Peritectic Alloys Research Articles

Investigation of the heterogeneous nucleation in a peritectic AlNi alloy

The goal of this research is a systematic investigation aiming at a quantitative understanding of the four different stages of peritectic solidification: 1) heterogeneous nucleation, 2) peritectic reaction, 3) peritectic transformation and 4) direct growth of the peritectic phase. Solidification experiments of the peritectic alloy Al-36wt.% Ni are carried out in differential scanning calorimetry (DSC), and the recently developed multi-phase-field techniques are employed to quantitatively model and simulate the peritectic phase transformation, by taking into account the precise thermodynamic description of the Al-Ni system. A previously developed phase-field model that had been successfully employed in the Al-Ni system is adapted here and calibrated by a novel method that simulates not only the final solidification microstructure but also DSC traces. The resulting kinetics of the phase-transformation processes are presented underlying the above four stages of phase transformation in detail.

Semisolid processing of short and long freezing range Cu alloys by electric current

The present study reports the effect of ac (50 Hz) electric current treatment (ECT) on short freezing range (Cu–Ni) and long freezing range (Cu–Sn) alloys. This treatment was conducted during the semisolid state of the alloys, i.e. during casting in sand moulds. In isomorphous Cu–Ni alloy, with gradual increase in the electric current, first, the dendrites are refined; then, the refined dendrites are fragmented; and finally, the fragmented dendrites are refined. The ECT also reduces homogenisation treatment time. It is observed that a small amount of electric energy spent during sand casting saves considerable amount of energy for homogenisation treatment. The sand castings of Cu–Sn (peritectic) alloys are very much prone to interdendritic shrinkage. Thus, the density of castings is reduced. However, the ECT during casting increases the density of castings by reducing interdendritic shrinkage, modifies phase diagram of Cu–Sn alloy, increases volume fraction of the primary α phase (Cu) and reduces volume fraction of the peritectic phase. The ECT also refines the peritectic phase that transforms to δ phase on cooling to room temperature.

An analysis of non-equilibrium peritectic reaction driven by solute diffusion under a temperature gradient

Experiments consisting of melting followed by thermal stabilization under a temperature gradient have been performed on Al–Ni peritectic alloys. A non-planar interface around the peritectic temperature has been observed. This is found to be induced by the primary Al3Ni2 phase dissolution above the equilibrium peritectic temperature TP. A solute depleted zone in a distance above TP and a solute enriched zone below TP have been characterized. The dissolution perturbation is explained in light of the constitutional superheating driven by solute diffusion under a temperature gradient.

Modeling structural transformations in binary alloys with phase field crystals

We develop a phase field crystal model for structural transformations in two-component alloys. In particular, the interactions between components are described by direct correlation functions that are an extension of those introduced by Greenwood et al. [Phys. Rev. Lett. 105, 045702 (2010)] for pure materials. These correlation functions result in broad density modulations that can be treated with high numerical efficiency, hence enabling simulations of phase transformations between a wide range of crystal structures. A simplified binary alloy model is shown to describe the equilibrium properties of eutectic and peritectic binary alloys in two and three dimensions. The robustness and versatility of this method is demonstrated by applying the model to the growth of structurally similar and dissimilar eutectic lamella and to segregation to defects.

Dendritic growth and solute trapping in rapidly solidified Cu-based alloys

Rapid solidification of binary Cu-22%Sn peritectic alloys and Cu-5%Sn-5%Ni-5%Ag quaternary alloys was accomplished by glass fluxing, drop tube and melt spinning methods. The undercooled, by glass fluxing method, Cu-22%Sn peritectic alloy was composed of α(Cu) and δ(Cu41Sn11) phases. If rapidly solidified in a drop tube, the alloy phase constitution changed from α(Cu) and δ(Cu41Sn11) phases into a single supersaturated (Cu) phase with the reducing of droplet diameter, and the maximum solubility of Sn in (Cu) phase extended to 22%. The Cu-5%Sn-5%Ni-5%Ag quaternary alloy was composed of (Cu) and (Ag) phases under the containerless processing condition in a drop tube, and the solute microsegregation of (Cu) phase was obvious. When the Cu-5%Sn-5%Ni-5%Ag quaternary alloy was solidified by melt spinning method, microsegregation was suppressed and solute trapping occurred. The experimental results show that the microstructures of primary (Cu) phase in the two alloys transfer from coarse dendrites into equiaxed grains with the increase of cooling rate and undercooling, which is accompanied by the grain refinement effect.

