Effect of peritectic reaction on dendrite coarsening in directionally solidified Sn–36 at.%Ni alloy

Effect of peritectic reaction on dendrite coarsening was investigated in directionally solidified Sn–36 at.%Ni peritectic alloys at different growth rates (2~200 μm/s) under constant temperature gradient. A coarsening model was used to characterize the coarsening process in terms of both the secondary dendrite arm spacing (λ ₂) of the primary Ni₃Sn₂ phase and the specific surface area (S _V ) of dendrites. It was shown that peritectic reaction could retard the increase of λ ₂ and decrease of S _V during coarsening, which resulted from decelerating solute transport rate between adjacent dendrite arms caused by the peritectic phase enclosing the primary phase. The kinetics of the peritectic reaction that was found to be crucial to determine the coarsening process was characterized by the reaction constant (f) which not only changed with growth rates but also with solidification time in the real solidification process at a given growth rate.     

A study of the heat parameters during bidirectional solidification

The temperature field and heat parameters are important in controlling metal liquid crystallinity in unidirectional and bidirectional solidification. The temperature field can be divided into three cases: a liquid temperature field; solid temperature field; and a temperature field on the solid-liquid (S–L) interface. Heat parameters can be divided into two cases: technical heat parameters; and solidification heat parameters. The temperature field on the S–L interface and solidification heat parameters are the most important for the structures and properties of materials. The temperature field on the S–L interface is determined by the alloy system, and solidification heat parameters are related to the temperature ield of the environment and technical heat parameters. The temperature ield on the S–L interface is closely related to the solidiication heat parameters.

A theoretical model describing precisely the temperature field on the S–L interface during bidirectional solidification was proposed. A series of heat parameters, including temperature gradients G, solidification rate R, cooling velocity V and characteristic temperature T_c have been derived from this model. A superalloy has been chosen as the experimental object in order to verify the theoretical model. The theoretical calculations are found to be in agreement with the experimental results.     

Microstructures and phase selection in rapidly solidified Zn-Mg alloys

The Zn-Mg system has potential glass-forming ability, and therefore studies were made of rapidly solidified zinc-based Zn-Mg alloys containing up to 6 wt% Mg. These alloys exhibited interesting eutectic phase selections and structural transitions across the ribbon thickness which are represented on a microstructure selection diagram for rapid solidification conditions. Although rapid solidification is known in many cases to produce metastable phases, in this case the equilibrium eutectic mixtures of Zn-Mg₂Zn₁₁ are observed after rapid solidification, whereas the metastable eutectic mixture Zn-MgZn₂ forms under normal solidification conditions. However, in the melt-spun Zn-Mg alloy which is exactly at eutectic composition, three different structures are observed across the ribbon thickness. These three structures do not exist simultaneously in the same region, but structural transitions occur as the thickness increases from the wheel side to the free side. Eutectic and hypereutectic alloys show a tendency to form a metallic glass. In these alloys a critical growth velocity exists beyond which eutectic solidification is not possible, suggesting a possible transition from eutectic solidification to amorphous phase formation. The eutectic phase selection and the extent to which a specific microstructure is present depends on the variation in growth rate and solid-liquid interface stability during rapid solidification.[/p]     

Phase formation sequence of high-temperature Zn–4Al–3Mg solder

Eutectic Zn–4Al–3Mg alloy is one of the potential candidates as high-temperature lead-free solders. The phase formation sequence of eutectic Zn–4Al–3Mg alloy under different solidification conditions were investigated in this work. The results show that the microstructure is strongly affected by the difference of solidification conditions. The microstructure of the furnace-cooled eutectic Zn–4Al–3Mg alloy with a lamellar eutectic structure is composed of (α-Al + η-Zn)_eutectoid, Mg₂Zn₁₁ and η-Zn three phases, while the metastable MgZn₂ phase acts as primary phase during the rapid solidification of the air-cooled and water-cooled alloy specimens, and it evolves into the Mg₂Zn₁₁ phase later through a peritectic reaction ( $ {\text{MgZn}}_{ 2} + {\text{L}} \to {\text{Mg}}_{ 2} {\text{Zn}}_{ 1 1} $ ). Actually, the final solidified microstructure exhibited a feature of the primary MgZn₂ phase surrounded by the Mg₂Zn₁₁ phase due to the incompleteness of the peritectic transformation. Compared with the air-cooled eutectic Zn–4Al–3Mg alloy specimen, the water-cooled eutectic Zn–4Al–3Mg alloy microstructure displayed a dendritic structure resulting from more rapid cooling rate. Furthermore, the difference between the microhardness in the eutectic Zn–4Al–3Mg alloy under various solidification conditions was mainly attributed to the high-hardness phases concluding Mg₂Zn₁₁ and MgZn₂.     

