Peritectic Alloys Research Articles

PHASE SELECTION AND THE SOLIDIFICATION CHARACTERISTICS OF TiAl BASE ALLOYS IN THE NONEQUILIBRIUM SOLIDIFICATION

The Al content and thestabilizers in TiAl base alloys were investigated, which have an important effect on the solidification characteristics and phase selection. TiAl base alloys with single �-solidifying have the characteristics of low Al content and narrow crystallization temperature range. It is attributed to forming a homogeneous fine-grained microstructure. Dendrite morphology in TiAl base alloys is investigated by SEM-BSE in the nonequilibrium solidification conditions. The relationships between undercooling and nucleation of two phases are investigated systematically by using the classical nucleation theory in the undercooled Ti48Al2Cr2Nb (atomic fraction, %) peritectic alloy. By calculating the interfacial critical nucleation work and steady state nucleation rates ofphase (bcc) andphase (hcp), it is obtained thatphase is always prior to nucleate from the Ti48Al2Cr2Nb undercooled melts at the cooling rate about 15 K/s in the achieved undercooling range. Due to narrow crystallization temperature range, microstructures of TiAl base alloys with high Al content as the hyper-peritectic solidification path take on the gradual solidification style and finer dendrite morphology. It is conducive to reducing and eliminating shrinkage porosity and hot cracking caused by mushy solidification. Thestabilizers have an important effect on solidification path depending on its Al equivalent and will change the solidification path of TiAl alloy from hyper-peritectic solidification to hypo-peritectic solidification. The overmuch Al content in TiAl base alloys leads to the formation

Influence of high magnetic fields on phase transition and solidification microstructure in Mn-Sb peritectic alloy

以Mn-56.5 wt%Sb包晶合金为研究对象, 进行了不同磁场、不同冷速条件下的凝固实验. 通过对液相线温度、包晶温度的考察, 发现强磁场可以提高Mn-56.5 wt%Sb合金的液相线温度, 且该上升值随磁感应强度的增加而增加, 当所施加的磁感应强度为11.5 T时, 液相线温度升高大约3 ℃, 但施加磁场后包晶反应温度没有明显改变. 对该合金的凝固组织进行定量金相分析发现, 施加磁场后MnSb相明显减少, 该结果与磁场对相变温度的影响相一致. 另外通过X射线衍射分析发现, 强磁场诱发包晶反应生成相MnSb的c轴垂直于磁场方向取向, 而Mn2Sb相的(311)面平行于磁场方向取向. 对不同冷速凝固的Mn-56.5 wt%Sb合金组织进行定量金相分析结果显示, 强磁场对合金凝固过程的作用效果受到冷却速度的影响. 随着冷却速度的增加, 强磁场对该合金凝固组织中MnSb相的相对含量变化影响效果减弱.

Effect of peritectic reaction on the migration of secondary dendrite arms in the presence of tertiary dendrites: analysis of a directionally solidified Sn–36 at.%Ni peritectic alloy

Directional solidification experiments have been performed on Sn–36 at.%Ni peritectic alloy in a constant temperature gradient at different growth velocities. Experimental result shows that a “sawtooth” morphology forms on secondary dendrite arms during the migration of secondary dendrites in the presence of tertiary dendrite arms. A theoretic model is therefore proposed to describe the formation of this “sawtooth” morphology in the peritectic solidification with tertiary dendrite arms taken into consideration. The migration of secondary dendrite arms is caused by remelting/solidification at the hot/cold sides of a liquid pool between secondary dendrite arms, which is a form of temperature gradient zone melting. And, the “sawtooth” morphology is ascribed to the difference in remelting velocity at the hot side of liquid pool during the migration of secondary dendrite arms due to the presence of tertiary dendrite arms. In addition, the proceeding of peritectic reaction can accelerate the formation of “sawtooth” morphology.

