Investigation on Ni3Al-Ni7Hf2 unidirectionally solidified eutectic composite

The formation of a Ni₃Al()+Ni₇Hf₂ unidirectionally solidified lamellar eutectic composite has been investigated in this paper. The results show that Ni-5.8Al-32Hf alloy, which has the Ni₃Al+Ni₇Hf₂ eutectic structure, is a suitable composition of D.S eutectic material. The melting range of this composition is 41 °C as determined by DTA. The critical ratio of G/R for Ni₃Al+Ni₇Hf₂ eutectic is found to be 5× 10⁵ °C · s · cm^–2, and the lamellar Ni₃Al+Ni₇Hf₂ eutectic aligned parallel to the direction of solidification was made with R = 5 m/s and G = 250 °C/cm. The investigation shows that the lamellar eutectic has a preferred crystallographic orientation between the Ni₃ Al and Ni₇Hf₂ lamellae, i.e., (111)_NiAl//(100)_NiHf and [110]_NiAl//[010]_NiHf. The lamellar Ni₇Hf₂ did not degrade or coarsen obviously, and no harmful phase formed in the interface of Ni₃Al/Ni₇Hf₂ after long time soaking of 1100 °C/110 h. This demonstrates that the Ni₃Al+Ni₇Hf₂ lamellar eutectic has high interface thermal stability.     

Interface temperature measurement of M2C and M6C eutectic carbides in the Fe–Mo–C system

The High speed cast iron, which is used for hot rolling parts, needs high fracture toughness and wear resistance. To improve these properties, the control of eutectic carbides, M₃C, M₇C₃, M₆C and MC is important by adding elements such as Cr, W, V and Mo.The aim of this study is to estimate which carbide will solidify under certain solidification conditions and compositions. This prediction criterion can be gained by measuring the interface temperature of each carbide in various samples with different solute elements, composition and growth rate.In this report, the solidified temperature of γ+M₂C and γ+M₆C eutectic carbide in the Fe–Mo–C ternary system in the composition range near to the eutectic monovariant line, was measured during the unidirectional solidification process. The relationship between solidified interface temperature and growth rate was obtained. In eutectic solidification along the γ+M₆C monovariant line, a coefficient of undercooling, the k value, was obtained.The authors have already measured the k values of other eutectic carbides, such as γ+M₃C, austenite+M₇C₃, and γ+VC in Fe–Cr–C and Fe–V–C system. The paper also discusses the relationships between these properties of eutectic carbides.     

Three-dimensional reconstruction of anomalous eutectic in laser remelted Ni-30 wt.% Sn alloy

Abstract

Laser remelting has been performed on Ni-30 wt.% Sn hypoeutectic alloy. An anomalous eutectic formed at the bottom of the molten pool when the sample was remelted thoroughly. 3D morphologies of the α-Ni and Ni₃Sn phases in the anomalous eutectic region were obtained and investigated using serial sectioning reconstruction technology. It is found that the Ni₃Sn phase has a continuous interconnected network structure and the α-Ni phase is distributed as separate particles in the anomalous eutectic, which is consistent with the electron backscatter diffraction pattern examinations. The α-Ni particles in the anomalous eutectic are supersaturated with Sn element as compared with the equilibrium phase diagram. Meanwhile, small wavy lamella eutectics coexist with anomalous eutectics. The Trivedi–Magnin–Kurz model was used to estimate undercooling with lamellar spacing. The results suggest that the critical undercooling found in undercooling solidification is not a sufficient condition for anomalous eutectic formation. Besides, α-Ni particles in the anomalous eutectic do not exhibit a completely random misorientation and some neighboring α-Ni particles have the same orientation. It is shown that both the coupled and decoupled growth of the eutectic two phases can generate the α-Ni + Ni₃Sn anomalous eutectic structure.     

