ≈0.18 has optimum defect concentration within the framework of the cluster model. In the term of λ n criterion, an alloy with λ n of about 0.18 will be one of bulk glass former candidates, which is verified by the experimental result of ZrAlNi system.

The relation between formation of compounds and glass forming ability for Zr–Al–Ni alloys

Abstract This paper reports the formation of compounds during the solidification process of liquid Zr 85− x Al 15 Ni x ( x =15, 20, 25, 30, 35) alloys studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The results indicate that compound AlNi 4 Zr 5 forms when liquid alloys containing 20, 25 and 30 at.% nickel solidify. It is suggested in this work that the large glass forming ability (GFA) of the Zr–Al–Ni system is attributed to the difficulty of formation of compound AlNi 4 Zr 5 as a primary phase during solidification process of liquid alloys. In comparing the phases appearing in the solidification structure of the liquid Zr 60 Al 15 Ni 25 alloy with the products formed during crystallization process of this amorphous alloy, it is assumed that Al 2 NiZr 6 is a metastable phase, which decomposes into equilibrium phases during the subsequent cooling process.

Kinetic effect of crystal growth on the absolute stability of a planar interface in undercooled melts

The stability of a planar interface during the non-equilibrium solidification of undercooled melts is reexamined by the linear stability theory of Mullins and Sekerka. The incorporation of the kinetic coefficient, which is dependent on the local interfacial temperature, into the analysis leads to a striking change of the absolute stability criterion proposed by Trivedi and Kurz. It is also shown that whether the absolute stability of a planar interface exists relies on a dimensionless parameter q which is a function of the physical properties of the melts. Only the melts whose q is smaller than 1 can solidify into a stable planar interface when the crystal growth velocity exceeds the critical value.

Effect of repeated melting of the ingots on the glass-forming ability of Zr-based alloys

Abstract This letter reports the effect of repeated melting of the ingots on the glass-forming ability (GFA) of Zr 60 Al 15 Ni 25 and Zr 65 Al 7.5 Ni 10 Cu 17.5 alloys. The results indicate that the glass transition temperature T g and the onset temperature of crystallization T x of Zr 60 Al 15 Ni 25 alloy increase as melting times increase. However, the repeated melting of the ingot brings only a little change to Zr 65 Al 7.5 Ni 10 Cu 17.5 glassy alloy. Within the framework of structure heredity and thermodynamics, the reason for the improvement of the GFA of alloys is discussed.