Late Stage Of Solidification Research Articles

Microstructure evolution in laser alloyed layer of Cu–Fe–Al–Si on Cu substrate

Laser alloyed layers of Cu–Fe–Al–Si were prepared using an 8 kW $CO_2$ laser on copper substrate and a detailed TEM investigation of the cross-section microstructure was carried out. This paper elucidates on the sequence of evolution of various phases constituting a complex microstructure due to non-equilibrium solidification of the laser alloyed layer. The formation of micron size Fe-rich globules having $DO_3$ structure within Cu-rich fcc matrix suggests access to a submerged miscibility gap during the laser processing. The solute rejection processes during later stages of solidification and solid state cooling result in the evolution of submicron size Cu-rich dispersoids inside the Fe-rich globules and the Fe-rich nanoscale fully coherent precipitates within the surrounding Cu-rich matrix.

Experimental determination of the interfacial heat transfer during cooling and solidification of molten metal droplets impacting on a metallic substrate: effect of roughness and superheat

The interfacial heat transfer between a solidifying molten metal and a metallic substrate is critical in many processes such as strip casting and spray deposition. As the molten metal cools down and solidifies, the interface undergoes a change from the initial liquid/solid contact to a solid/solid contact, leading to very dynamic variations in the rate of interfacial heat transfer. This article presents the results of an experimental study of the contact heat transfer when molten nickel or copper droplets are dropped on an inclined metallic substrate. The interfacial heat transfer coefficient, h, between the melt and the substrate is evaluated by matching model calculations with the top splat surface temperature history measured by a fast-response pyrometer. The results suggest that a high value of the interfacial heat transfer coefficient h (10 4 to 3×10 5 W/m 2 K) is achieved when the molten splat is in contact with the substrate, followed by a smaller value (<10 4 W/m 2 K) during the later stages of solidification and the solid cooling phase. A parametric study was performed to investigate the effect on h of the metal/substrate materials combination, the melt superheat, and the substrate surface roughness, and some of the results are also presented.

Direct observation of the crystal-growth transition in undercooled silicon

Using an electromagnetic levitation facility with a laser heating unit, silicon droplets were highly undercooled in the containerless state. The crystal morphologies on the surface of the undercooled droplets during the solidification process and after solidification were recorded live by using a high-speed camera and were observed by scanning electron microscopy. The growth behavior of silicon was found to vary not only with the nucleation undercooling, but also with the time after nucleation. In the earlier stage of solidification, the silicon grew in lateral, intermediary, and continuous modes at low, medium, and high undercoolings, respectively. In the later stage of solidification, the growth of highly undercooled silicon can transform to the lateral mode from the nonlateral one. The transition time of the sample with 320 K of undercooling was about 535 ms after recalescence, which was much later than the time where recalescence was completed.

Quasicrystals and related approximant phases in Mg–Zn–Y

As-cast microstructure of Mg-rich Mg 68Zn 28Y 4 has been investigated by a detailed transmission electron microscopy study. The as-cast Mg 68Zn 28Y 4 alloy consisted of three different types of phases: 10–20 μm size primary solidification phase, dendritic phase grown from the primary phase and a eutectic structure formed at the later stage of solidification. The primary solidification phase has an icosahedral structure with a large degree of phason strain. 1/1 rhombohedral approximant phase with lattice parameters a=27.2 A ̊ and α=63.43° is first observed in Mg–Zn–Y system. The rhombohedral structure can be obtained by introducing phason strain in the six-dimensional face centered hyper-cubic lattice. The decagonal phase nucleates with orientation relationship with the icosahedral phase, and Mg 4Zn 7 nucleates with orientation relationship with the decagonal phase, indicating a close structural similarity between the three phases. Gradual depletion of Y during solidification plays an important role in heterogeneous nucleation of decagonal and Mg 4Zn 7 phases from icosahedral and decagonal phases, respectively.