Tensile and fatigue behaviour of Ni–Ni3Si eutectic in situ composites

A Ni–Ni₃Si composite was fabricated via a eutectic reaction (Ni–Ni₃Si) using a rapidly cooled directional solidification technique at a solidification rate of 40?μm?s^?1. The composite consisted of approximately 62.2% Ni–Si solid solution and 37.8% Ni–Ni₃Si eutectic phase in volume. Four-point bend fatigue tests were carried out on the composite. The fatigue strength of the alloy was measured to be 520?MPa (maximum cyclic stress). It was found that the fatigue cracks were preferably initiated in the Ni–Ni₃Si eutectic phase, and that the Ni matrix was fractured in a cleavage fashion. It was probably attributed to the high level of supersaturated Si in the Ni matrix, which led to inducing the embrittlement of the Ni matrix.     

Three-dimensional reconstruction of anomalous eutectic in laser remelted Ni-30 wt.% Sn alloy

Laser remelting has been performed on Ni-30 wt.% Sn hypoeutectic alloy. An anomalous eutectic formed at the bottom of the molten pool when the sample was remelted thoroughly. 3D morphologies of the α-Ni and Ni₃Sn phases in the anomalous eutectic region were obtained and investigated using serial sectioning reconstruction technology. It is found that the Ni₃Sn phase has a continuous interconnected network structure and the α-Ni phase is distributed as separate particles in the anomalous eutectic, which is consistent with the electron backscatter diffraction pattern examinations. The α-Ni particles in the anomalous eutectic are supersaturated with Sn element as compared with the equilibrium phase diagram. Meanwhile, small wavy lamella eutectics coexist with anomalous eutectics. The Trivedi–Magnin–Kurz model was used to estimate undercooling with lamellar spacing. The results suggest that the critical undercooling found in undercooling solidification is not a sufficient condition for anomalous eutectic formation. Besides, α-Ni particles in the anomalous eutectic do not exhibit a completely random misorientation and some neighboring α-Ni particles have the same orientation. It is shown that both the coupled and decoupled growth of the eutectic two phases can generate the α-Ni + Ni₃Sn anomalous eutectic structure.     

Interface temperature measurement of M2C and M6C eutectic carbides in the Fe–Mo–C system

The High speed cast iron, which is used for hot rolling parts, needs high fracture toughness and wear resistance. To improve these properties, the control of eutectic carbides, M₃C, M₇C₃,M₆C and MC is important by adding elements such as Cr, W, V and Mo.

The aim of this study is to estimate which carbide will solidify under certain solidification conditions and compositions. This prediction criterion can be gained by measuring the interface temperature of each carbide in various samples with different solute elements, composition and growth rate.

In this report, the solidified temperature of γ + M₂C and γ + M₆C eutectic carbide in the Fe–Mo–C ternary system in the composition range near to the eutectic monovariant line, was measured during the unidirectional solidiication process. The relationship between solidified interface temperature and growth rate was obtained. In eutectic solidification along the γ + M₆C monovariant line, a coefficient of undercooling, the k value, was obtained.

The authors have already measured the k values of other eutectic carbides, such as γ + M₃C, austenite + M₇C₃, and γ + VC in Fe–Cr–C and Fe–V–C system. The paper also discusses the relationships between these properties of eutectic carbides.     

Modeling of free eutectic growth and competitive solidification in undercooled near-eutectic alloys based on in situ measurements

Free eutectic growth and its competition with single-phase growth in solidification of undercooled near-eutectic alloys are not yet fully understood. In this paper, the historical development of eutectic growth models was reviewed. The LZ model of free eutectic growth was evaluated using recent data of eutectic growth velocities in an undercooled Ni_81.3Sn_18.7 eutectic composition. An excellent agreement was achieved between the LZ model and the data. Crystal growth velocities in off-eutectic Ni₈₃Sn₁₇ and Ni₈₀Sn₂₀ compositions were measured using a high-speed camera technique. The present data of the off-eutectic compositions and the recent data of the eutectic composition were modeled using the LZ model and the LKT/BCT model of free dendritic growth. The modeling revealed that the competition between the free eutectic growth and the single-phase growth is controlled by the highest interface temperature criterion. A coupled zone of the α-Ni-Ni₃Sn eutectic was calculated using this criterion. The coupled zone agrees well with studies of solidified structures of undercooled samples.     