Primary dendrite distribution in directionally solidified Sn–36 at.% Ni peritectic alloy

Abstract

Unsteady-State Directional Solidification of a Hypoperitectic Pb-9.5wt%Bi Alloy

Although considerable attention has been paid to studies on the unidirectional solidification of peritectic alloys, most of these investigations are carried out under steady-state solidification, where both the growth rate and the thermal gradient can be independently controlled and held constant in time. In this work, a hypoperitectic Pb-9.5wt%Bi alloy was directionally solidified under unsteady-state heat flow conditions and the microstructure evolution was analyzed. Continuous temperature measurements in the casting were monitored during solidification, using a data acquisition system and a bank of six type J thermocouples positioned along the casting length. Thermal parameters such as the growth rate (v) and the cooling rate () were experimentally determined by the experimental cooling curves. The solidification microstructure was characterized by a dendritic morphology along the entire casting length. The primary (l1) and secondary (l2) dendrite arm spacings were measured and experimental growth laws relating them to the solidification thermal parameters v and are proposed.

Microstructure Evolution in Directionally Solidified Fe-Ni Alloys in Diffusive Regime

Directional solidification experiments have been carried out in Fe-Ni peritectic alloys to study microstructure evolution in diffusive regime. A numerical modeling of melt convection was developed and discussed to investigate the convective velocity in samples of different diameters. The simulation results show that convection effects can be reduced by decreasing the sample diameter, and diffusion-controlled growth can be achieved if the sample diameter is smaller than about 1 mm. Based on the simulation results, experiments were performed in thin samples of 1 mm diameter to obtain microstructures in diffusive regime. Different kinds of microstructure evolutions were observed in the directionally solidified Fe-Ni alloys. The time-dependent microstructure evolution implies that the solidification process seemed to be in non-steady state rather than in steady state. Based on the transient model, solute distributions in the liquid ahead of the primary and peritectic phases were discussed to reveal the microstructure evolution. Since the solute partition coefficients of the primary and peritectic phases are different, the magnitudes of solute element rejected by the two solid phases are different in two-phase growth conditions. And within the boundary layer, the solute fields ahead of the primary and peritectic phases are different owing to the weak effect of solute mixing in diffusion-controlled regime. Furthermore, microstructure evolution in peritectic alloys was discussed based on the analysis of solute redistribution.

Influence of initial solid–liquid interface morphology on further microstructure evolution during directional solidification

Preparation of the initial solid–liquid interface on which growth is started is a very critical step in directional solidification experiments. Dedicated experiments concerning preparation of the initial solid–liquid interface morphology and its influence on further directionally solidified microstructure were performed on Cu-20 wt% Sn peritectic alloy in a Bridgman-type furnace. To verify the morphology of the initial solid–liquid interface, steady-state directional dendritic growth was interrupted by thermal stabilization ranging from 0 to 1 h prior to quenching. With thermal stabilization duration increase, the solid–liquid interface morphology degenerated from dendritic to cellular and finally to planar. To verify the influence of the initial state on further solidification microstructure, directional solidification experiments were performed at a low pulling rate of 1 μm/s with different initial solid–liquid interface morphologies. The initial state affects solute redistribution and formation of peritectic coupled growth structure in the subsequent directional solidification process.

Banding structure formation during directional solidification of Pb–Bi peritectic alloys

Directional solidification experiments on Pb–Bi peritectic alloys were carried out at very low growth rate (v=0.5 μm/s) and high temperature gradient (G=35 K/mm) in an improved Bridgman furnace. The banding structures were observed in both hypoperitectic and hyperperitectic compositions (Pb–xBi, x=26%, 28%, 30% and 34%). Tree-like primary α phase in the center of the sample surrounded by the peritectic β phase matrix was also observed, resulting from the melt convection. The banding microstructure, however, is found to be transient after the tree-like structure and only the peritectic phase forms after a few bands. Composition variations in the banding structure are measured to determine the nucleation undercooling for both α and β phases. In a finite length sample, convection is shown to lead only to the transient formation of bands. In this transient banding regime, only a few bands with a variable width are formed, and this transient banding process can occur over a wide range of compositions inside the two-phase peritectic region.