Formation of a single quasicrystalline phase in an Al55Cu25.5Fe12.5Be7 alloy

An alloy with a single quasicrystalline icosahedral phase (I phase) can be produced by partial replacement of Al with Be (7at.%) in an Al62Cu25.5Fe12.5. alloy. The I phase nucleates first and then grows in a dendritic manner and the remaining liquid solidifies into the metastable t phase at a later stage of solidification during conventional casting with a Cu mould. With a lower cooling rate (crucible cooling), the liquid melt solidifies into an almost single I phase without heat treatment. The present study shows that the addition of Be stabilizes the I phase and expands the I-phase region. The Al55Cu25.5Fe12.5Be7I phase exhibits a highly ordered F-type structure like the Al-Cu-Fe I phase, and the intensities of the superlattice peaks in the electron or X-ray diffraction patterns do not vary on the addition of Be.

Numerical Simulation of Interface Shape around an Insoluble Particle for Fe-C Alloys Using a Phase-field Model.

The first trial for numerical simulations of solid/liquid interface shape change around an insoluble particle for Fe-C alloys has been carried out. Real-time interface shape evolution during unidirectional solidification has been calculated using the phase-field model with parameters determined by thin interface limit condition. The interface becomes concave near the particle because of solute enrichment in the liquid between interface and a particle. Liquid trench ahead of the particle and thin liquid film along its surface remain until the late stage of solidification. The results are qualitatively in good agreement with the reported experimental observations. The effects of interface velocity and initial carbon content on the interface shape are also examined. Using the obtained interface shapes, the forces to the particle are estimated and the particle engulfment behavior by interface is discussed.

Solidification of binary hypoeutectic alloy matrix composite castings

We consider a binary hypoeutectic alloy casting which solidifies in dendritic form in an unreinforced engineering casting and seek to predict its microstructure in a metal matrix composite. We focus on the case where the reinforcement is fixed in space and fairly homogeneously distributed. We assume that the reinforcement does not catalyze heterogeneous nucleation of the solid. We show that the reinforcement can cause several microstructural transitions in the matrix alloy, depending on the matrix cooling rate, the width, A, of interstices left between reinforcing elements, and the initial velocityV of the solidification front. These transitions comprise the following: (1) coalescence of dendrite arms before solidification is complete, causing solidification to proceed in the later stages of solidification with a nondendritic primary phase mapping the geometry of interstices delineated by reinforcement elements; (2) sharp reduction or elimination of microsegregation in the matrix by diffusion in the primary solid matrix phase; and (3) a transition from dendrite to cell formation, these cells featuring significant undercoolings or a nearly plane front configuration when reinforcing elements are sufficiently fine. Quantitative criteria are derived for these transitions, based on previous work on composite solidification, observations from directional solidification experiments, and current solidification theory. Theory is compared with experimental data for aluminum-copper alloys reinforced with alumina fibers and for the dendrite to cell transition using data from directional succinonitrile-acetone solidification experiments. Theory and experiment show good agreement in both systems.

Precipitation Behavior of Fe-Al-O Inclusions under Unidirectional Solidification of Fe-30mass%Ni Alloys Saturated with CaO-Al2O3 Slags.

The inclusions composition, size, morphology and distribution in an Fe-30mass%Ni alloy were studied as a function of the initial solute contents, which were controlled by the CaO-Al2O3 slags, and of the solidification parameters. The Al2O3-rich Al2O3-CaO inclusions were observed at the initial contents of Al=300-400 and O<6, while the FeO and the FeO-rich FeO-Al2O3 inclusions were observed at the initial contents of Al=2-6 and O=20-130 on mass ppm base. The FeO-rich FeO-Al2O3 inclusions were located in later stage of solidification in the intercellular or the interdendritic region. And most of the Al2O3-rich Al2O3-CaO inclusions were located in later stage of solidification, but some distributed in the intercellular or the interdendritic region. The dependence of the composition and the size of these inclusions on the cooling rate was not observed. The precipitation of FeO and Al2O3 was discussed by using the solute segregation models.