Directional solidification of SiC-B4C eutectic: Growth and some properties

Directionally solidified SiC-B₄C eutectic formed lamellar microstructures with no colonies if the solidification rate (R) was below 2cm/hr. It was determined that 〈001〉 of B₄C and 〈111〉 of SiC were parallel to the eutectic growth direction, and that the interlamellar spacing was proportional to the inverse square root of solidification rate. The Knoop hardness increased with the increasing solidification rate. The minimum wear was found with compositions close to eutectic. At R ≈ 9cm/hr the most wear resistant specimens were obtained. SiC-B₄C eutectic ingot solidified at approximately 9cm/hr had considerably better wear resistance than solidified B₄C or hot pressed SiC.     

Critical splitting of the solidifying interface-geometrical model of spacing selection during directional solidification of lamellar eutectics

The steady-state coupling equations of directional solidification of Al-Al₂Cu (=0.94), Sn-Pb (=0.59)and Al-Si (=0.17)eutectics have been solved numerically. The profiles, splitting features and supercooling of the solidifying interface were investigated in detail as functions of the lamellar spacing. It was found that when supercooling, T ₀, at the three-phase conjunction point reaches its minimum value, min (T ₀), the solidifying interface profile of one lamellar phase was in the critical splitting state (marginally stable state) and that of the other lamella was super-stable. Langer's marginal stability theory of lamellar eutectic solidification with a planar interface was extended to the case with a curved interface, and a geometrical model of the spacing selection has been suggested (critical splitting state of one lamella's solidifying interface). The general scaling law of the spacing, derived according to this theory, was found to be consistent with the similarity law recently derived by Kassner and Misbah; it is also supported by experimental results. Another geometrical model of the spacing selection was found, where the solidifying interface profile of one lamella became split and other lamella's profile was in the critical splitting state. The experimental data for directional solidification of Al-Si eutectic showed that the irregular eutectics have a spacing selected according to this mode.     

Influence of additions on the solidification behaviour of Ni-B alloys — crystallography of Ni-Ni3B eutectic

The influence of small additions (1 to 2 at.%) of some elements (chromium, copper, silicon, aluminium, iron, titanium, vanadium) on the solidification behaviour and crystallography of Ni()-Ni₃B eutectic composition is examined by differential thermal analysis, X-ray and electron diffractions, scanning and transmission electron microscopy. With the exception of Fe-doped eutectic alloys, all the alloys whether doped or undoped exhibit a very large undercooling for the nucleation of Ni₃B. We give an interpretation of the controversial hypothesis of this undercooling. We propose a crystallographic orientation relationship between the cubic phase Ni() and the orthorhombic phase Ni₃B in the lamellar eutectic Ni-Ni₃B. We also examine a complex transformation which occurs in Ni₃B during slow cooling.     

Microstructures formed by directional solidification of two ternary eutectic alloys of Bi–Cd–In system

Abstract

Microstructures of the two ternary eutectic alloys of the Bi–Cd–In system were studied using slow unidirectional solidification, followed by quenching to form a representative solid/liquid interface for subsequent observation. The eutectic reactions were found to take the form L?BiIn+BiIn₂+Cd at 77.5°C and L?BiIn₂+_?+Cd at 61.5°C. The 77.5°C eutectic was observed to be of the faceted (BiIn)–faceted (Cd)–non-faceted (BiIn₂) type, while all three phases of the 61.5°C eutectic showed faceting. The BiIn and BiIn₂ phases of the 77.5°C eutectic formed a quasiregular microstructure with the Cd phase growing relatively independently. The phases of the 61.5°C eutectic tended to form a lamellar microstructure with a BiIn₂–?–Cd–_?–BiIn₂ phase sequence. Both eutectics were observed to obey the usual phase spacing law, λ²R=constant, where λ is the phase spacing and R is the growth rate.     