The effect of the flow driven by a travelling magnetic field on solidification structure of Sn–Cd peritectic alloys

The effect of the flow driven by a travelling magnetic field (TMF) on solidification structure of Sn–1.8wt% Cd peritectic alloy was investigated numerically and experimentally. A new TMF generator consisting of three co-coils and a cooling system has been designed and applied successfully. The corresponding radial and axial magnetic field intensity in the inner space of the TMF generator are measured by teslameter. Numerical results indicate that the flow velocity under a downward TMF is smaller than that under an upward TMF. The experimental results demonstrate that α island structures in β matrix are mainly observed under the 6.3mT and 10.3mT upward TMF, and the number of α island phase increases with increasing magnetic field intensity. Banded structure is observed under the 6.3mT and 10.3mT downward TMF, and the number of bands increases with increasing magnetic field intensity. This should be attributed to the different flow intensity at the interface front under an upward/downward TMF.

Cellular growth during the transient directional solidification of Zn-rich Zn–Cu monophasic and peritectic alloys

Zn–Cu alloys located in the monophasic and hypoperitectic ranges of compositions of Zn-rich Zn–Cu alloys were directionally solidified under unsteady-state heat flow conditions. The experimental cooling curves allow solidification thermal parameters: tip cooling rate (Ṫ), tip growth rate (VL) and temperature gradient (GL) to be experimentally determined. The observed microstructural evolution of both alloys has shown that a regular cellular morphology prevails along the whole castings lengths. Only the regions very close to the cooled casting surface showed the presence of plate-like cells due to very high cooling rates (higher than 25K/s). The cell spacing (λc) was measured along the castings lengths, and experimental correlations between λc and experimental solidification thermal parameters have been established. Power laws with −0.55 and −1.1 exponents expressing λc as a function of Ṫ and VL, respectively, were found to better represent the growth of cells under transient heat flow conditions for both alloys experimentally examined. The predictions furnished by the Hunt–Lu model underestimate the experimental cell spacings found for both alloys examined. It was shown that the proposed equations relating λc as a function of Ṫ are able to represent both the steady-state and unsteady-state cellular growth of monophasic and peritectic Zn-rich Zn–Cu alloys.

Electrochemical behavior of Zn-rich Zn–Cu peritectic alloys affected by macrosegregation and microstructural array

The aim of this experimental investigation is to evaluate the electrochemical behavior of an as-cast Zn-rich Zn–Cu peritectic alloy as a function of both the solute macrosegregation profile and the microstructure cellular spacing. A directional solidification apparatus was used to obtain the as-cast samples. Electrochemical impedance spectroscopy (EIS), potentiodynamic polarization techniques and an equivalent circuit analysis were used to evaluate the corrosion resistance in a 0.5M NaCl solution at 25°C. It was found that both copper content and cell spacing are significantly affected by the position along the casting length from the cooled bottom of the casting. These parameters play interdependent roles on the resulting corrosion behavior. Samples which are closer to the casting cooled surface are more affected by the solute inverse segregation, i.e. have a Cu content that is higher than the alloy nominal composition. This is shown to be a driving-force leading to a decrease in the corrosion resistance. However, for other samples with similar Cu content and close to the nominal alloy composition, the cell spacing seems to be the driving-force associated with the corrosion resistance and for these cases finer microstructures are shown to be related to higher corrosion resistance.

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.

Competition of the primary and peritectic phases in hypoperitectic Cu–Sn alloys solidified at low speed in a diffusive regime

Directional solidification experiments on hypoperitectic Cu–Sn alloys have been performed at very low velocity in a high thermal gradient to ensure planar front growth of both phases. The diameter of the sample has been reduced to 500μm to strongly reduce convection. Lamellar and fibrous peritectic cooperative growth of the primary α- and peritectic β-phases has been observed on length spanning several millimeters. For the first time in a high solidification interval peritectic alloy, a quenched interface of both phases in contact with the liquid has been obtained. An unexpectedly high volume fraction of the primary phase, which furthermore fluctuates over time, has been observed. This is attributed to the transient state of the (α+β) growth front to a steady state and the associated evolution of the large diffusion layer ahead of the solid–liquid interface.