Analysis on fluid permeability of dendritic mushy zone during peritectic solidification in a temperature gradient

Compared with the growing applications of peritectic alloys,none research on the fluid permeability K of dendritic network during peritectic solidification has been reported before.The fluid permeability K of dendritic network in the mushy zone during directional solidification of Sn-Ni peritectic alloy was investigated in this study.Examination on the experimental results demonstrates that both the temperature gradient zone melting (TGZM) and Gibbs-Thomson (G-T) effects have obvious influences on the morphology of dendritic network during directional solidification.This is realized through different stages of liquid diffusion within dendritic mushy zone by these effects during directional solidification.The TGZM effect is demonstrated to play a more important role as compared with the G-T effect during directional solidification.Besides,it is shown that the evolution of dendrite network is more complex during peritectic solidification due to the involvement of the peritectic phase.Through the specific surface Sv,analytical expression based on the Carman-Kozeny model was proposed to analyze the fluid permeability of dendritic mushy zone in directionally solidified peritectic alloys.In addition,it is interesting to find a rise in permeability K after peritectic reaction in both theoretical predication and experimental results,which is different from that in other alloys.The theoretical predictions show that this rise in fluid permeability K after pedtectic reaction is caused by the remelting/resolidification process on dendritic structure by the TGZM and G-T effects during peritectic solidification.     

Role of Ti diffusion on the formation of phases in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> brazed interface

Diffusivities of Ti, Cu, Al and Ag in the interface of Al₂O₃–Al₂O₃ braze joints using Ag–Cu–Ti active filler alloy, have been calculated by Matano–Boltzman method. The Matano plane has been identified for each elemental diffusion at various brazing temperatures. The diffusivities of Ag, Cu and Al are almost insignificant on formation of interface during brazing, whereas the diffusivity of Ti changes significantly with the brazing temperature and controls the formation of different reaction product in the interface. Presence of TiO and Ti₃Cu₃O phases in the interface has been confirmed by transmission electron microscopy (TEM).     

Phase relationships in the system NiO-WO3

The phase diagram of the system NiO-WO₃ was established by means of differential thermal analysis (DTA) supported by photoemission electron microscopy (PhEEM) and X-ray diffraction (XRD). The only compound, NiWO₄, melts incongruently (peritectic decomposition) at 1420° C and forms a eutectic with WO₃ at 73 mol % WO₃ and 1245° C. Primary phase crystallization as well as eutectic and peritectic solidification were studied by the comparison of the DTA measurements and the phase intergrowth morphologies as observed by PhEEM. The experimental liquidus lines are briefly discussed with reference to the calculated values.     

On secondary dendrite arm coarsening in peritectic solidification

A model for isothermal coarsening of secondary dendrite arms in peritectic reaction and transformation (liquid + primary-phase → peritectic-phase) is proposed to evaluate the secondary dendrite arm spacing (λ₂) of the primary phase in directional solidification of peritectic alloys. The model defines three stages for thin-arm dissolution (or thick-arm coarsening), i.e. the initial, intermediate and final stages: the initial thin-arm dissolution through the primary phase is sustained solely by the Gibbs–Thomson effect; the intermediate thin-arm dissolution through the peritectic phase is driven by Gibbs–Thomson effect but retarded by the peritectic reaction and transformation; the final dissolution through the primary and peritectic phases is enhanced by the Gibbs–Thomson effect and the phase transformation. The kinetics of peritectic reaction and transformation were found to be crucial to determine the thin-arm dissolution, which were characterized by the reaction constant (f) and the diffusion coefficient of solute in solid peritectic-phase (D_S), respectively. The present model shows that λ₂V^m is constant for a given Pb–Bi peritectic alloy, where V is growth velocity, and the factor, m, ranges from 1/3 to 1/2, rather than that normally observed (e.g. 1/3) for single-phase solidification. It is also notable that the calculated λ₂ for a Zn–7.37 wt.% Cu peritectic alloy was reasonably consistent with our earlier experiments for various growth velocities.     

Interdendritic microsegregation development of high-strength low-alloy steels during continuous casting process

A 1-D model based on DICTRA software was used to simulate Mn microsegregation in high-strength low-alloy steels during continuous casting. The experimentally determined (using a cumulative profiling method) segregation results were in good agreement with the modelling results. Steels undergoing solidification via a peritectic reaction had a larger segregation range than non-peritectic steels ascribed to trapping of the alloying atoms in liquid by austenite acting as a diffusion barrier. Subsequent, post-solidification cooling through the single phase austenite field decreased the microsegregation level, although the last γ→α phase transformation did affect the segregation profile in the solute-depleted dendrite centre. Simulation indicates that segregation levels could be reduced by decreasing either secondary dendrite arm spacing or adopting faster cooling rates through solidification.     

