Search

Abstract: Ni54.6Pd45.4, a solid solution alloy without crystallization temperature range, has been undercooled by glass flux method up to the hypercooling limit. It was found that it solidified as pure metals. As undercooling increases, the secondary dendrite arm spacing decreases monotonously until a undercooling of 135K, above which the secondary branches considerably degenerate. The solidification with undercoolings higher than 160K leads to grain-refined structures. In this case, their average grain sizes continuously decrease with increasing undercooling even if the alloy has been hypercooled. The fact that grain refinement still takes place in the solidification of the hypercooled alloy indicates that the solidification time after recalescence is not a factor to dominate the grain refinement at large undercooling. Recrystallization of the solid in the post-solidification period should be responsible for the grain refinement. [Copyright &y& Elsevier]

Anomalous eutectics are argued to form due to the remelting of the primary solid during solidification of deeply undercooled eutectic alloy melts. As an indicator of the tendency for anomalous eutectic formation, the remelted fraction of the primary eutectic was analyzed systematically based on the eutectic dendrite growth theory. For eutectic alloys with either larger equilibrium solute distribution coefficients or gentle liquidus slopes, the primary eutectic is highly supersaturated with solute and more prone to remelting. When the eutectic composition is set to different values, (e.g. the eutectic point is closer to one phase of the eutectic), the two eutectic phases under rapid growth change their compositions simultaneously, but their remelted fractions during temperature recalescence do not vary significantly. Three representative binary eutectic alloys Ag–39.9 at.%Cu, Ni–19.6 at.%P and Pd–16.0 at.%P – their eutectic products are solid solution–solid solution, solid solution–stoichiometric intermetallic compound and stoichiometric intermetallic compound–stoichiometric intermetallic compound, respectively, were solidified at large undercooling. Anomalous eutectics were observed in the first two eutectic alloys whose eutectic structure consists of at least one solid solution phase, whereby the stoichiometric intermetallic compound phase remained highly oriented whereas the solid solution phase had a near random distribution of orientations. For the Pd–16.0 at.%P alloy, however, the primary eutectic retained its original morphology as the final solidification structure regardless of the degree of undercooling. All these experimental results validate the argument that anomalous eutectics result from the remelting of the primary solid. [ABSTRACT FROM AUTHOR]

Abstract: How to explore an inherent linkage between non-equilibrium liqulid/solid transformation (LST) and subsequent solid-state transition (SST) is becoming more and more important for material preparation. Applying highly undercooled solidification and melting spinning, single-phase supersaturated solid solution (SSSS) was prepared for immiscible Fe–Cu and low-solid-solubility Fe–B alloys, respectively. By performing isothermal annealings at temperatures for the above SSSS, an inherent linkage between non-equilibrium LST and solid-state grain growth has been studied, in terms of an extended dendrite growth model and solute trapping model (for non-equilibrium solidification) and a numerical and an analytical thermo-kinetic model (for solid-state grain growth). For Fe–Cu SSSS, a relation between the metastable equilibrium grain size and the initial melt undercooling was derived, whereas, for Fe–B SSSS, a relation between the metastable equilibrium grain size and the ribbon thickness/rotational speed (in melt spinning) was deduced. The model prediction is consistent with the experimental result, indicating that the solid-state grain growth stops as a result of decrease in GB energy due to solute segregation to GBs. This further certifies that, for the alloys considered, the final microstructure is determined by both the non-equilibrium LST and the solid-state grain growth including GB segregation. [ABSTRACT FROM AUTHOR]

Abstract: Assuming thermal balance and solute conservation, a numerical model has been proposed to describe the recalescence behavior of bulk-undercooled Cu–Ni melts. Applying a finite-difference scheme, the transformed solid fraction upon recalescence is given as a function of the liquid temperature, while the average liquid concentration can be tracked by calculation of the liquid/solid (L/S) Gibbs energy difference, in combination with a dendrite growth model. Accordingly, a transition from non-equilibrium to equilibrium process has been described from the evolution of L/S Gibbs energy difference. Applying the present model, the experimentally observed maximum recalescence temperature can be well predicted. [Copyright &y& Elsevier]

Peritectic Fe–Ni alloys were undercooled using glass fluxing. When subjected to an undercooling (ΔT) beyond a critical value (ΔT ∗ ≈130K), the peritectic reaction, L +δ→γ, normally occurring after primary recalescence, was found to be suppressed. This can be ascribed to a longer incubation time for the occurrence of the peritectic reaction than the time required for solidification of δ phase during post-recalescence, for ΔT >ΔT ∗. [Copyright &y& Elsevier]