Directional solidification of Cu–20Sn alloy at low speed: From peritectic coupled growth to banding

Bridgman-type directional solidification experiments have been carried out in Cu–20Sn peritectic alloy. Peritectic coupled growth and banding structures have been observed at low growth rates (1.5 and 2 μm/s) under a temperature gradient up to 40 K/mm. The peritectic coupled growth structure, containing rod dendrite primary α phase plus peritectic β phase, forms initially. As solidification proceeds, peritectic coupled growth is overgrown by banding or island banding structures. The formation of banding structure from coupled growth is explained by a model involving Sn concentration change at nucleation of the secondary phase ahead of the solid/liquid interface. It is found that the competitive growth between the α and β phases also plays a critical role on the formation of banding structures.

包晶合金定向凝固过程中的对流效应及带状组织形成机制 II.理论分析

The axial macrosegregation and relevant microstructure evolution of Fe-Ni peritectic alloys directionally solidified under different convection strength were discussed using pure diffusion model,boundary layer convection model and the Scheil equation.Comparing with the experiments of Fe-Ni alloys conducted in resistance heating and induction heating Bridgman system,it was found that the predictions of boundary layer convection model agreed well with the experimental results of induction heating directional solidification when the convection strength parameters lies in the region 1.7μ≤2,indicating that the model can be used to predict the axial macrosegregation and microstructures evolution of peritectic alloys.

Simulation of microsegregation and the solid/liquid interface progression in the concentric solidification technique

A concentric solidification technique was employed to simulate experimentally the segregation of alloying elements during solidification at the centerline of continuously cast steel. Microstructural development of low carbon steel upon solidification has been observed in situ in a laser-scanning confocal microscope. Microscopic analyses following in situ observations, demonstrate that segregation occurring at steel slabs can reasonably be simulated by the use of the concentric solidification technique. The validity of these experimental simulations has been correlated with mathematical analyses using the Thermo-Calc and DICTRA (Diffusion Controlled Transformation) modeling tools. The effect of cooling rate on the sequence of events during solidification of Fe–0.18%C and Fe–4.2 wt%Ni peritectic alloys was studied and compared with the experimental observations.

Effects of Geometric Constraint on Phase Selection and Segregation in Cast TiAl

ABSTRACTSimulations of directional solidification of binary peritectic TiAl alloys through ceramic preforms consisting of narrow straight channels with spacings on the order of the dendritic tip radius were performed with a modified cellular automaton coupled to a finite volume calculation of solute diffusion. Depending upon the channel spacing, the microstructure may be refined after dendritic growth through the preform. As the channel width decreases, the lateral solute diffusion in the liquid is more constrained, changing the morphology of the primary phase from dendritic to cellular. In narrow channels, the liquid concentration gradient ahead of the solid/liquid interface decreases resulted in lower growth velocities and higher undercoolings. For hypo-peritectic TiAl alloys, growth conditions that lead to growth of β dendrites can result in α nucleation and growth in narrow channels due to the geometric constraint on solute diffusion.

Morphological instability of interface, cell and dendrite during directional solidification under strong magnetic field

The effects of a strong magnetic field on the interface shape and the cellular and dendritic morphology were investigated in the directionally solidified Al–Cu, Zn–Cu and Al–Ni alloys experimentally. The results show that morphological instability of the interface, cell and dendrite has occurred during the directional solidification under the strong magnetic field. Indeed, the magnetic field caused the breakdown of a planar interface into a cellular undulation and formed an irregular shape. Especially, for the Zn–2.0wt%Cu peritectic alloy, the wavy band-like structure appears under a strong magnetic field. Moreover, it has been found that the application of a strong magnetic field caused the cell and dendrite to twist and deflect from the solidification direction during directional solidification. The stresses in the solid near the solid/liquid interface under the strong magnetic field were analyzed, measured and numerically simulated. The magnetization force (MF) and thermoelectric magnetic force (TEMF) may be responsible for the irregularity and instability of the interface, cell and dendrite during directional solidification under a strong magnetic field.