Cumulates from strongly depleted mid-ocean-ridge basalt

RECENT studies of abyssal peridotites1, mid-ocean-ridge basalts (MORBs)2 and their entrained melt inclusions3,4 have shown that fractional melting of the upwelling sub-oceanic mantle produces magmas with a much wider range of compositions than erupted MORBs. In particular, it seems that strongly depleted primary magmas are routinely produced by melting beneath ridges1. The absence of strongly depleted melts as erupted lavas prompts the question of how long such magmas survive beneath ridges, before their distinctive compositions are concealed by mixing with more enriched magmas. Here we report mineral compositions from a unique suite of oceanic cumulates recovered from DSDP Site 334 (ref. 5), which indicate that the rocks crystallized from basaltic liquids that were strongly depleted in Na, Ti, Zr, Y, Sr and rare-earth elements relative to any erupted MORB. It thus appears that the magmatic plumbing system beneath the Mid-Atlantic Ridge permitted strongly depleted magmas to accumulate in a magma chamber and remain sufficiently isolated to produce cumulate rocks. Even so, spatial heterogeneity in the compositions of high-calcium pyroxenes suggests that in the later stages of solidification these rocks reacted with infiltrating enriched basaltic liquids.

Microstructure of Ni-10Co-8Cr-4W-13Zr alloy and its bonding behaviour for single-crystal nickel-base superalloy

In the later stages of solidification of zirconium-containing superalloys, the concentration of zirconium in the interdendritic melts is above 10 wt%. The γ dendrites formed in the early stage of solidification may be considered to be joined by the interdendritic zirconium-rich melt. Based on the composition of the zirconium-rich melt, an Ni-10Co-8Cr-4W-13Zr (wt %) alloy was selected as an interlayer alloy for brazing and transient liquid-phase (TLP) bonding of single crystal superalloys. All the elements in the interlayer alloy are beneficial to the single-crystal superalloys. A bonding microstructure which is very similar to that of the base alloy was obtained by means of the TLP process using this interlayer alloy. In the present work, the microstructural characteristics of the interlayer alloy and the phase relationships in the bond during brazing and TLP processes were investigated.

Characterisation of interfaces in Al–Zn–Mg alloy reinforced with Sicabo fibres

The microstructure of a metal matrix composite consisting of an Al–Zn–Mg alloy reinforced with SiC coated boron fibres has been examined by electron microscopy, electron probe microanalysis, and by optical microscopy. Considerable amounts of Mg2Si phase were found to be segregated at the fibre/matrix interface. This intermetallic was not formed by a reaction between the fibre and matrix during the fabrication process, a liquid infiltration technique, but as a result of silicon impurities present as contaminants in the melt. It was concluded that the interface phase was precipitated from the metal matrix in the later stages of solidification without any nucleation role being played by the fibre. The Mg2Si phase appears to be brittle and was present in amounts likely to have a deleterious effect on the strength of the composite.MST/871

Characterisation of interfaces in Al–Zn–Mg alloy reinforced with Sicabo fibres

The microstructure of a metal matrix composite consisting of an Al–Zn–Mg alloy reinforced with SiC coated boron fibres has been examined by electron microscopy, electron probe microanalysis, and by optical microscopy. Considerable amounts of Mg2Si phase were found to be segregated at the fibre/matrix interface. This intermetallic was not formed by a reaction between the fibre and matrix during the fabrication process, a liquid infiltration technique, but as a result of silicon impurities present as contaminants in the melt. It was concluded that the interface phase was precipitated from the metal matrix in the later stages of solidification without any nucleation role being played by the fibre. The Mg2Si phase appears to be brittle and was present in amounts likely to have a deleterious effect on the strength of the composite.MST/871

Geochemistry of the granitic rocks from the Kwandonkaya Complex, northern Nigeria