Microstructure development in isothermally melt-textured 123–211 composite materials

Microstructures of isothermally melt-textured 123–211 composite materials have been examined during the solidification process from quenched specimens. The distribution of 211 particles, the superconducting 123 grain boundary and the solid-liquid interface features are emphasized. The quality of the materials has been investigated by thermopower and thermal conductivity measurements. A seeded-melt texturing technique has been also tested in order to grow large single-domain of DyBa₂Cu₃O_7-y. The chosen seed was a Dy₂O₃ single crystal.     

Solidification parameters of the solid-liquid interface in crystal growth in response to vibration

The influence of vibration on the solidification parameters of the solid-liquid interface was investigated by applying a stable longitudinal sinusoidal vibration with different orders of resonant frequencies to the solidification system during crystal growth. Experimental results including the temperature profiles, temperature gradients of the liquid in front of the solid-liquid interface and the growth rates of the solid-liquid interface, are presented and analysed.     

Reactions of Hf-Ag and Zr-Ag alloys with Al2O3 at elevated temperatures

We are studying reactions of Ti, V, Zr, and Hf with ceramics as part of a program to understand fundamental reaction and bonding mechanisms in active metal brazing of ceramics. In this paper we present results of experiments with model systems comprising Ag alloys that contain different amounts of Hf or Zr that were reacted with sapphire or 99.6% alumina for different times and temperatures in a controlled atmosphere furnace. In these alloys the Ag functions as an inert solvent, which allowed us systematically to determine the effects of changes in concentration of the active element. We observed qualitative wetting and spreading tendencies of the alloys during heating and examined cross sections after cooling using electron analytical techniques. For all reaction times studied, the Hf/Ag alloys formed a discontinuous reaction layer, which was consistent with earlier high-resolution electron microscopy that showed sub-micrometer HfO₂ particles embedded in the surfaces of the Al₂O₃ grains. By contrast, initial reaction of the Zr/Ag alloys with Al₂O₃ produced a continuous interface layer. With longer reaction times, the ZrO₂ reaction product became much thicker and exhibited three distinct zones at the interface. The results suggest that the rate limiting step in the Zr/Ag reaction is the chemical reaction at the interface, whereas with Hf/Ag reaction diffusion of products away from the interface is rate limiting.     

Evolution of Ag3Sn compound formation in Ni/Sn5Ag/Cu solder joint

Ag₃Sn compound formation was observed to evolve from the Cu side to the Ni side in Ni/Sn5Ag/Cu solder joint upon reflow process. The transformation of the interfacial compound phase at the Sn5Ag/Ni interface, i.e., from Ni₃Sn₄-based compound to Cu₆Sn₅-based compound, changes the interfacial energy state, which is the main driving force for the evolution of Ag₃Sn compound formation. The evolution consequence of Ag₃Sn compound suggests that Sn5Ag/(Cu,Ni)₆Sn₅ interface is the best energy-preferential heterogeneous nucleation site for the Ag₃Sn compound phase, comparing to the Sn5Ag/Cu₆Sn₅ interface and the Sn5Ag/Ni₃Sn₄ interface. In addition, the lower Cu concentration near the Ni side is another added driving force for the heterogeneous nucleation of Ag₃Sn phase at Sn5Ag/(Cu,Ni)₆Sn₅ interface.     

Microstructure evolution and interfacial reaction of TiAl–Si alloy solidified in alumina crucible

The microstructure evolution of Ti–43Al–3Si (at-%) alloy solidified in alumina crucible was investigated by directional solidification technology. After directional solidification, the microstructure of the alloy is consisted of γ-TiAl, α₂-Ti₃Al, ξ-Ti₅Si₃ phases and Al₂O₃ particles. There are three morphologies of ξ phases formed in the alloy, namely, long rod-like, cluster-like with eutectic morphology, and needle-like shape. The volume fraction of ξ phases decreases with increasing growth rates. Al₂O₃ particles broke from the crucible and enter into the melt by the thermal physical erosion. Al₂O₃ particles enrich in the liquid phase with the moving of solid-liquid interface, and are captured or entrapped by dendrites during solidification. The Al₂O₃ particles mainly distributed in the interdendritic region, and some particles exist in dendrites.     