Influence of thermal stabilization on the solute concentration of the melt in directional solidification

Experiments on Al–25 at%Ni peritectic alloy consisting of melting followed by thermal stabilization ranging from 0 to 2 h were carried out in a Bridgman-type furnace. Temperature distribution, microstructure evolution and solute concentration in the mushy zone are characterized. An analytical model is proposed to evaluate the Ni concentration of the melt after thermal stabilization. Effect of temperature gradient and volume fraction of liquid phase in the mushy zone on the Ni concentration of the melt is discussed. The steady state Ni concentration of the melt is inappropriately below the initial Ni concentration of the sample. The deviation increases with decreasing temperature gradient. Finally, the influence of thermal stabilization on the solute concentration of the melt is discussed based on a comparison of Al–Ni peritectic alloys with Al–Ni hyper-eutectic alloys and Al–Cu hypo-eutectic alloys.

Preparation of the initial solid–liquid interface and melt in directional solidification of Al–18 at%Ni peritectic alloy

The preparation of the initial solid–liquid interface and melt is a critical step in directional solidification process. Experiments on Al–18 at%Ni peritectic alloy consisting of melting followed by thermal stabilization ranging from 0 to 2 h were carried out in a Bridgman-type furnace. In the directional melting process, due to the temperature gradient imposed on the rod, a mushy zone was created between the complete liquid zone and non-molten solid zone parallel to the temperature gradient. With the thermal stabilization time increase, the volume fraction of the liquid inclusions in the mushy zone decreased. After 2 h of thermal stabilization, the initial solid–liquid interface was corrugated, and the Ni concentration in the complete liquid melt zone was uniform but below the original composition. A solute diffusion transport model in a temperature gradient was proposed to describe the concentration evolution both in the mushy zone and the complete liquid melt zone during thermal stabilization.

Phase Selection in Undercooled Melts of Peritectic Cu-Ge Alloys

Phase selection in undercooled melts of Cu-14.8Ge and Cu-18Ge compositions was investigated using the electromagnetic levitation technique in combination with substrate quenching and a drop tube. The results showed that the levitated and gas-cooled samples were all solidified into a microstructure consisting of primary -Cu plus peritectic -Cu5Ge. However, the samples quenched onto a copper substrate showed a segregation-free microstructure in the chilled zone, suggesting direct crystallization of the peritectic -Cu5Ge phase from a highly undercooled liquid. The Cu-18Ge samples quenched onto a glass substrate as well as those solidified in the drop tube also showed a segregation-free microstructure. An analysis of the nucleation kinetics revealed that the -Cu5Ge phase had a larger nucleation barrier than that of -Cu for all accessible undercoolings. It was suggested that phase selection in the undercooled Cu-Ge liquid might be controlled by transient nucleation kinetics.

Phase-Field Simulation of Solidifying Microstructure in the Presence of a Convective Field

Here we present a contribution to the microstructure based design of new steel alloys by a careful investigation of multiphase microstructure solidification in a convective field. To this end we present an extension of the quantitative phase-field model proposed in [1] to investigate the influence of hydrodynamic convection on the growth of eutectic/peritectic alloys. We study directional solidification of a eutectic alloy under the influence of a shear flow ahead of the solidifying front. We mainly investigate the growth of a eutectic lamellar structure. We show that the imposed flow tilts the whole lamellar structure away from the flow direction. Moreover, we show that forced flow alters the growth morphology of Fe-Ni peritectic alloys, having a strong influence on the nucleation of the peritectic phase. Based on these investigations, a scale relation between the strength of flow and the solid volume fraction is derived. Further we propose an application of these models as an alternative approach to study heterogeneous nucleation kinetics in the solidification of peritectic materials systems.