The Kwandonkaya Complex forms one of the most prominent and typical ring structures in the Centre of Jos, Plateau State, Nigeria. Lithologically, it is composed of amphibole and biotite granites emplaced within Archean basement gneisses and migmatites. The chemical and mineralogical characteristics of this complex suggest that it was derived from crustal source materials. Trace elements are high in Rb, Li, F, Sn, Nb, W and low in Sr and Ba compared to the world average for rocks of similar composition indicating a high degree of fractionation. However, the K Rb ratios and petrographic evidence indicate that recrystallization may have redistributed the elements after the initial solidification of the rocks particularly in the biotite granites; and that the distribution of tin may have been initially controlled by crystal-silicate melt equilibra. Tin probably preferentially entered the aqueous phase during the late stages of solidification. It was the interaction between this phase and early formed minerals that led to greisenization and tin-tungsten mineralization. Although total rock chemical composition show no marked trends the composition of the biotites and hornblende define a trend in the compositional triangle Fe 3+Fe 2+Mg. When the biotites are related to the experimentally studied ternary system KFe 3 3+AlSi 3O 12(H−1)KFe 3 2+AlSi 3O 10(OH) 2KMg 3AlSi 3O 10(OH) 2 and coupled with the estimated position of biotite solid solutions for different oxygen buffers, the trend suggests that oxygen fugacities in the magmas during biotite formation were slightly higher than those defined by the Fe 2SiO 4SiO 2Fe 3O 4 buffer. The ratio of ferric to ferrous iron in pairs of the minerals from the Kwandonkaya Complex is the same for both hornblende and biotite and suggests crystallization in a closed but buffered environment.

Rapid solidification microstructures in austenitic FeNi alloys

The microstructures of rapidly solidified austenitic FeNi alloys have been investigated by a combination of scanning and transmission electron microscopy, X-ray diffraction and X-ray microanalysis. Temperature measurements during rapid solidification have been used to interpret the results. Rapid solidifiation is shown to be a non-Newtonian cooling process, with high liquid undercooling before the onset of solidification, followed by adiabatic recalescence while solidification takes place. Detailed composition measurements provide direct evidence that high liquid undercooling causes massive segregation-free solidification, but recalescence can produce a breakdown in the later stages of solidification to a cellular, segregated solidification structure.

Origins of dendritic AIN precipitates in aluminium-killed-steel castings

The precipitation of aluminium nitride on as-cast grain boundaries can be a major cause of intergranular fracture in aluminium-killed-steel castings. It has long been known that AIN can exist in two idiomorphic forms (plates and dendrites) and it had been implied that both are precipitated in the solid state. In this paper directional-solidification studies, which were performed using a 0·30C-1·50Mn- 0.17Al-0·017N steel, are described. The results show that the dendritic form of AIN precipitates from the residual liquid during solidification, and this has been modelled using available solubility data. It has been demonstrated that AIN precipitation can occur in the solute-enriched liquid at a late stage of solidification, even in cases where the bulk AI and N contents are relatively low.

An experimental study of the decomposition of steenstrupine in Na2CO3 solution

This study is part of a more detailed investigation of the stability and the natural decomposition process of steenstrupine, initiated to help delineate the conditions during the later stages of solidification of the Ilimaussaq rocks. Interest in steenstrupine also arises from the potential importance regarding Kvanefjeld as a uranium ore reserve, steenstrupine being the most important uranium-bearing mineral in the Kvanefjeld area (Sørensen et al., 1974; Makovicky, 1981). Major emphasis has been placed on the decomposition of steenstrupine in Na2CO3 solution in the 200° to 600°C range at pressures up to 1000 bars.

Cu-Sn合金における包晶&amp;beta;相の成長

The non-equilibrium peritectic transformation of Cu-Sn alloys was verified by experiment and calculation assuming a diffusion controlled thickening process of the peritectic β-phase. The unstable liquid and the α-solid solution remain in the hypo- and hyper-peritectic alloys respectively, when cooled down to just below the peritectic isotherm. The fact shows occurance of the non-equilibrium peritectic transformation. The rate of peritectic β-phase thickening by diffusion in the β-phase increases with undercooling from the peritectic isotherm 1072 K during holding at the temperature between two peritectic isotherms (1072 and 1029 K) and with the cooling rate during continuous cooling. If the peritectic β-phase to be formed is of large thickness or the cooling rate is great, solidification of Cu-Sn alloys completes with direct crystallization of secondary β-phase from super-cooled liquid at the later stage of solidification.