The solidification behaviour of melts in the system Zn2SiO4-Mg2SiO4

The melting and solidification behaviour of various compositions across the system Zn₂SiO₄- Mg₂SiO₄ (willemite-forsterite) was studied by means of photoemission electron microscopy, X-ray diffraction and DTA. The results revealed the peritectic type of the phase system instead of the earlier assumed eutectic one. During the formation of willemite solid solutions by rapid solidification of zinc-silicate-rich melts, grain-boundary enrichment of zinc oxide besides magnesium silicate takes place. From melts richer in magnesium silicate, forsterite solid solution forms primarily. The sluggish process of its peritectic decomposition during cooling and the preferred subsolidus formation of zinc silicate give rise to the formation of a ternary non-equilibrium phase assemblage even at slow cooling.     

Formation of intermetallic compounds in Mg–Ag–Al joints during diffusion bonding

In this study, we conducted the diffusion bonding of Mg and Al alloys using a 30-μm-thick pure silver foil interlayer at median temperatures between 390 and 490 °C. We obtained a multilayered structure across the Mg–Ag–Al joint: Mg/Mg(ss, Ag)/Mg₃Ag/MgAg/Ag/Ag(ss, Al)/Ag₂Al/Al. The silver diffusion barrier prevented the formation of brittle intermetallics between Mg and Al. Intermetallics identified at the joint interface include the more ductile types between Mg and Ag, ε-Mg₃Ag and β′-MgAg, and Ag and Al, δ-Ag₂Al. As the bonding temperature increased, Ag₂Al, followed by MgAg, favored the growth of Mg₃Ag IMC layer. The shear strength of the joints increased with the rising bonding temperature to a maximum value of 11.8 MPa at 470 °C. Fracture failure in the joints mainly occurred in the Ag₂Al layer. The formation mechanism for interfacial layers in the joints is believed to consist of four stages: (1) solid-solution formation, (2) Mg–Ag IMC formation, (3) Ag–Al IMC formation, and (4) growth of Mg–Ag and Ag–Al IMCs.     

Solidification and microstructure of ZA27/SiCp composite fabricated by mechanical–electromagnetic combination stirring process

Abstract

The solidification behaviours and microstructural characteristics of both ZA27/SiC_p composites and monolithic ZA27 alloy were studied by using differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and X-ray diffraction. It was found that there were differences in the transformation temperature and volume fraction of the phases, although the solidification process was almost identical for the composite and the monolithic alloy. The incorporation of SiC particles in the ZA27 alloy led to slight refinement of primary grains and reduced volume fraction of eutectic-like phase. The SiC particles obstructed Zn diffusion in the residual melt during the formation of proeutectic β phase, but promoted Zn diffusion from (Al) to η (Zn) phase during eutectoid transformation. During solidification, Cu was mainly segregated in the final solidification regions; Mg was present not only in the matrix but also on SiC particles; and oxide inclusions were mainly distributed around SiC particles. The matrix microstructure for both materials mainly consisted of primary cores of Al rich +η eutectoid; β′ phase resulting from the eutectoid transformation of the proeutectic β phase; and Zn rich +η eutectoid resulting from the eutectoid transformation of the eutectic-like phase. The SiC particles were mainly distributed around the primary grains. Several new phases based on the Al–Zn–Mg–Cu system and interfacial reaction products, including Al₂₁Fe₃Si, Cu₅Zn₈, Mg₆Cu₃Al₇, MgAl₂O₄, and amorphous oxide inclusions, were identified in the final solidification regions. The nucleation of both primary phase and eutectic-like +η phase at the surface of SiC particles and their crystallographic orientation relationships were investigated theoretically and experimentally. No distinct crystallographic orientation relationship between the matrix and SiC has been identified, although the mismatch between (0001)_SiC and (111)_ was calculated to be as small as 7·6%.     

Selective laser melting of Zn–Ag alloys for bone repair: microstructure,mechanical properties and degradation behaviour

Zn possesses good biodegradability and biocompatibility, but its strength and hardness are insufficient for bone implants. In this study, Ag was introduced into Zn to improve the mechanical properties by selective laser melting. The results showed that Ag was dissolved in Zn, which generated constitutional undercooling in front of the advancing solid/liquid interface during solidification, making more nucleation events occur and thus refining the grains. When Ag content exceeded its solid solubility in Zn, AgZn₃ phase is formed, which acted as active nucleation sites for Zn grains, further refining the grains. The refinement of the grains effectively hindered the plastic deformation and dislocation. As a result, the compressive strength and hardness were improved by about 100% and 116%, respectively. When Ag content continued increasing and became excessive, AgZn₃ phase grew rapidly, coarsening the grains. Accordingly, the mechanical properties slightly decreased. These results demonstrated that the Zn–Ag alloys are potential implant biomaterials.