Secondary dendrite arm migration caused by temperature gradient zone melting during peritectic solidification

During dendritic solidification of Al–Ni and Sn–Ni peritectic alloys in a temperature gradient, it is observed that a thick peritectic layer forms on the front edge of the secondary dendrite arm of the primary phase, while there is almost no peritectic phase on the back edge. This observation is explained satisfactorily by a new version of secondary dendrite arm migration caused by temperature gradient zone melting during peritectic solidification, which involves both primary and peritectic phases. Experimental and theoretical analysis in the present work demonstrates that temperature gradient zone melting can cause extensive melting/solidification of the primary and peritectic phase during peritectic solidification.

Dual solidification mechanisms of liquid ternary Fe-Cu-Sn alloy

Liquid ternary Fe47.5Cu47.5Sn5 alloy displayed dual solidification mechanisms when it was undercooled by up to 329 K (0.19TL). Below a critical undercooling of about 196 K, it solidified just like a normal peritectic alloy, even though metastable phase separation occurred to a microscopic extent. Once bulk undercooling exceeds 196 K, macroscopic segregation played a dominant role in solidification. In both cases, the solidification process was always characterized by two successive peritectic transformations: firstly primary γFe dendrites reacted with liquid phase to form (Cu) phase, and subsequently the (Cu) phase reacted with residual liquid phase to yield β-Cu5.6Sn intermetallic compound. The primary γFe dendrites achieved a maximum growth velocity of 400 mm/s and experienced a growth kinetics transition as a result of macrosegregation. Since the (Cu) phase was both the product phase of the first peritectic transformation and also the reactant phase for the second peritectic transformation, it appeared as two layers in solidification microstructures due to the microsegregation of Sn solute. The boundary continuity between the macroscopically separated Fe-rich and Cu-rich zones was enhanced with the increase of undercooling.

In-situ observation of coupled growth morphologies in organic peritectics

Systematic directional solidification experiments were done with TRIS-NPG organic peritectic alloys to investigate peritectic solidification close to the limit of constitution undercooling. The experiments were carried out as in-situ observation in a horizontal micro Bridgman furnace. Under specific conditions isothermal peritectic coupled growth (PCG) were obtained for alloys within the hypo-peritectic region. Isothermal PCG was obtained either by (i) reducing the growth velocity from above the critical value for morphological stability of both solid phases to a value below, or by (ii) long-time growing with a constant velocity below the critical value for morphological stability of both solid phases. The later happens via island banding, where nucleation and lateral growth of the peritectic phase compete with growth of the primary phase. Interphase spacings and the widths of α and β lamellae were measured as function of growth velocity for a fixed composition and as function of composition for a fix growth velocity.

Solidification researches using transparent model materials — A review

A review is given in the paper for solidification researches with transparent model materials. The effective experimental method was first proposed by Jackson and Hunt in 1965. The transparent model materials for solidification researches are a kind of non-faceted crystals known as “plastic crystals” or “globular molecules”, which have very low entropy of melting as that of metals. According to Jackson’s theory proposed in 1958, entropy of phase transformation will determine whether the phase interface morphology is smooth or rough in atomic scale, which will lead to faceted or nonfacted phase interface in microscopic and macroscopic scales. Succinonitrile (SCN) and its alloys with water, ethanol, acetone, and NH4Cl-H2O solution are most commonly used as transparent model materials for solidification researches of dendritic growth, anisotropy of solid-liquid interfacial energy, crystal nucleation, crystal grain formation, directional solidification, eutectic and peritectic solidification, solidification defects formation such as bubble, hot tearing, etc. Among these researches, the most impressive work was the critical test of dendritic growth theories with high purity succinonitrile by Glicksman et al., which gave positive answer to the Ivantsov’s analysis and negative answer to the ad hoc condition of the maximum velocity hypothesis. The future researches with transparent model materials could be suggested in three aspects: 1) accurate measurement of material properties and alloy phase diagrams in more plastic crystals, especially to find more transparent eutectic and peritectic alloys; 2) accurate measurement of the grain boundary groove shape to obtain precise data of the anisotropy parameters of the interfacial free energy in transparent model materials; 3) to get clear pictures of solidification processes with morphology details in a relatively large area, with continuous movement of liquid and particles, in order to give experimental support to numerical simulations aimming at accurate description of microstructure formation during solidification of multicomponent alloys under complex conditions of real casting and welding processes.