大型扁平鋼塊の内質におよぼす鋼塊形状の影響

Influence of shape, dimensions, and hot-topping conditions of 23 to 90 ton large flat ingots on the occurrence of heterogeneities such as channel-type and centerline segregations and centerline loose structure has been made clear to optimize the design of HSLA steel ingots for extra-heavy plates.Critical values of local solidification rate and local cooling rate for the formation of the channel-type segregation have been evaluated with the aid of heat-transfer calculation on these ingots. A speculative model for the later stage of solidification has also been presented which gives reasonable interpretation of the formation of the centerline heterogeneities characteristic in these large flat ingots.

Fractional crystallization and the origin of tin deposits in granitoids

A comparison between tin-bearing granitoids in an anorogenic setting (Bushveld Complex) and an orogenic setting (Blue Tier Batholith, Tasmania) reveals a number of genetically important similarities. These include: in situ fractional crystallization characterised by marked decrease in Ba and Sr and increase in Rb; the accumulation of late melt in a sheet-like form near the roof zone; the association of barren pegmatites overlying the ore; and of aplites; and the occurrence of conformable tin-bearing sheets, often exhibiting greisenization. These features allow the formulation of the following genetic model. A crustally-derived granitoid magma is emplaced and undergoes fractional crystallization from the margins inwards, with bottom crystallization dominating. Disruption of earlier formed solids by rest liquid commonly occurs. Continued fractional crystallization causes enrichment in volatiles and incompatible elements in the late rest melts, which have a sheet-like habit. The efficiency of enrichment of incompatible elements is critically dependant on the degree of separation of melt from solids throughout crystallization. An early, tin-poor vapour may separate after initial water-saturation of the magma is achieved, and this collects under the roof, commonly forming an impermeable barrier to later tin-bearing fluids. Continued fractional crystallization on the floor further enriches incompatible elements, and at a very late stage a Sn-rich vapour separates within the intercumulus phase and becomes concentrated by progressive crystallization of the intercumulus melt. At a late stage of solidification, this vapour loses equilibrium with the earlier formed feldspars and greisenization ensues, accompanied by the crystallization of cassiterite and other ore minerals. The nature of the mineralization changes if through-going fractures tap the late fluids. This model predicts systematic changes in trace element geochemistry with crystallization which provide useful tools for assessing the tin potential of a granitoid, and for indicating the direction of crystallization of the magma, and hence the location of possible ore.

Two-Dimensional Flow of Liquid During the Late Stage of Solidification of Alloys

This is a mathematical analysis of the two-dimensional interdendritic liquid flow occurring during late stages of solidification, i.e., when the liquidus front reaches the center of the slab. The liquid flow considered is that caused by density change accompanying freezing. The liquid-solid region is considered a porous medium where the liquid permeability is a function of the solid fraction. Analytical solution for the two-dimensional velocity and pressure fields in solidifying Al-4.5 percent Cu alloy is obtained from a mathematical model which includes the concept of conservation of mass and Darcy’s law to correlate the pressure and velocity. Assuming that there is no gas-bubble formation and that the solidified metal in the mushy zone remains rigid, the liquid moving with a creeping velocity in the interdendritic regions is under tension (when no external pressure is applied). The magnitude of this tension increases with increasing depth of the solidifying ingot and is a strong function of the cooling rate. For example, a negative pressure of the order of 100 atm in the interdendritic liquid of solidifying Al-4.5 percent Cu alloy is estimated at ∼90 percent solidification. On the basis of the present analysis, an estimate can be made of pressures required to suppress blow-hole formation during the later stages of freezing arising from the solidification shrinkage.