Peritectic transformation and primary α-dendrite dissolution in directionally solidified Pb–26%Bi alloy

Peritectic transformation is one of the main mechanisms during solidification of peritectic alloys, and accompanied with dissolution of primary phase. In the present work, the effect of solidification temperature on the peritectic transformation and dissolution of primary α-dendrite was investigated during directional solidification of Pb–26wt%Bi peritectic alloy. The resulting microstructures of the alloy in consideration of different solidification temperature have been investigated using optical microscopy. The volume fraction of peritectic β-phase and the average size of α-dendrite core were measured using image analyzing software SISC IAS V8.0. These results show that the increment in β-phase volume fraction and decrement in size of α-dendrite core first increase, then decrease with falling solidification temperature. These results can be explained from the viewpoint of a reduced solidification temperature leading to the rate of peritectic transformation first increases to a maximum, and then decreases.

Construction of a Cr3C2-C Peritectic Point Cell for Thermocouple Calibration

The melting points of Cr3C2-C peritectic (1826°C) and Cr7C3-Cr3C2 eutectic (1742°C) alloys as materials for high-temperature fixed point cells are investigated for the use of thermocouple c...

Microstructure Formation at Low Velocity during Directional Solidification of Pb-Bi Peritectic Alloys

Directional solidification experiments on Pb-Bi peritectic alloys have been conducted at very low velocity (V=0.5 μm/s) and high thermal gradient (G=25 K/mm). Incomplete banded and oscillatory structures have been observed in both of hypoperitectic and hyperperitectic compositions over several millimeters of growth. These structures resulted from the repeated nucleation and competition between properitectic α- and peritectic β-phases. The banded or oscillatory structures are found to be transient and the final steady-state phase was only the peritectic β-phases. With an increase in composition, β phase formed and α phase disappeared at a lower solidified distance. Composition variations in the banded structure are measured to determine the solute distribution along the growth direction.

Connectivity of Phases and Growth Mechanisms in Peritectic Alloys Solidified at Low Speed: an X-Ray Tomography Study of Cu-Sn

The variety of microstructures that form at low solidification speed in peritectic alloys, bands, and islands, or even coupled (or cooperative) growth of the primary α and peritectic β phases, have been previously explained by nucleation-growth mechanisms. In a recent investigation on Cu-Sn, a new growth mechanism was conjectured on the basis of two-dimensional (2-D) optical microscopy and electron backscattered diffraction (EBSD) observations made in longitudinal sections. In the present contribution, synchrotron-based tomographic microscopy has been used to confirm this mechanism: α and β phases totally interconnected in three dimensions and bands (or islands) can result from an overlay mechanism, rather than from a nucleation events sequence. When the lateral growth of a new layer is too fast, an instability can lead to the formation of a lamellar structure as for eutectic alloys.

A study of microstructure and solidification behavior of Zn-Cu alloy

Zn-1.5wt.%Cu peritectic alloy was prepared in a graphite crucible under a vacuum atmosphere. Unidirectional solidification of Zn-1.5wt.%Cu peritectic alloy was carried out with the Bridgman method by using metals of 99.99 % purity under the argon atmosphere and two different conditions; with a temperature gradient (G) range of 1.99–7.81 K mm �1 at constant growth rate (V ) and a growth rate range of 8.41–661.11 µ ms �1 at a constant temperature gradient. The microstructures of the directionally solidified Zn-1.5wt.%Cu peritectic samples were observed to be cellular. From both transverse and longitudinal sections of the samples, cellular spacing (λ), and cell tip radius (R) were measured and expressed as functions of solidification processing parameters (G and V ) using a linear regression analysis. The experimental results were also compared with values calculated according to the current theoretical and numerical models, and similar previous experimental results. K e y w o r d s : metals & alloys, peritectic solidification, cellular growth, crystal growth

Peritectic Alloy Rapid Solidification with Electromagnetic Convection

Peritectic Alloy Rapid Solidification with Electromagnetic Convection