Reason for the Beneficial Effect of Sulfur Additions on the Hypoperitectic Reaction in a 0.1 Pct Carbon and the Sulfur Content Needed to Establish the Effect

It has been shown that the addition of 0.3 pct sulfur to the 0.1 pct carbon, 0.9 pct manganese, and 0.3 pct silicon peritectic steel does not exhibit a rippled shell because of the accommodation of the buckling strains by the presence of a significant amount of low-melting point liquid when solid austenite and ferrite are formed. A similar solidification path will exist for sulfur contents down to approximately 0.1 pct for this alloy. A significant amount of low melting liquid will be present down to that sulfur concentration. The exact point for the disappearance of the peritectic effect still must be determined experimentally, but it certainly is in the range of 0.1 to 0.15 pct sulfur. For other peritectic alloys, the approximate sulfur content at which the effect of the peritectic reaction will disappear can be determined similarly to the method used in this study.

Fluid-flow effects on phase selection and nucleation in undercooled liquid metals

Fluid flow can have a profound effect on phase selection and nucleation in undercooled liquid metals.In phase selection experiments, the sample does not solidify directly to the thermodynamically stable phase, but instead first forms a metastable phase, then transforms to the stable phase after some delay. Previous experiments and modeling on phase selection in near-eutectic Fe-Cr-Ni stainless steels have shown that the lifetime of the metastable phase is limited by the incubation time for formation of a critical nucleus of the stable phase, and that for some conditions, fluid flow can change that incubation time by an order of magnitude or more. New microgravity experiments in this area will be performed on new materials, including peritectic alloys, in the Materials Science Laboratory Electromagnetic Levitator (MSL-EML) as a part of projects PARSEC and THERMOLAB. The models that predict the effect of fluid flow on phase selection will be extended to these new experiments.In another class of experiments, nucleation in quasicrystal- and glass-forming alloys, testing a new theory on nucleation kinetics requires control of the shear rate in fluid flow to prevent interference of the solute fields around the sub-critical nuclei. These theories will be tested in microgravity using the MSL-EML under the projects ICOPROSOL and THERMOLAB.

SOLIDIFICATION MECHANISM OF TERNARY QUASIPERITECTIC ALLOY OF Al-11.8Cu-24.22Mg

In the field of condensed matter physics and materials science,it is of great importance to investigate the microstructures,properties and solidification regularities of liquid metals.In the last few decades,the theories of solidification of binary alloys,such as dendritic growth and eutectic growths have been built.Great progress has also been made on the study of monetectic and peritectic alloys. But a solidification theory on ternary quasiperitectic alloys has not been established up to now.The study of the solidification process of quasiperitectic alloys will provide a basic work for solidification theories of ternary alloys. The master alloy of Al-11.8Cu-24.22Mg was prepared from pure Al(99.99%),pure Mg(99.99%) and Al-54.2Cu in a resistance furnace under CO_2 and SF_6(volume proportion is 40:1) atmosphere. The melted alloy(840—850℃) was pouring into different quenching graphite crucibles at the same time,and the cooling curves were recorded by a sixteen channels temperature recorder.The graphite crucible was quenched into cold-water immediately for rapid cooling at the preplanned quenching temperature.After the experiment,the microstructures of the sample were analyzed by SEM,with EDS analysis. The experiment result indicates that the primary phase is identified as S(Al_2CuMg) and the quasiperitectic phases areα-Al and T(Al_6CuMg_4).The solidification microstructure is composed of remnant primary phase,quasiperitectic phases,binary eutectic and ternary eutectic.Although the quasiperitectic phases and binary eutectic are composed of the same phases(α-Al+T(Al_6CuMg_4)), their structures are different.The former structure presents strip form and the later present dendritic form.The ternary eutectic reaction is suppressed and the remnant primary S phase is reserved in the matrix with non-equilibrating crystallization.