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.     

Precipitation behavior of Heusler phase (Ni2AlHf) in multiphase NiAl alloy

Precipitation behavior of Heusler phase (Ni₂AlHf) in a directionally solidified (DS) NiAl- 28Cr-5Mo-1Hf (at.%) alloy was examined using scanning electron microscope (SEM) and transmission electron microscope (TEM). In the as-cast alloy, the Ni₂AlHf phase generally appeared on the NiAl/Cr(Mo) interface, which degraded the NiAl/Cr(Mo) eutectic structure. In the heat-treated alloy, the density of the intercellular Ni₂AlHf phase was slightly reduced. In addition, the spherical Ni₂AlHf phase precipitated heterogeneously in the NiAl matrix, but the Ni₂AlHf phase did not precipitate in the lamellar Cr(Mo) phase. The precipitation behavior of the Ni₂AlHf phase could be explained in terms of the interfacial energy. A lattice model was also proposed to explain the NiAl↔Ni₂AlHf phase transformation.     

Phase and microstructure pattern selection of Zn-rich Zn–Cu peritectic alloys during laser surface remelting

Peritectic solidification has attracted increasing attention as a lot of important binary alloys, such as Fe–Ni, Zn–Cu, Fe–C and Ti–Al, exhibit peritectic reaction during solidification. In order to investigate the solidification behavior of Zn-rich Zn–Cu peritectic alloy containing nominally up to 7.8 wt.% Cu, a series of laser surface remelting experiments were performed. With the increase in growth velocity, Zn–Cu alloys with Cu content below 3.0wt.% showed an evolutional sequence from low-velocity η planar interface?→?lamellar structures?→?η shallow cells and finally to high-velocity η planar interface. The Zn-4.0 wt.%Cu alloy showed a similar transitional sequence except that irregular η cells appeared when low-velocity planar interface became unstable. In contrast, ε cell/dendrite was the typical microstructure of the Zn-7.8 wt.% Cu alloy within the whole scanning velocity range. Based on the maximum interface temperature criterion, a eutectic growth model under rapid solidification conditions (TMK model) and a self-consistent numerical model for the cellular and dendrite growth were applied to establish a phase and microstructure pattern selection map, which drew a clear whole picture of the relationship between phase/microstructure and solidification conditions of this series of alloys. Regarding the microstructure feature, our investigation revealed the range of the solidification velocity and chemical composition of lamellar structures as dominant microstructure and their lamellar spacing displayed a considerable range of the average value as a function of growth velocity. The relationship between the lamellar spacing and the growth velocity was further analyzed by using the TMK eutectic model, and the results showed the same overall trend as the experimental results.

Graphical abstract

A phase and microstructure pattern selection map of Zn-rich Zn–Cu peritectic alloys. Regression analysis of the average spacing of lamellar structures.

     

Fabrication and characterization of large size LiF/CaF2:Eu eutectic composites with the ordered lamellar structure

As alternative candidates for the ³He neutron detectors, ⁶LiF/CaF₂:Eu eutectic composites were fabricated and their scintillation properties were evaluated. Large size LiF/CaF₂:Eu eutectic composites of 58 mm diameter and 50 mm thickness were produced by Bridgman method. The composites had a finely ordered lamellar structure along the solidification direction. The lamellar structure was controlled by the direction and the rate of solidification, and it was optimized to improve the scintillation properties. Better results were achieved when thinner lamellar layers were aligned along the scintillation light path.     

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 field of the environment and technical heat parameters. The temperature field on the S–L interface is closely related to the solidification 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.     

Development of the chitosan tube prepared from crab tendon for nerve regeneration

Microstructure of a joint between a Pb–Sn eutectic solder and an electroless Ni–8 mass% P has beenexamined using transmission electron microscopy. Four layers, i.e. Ni₃Sn₄, Ni₄₈Sn₅₂, Ni₂SnP and Ni–20 mass% P, are formed between the solder and the electroless Ni–8 mass% P. Among them, Ni₄₈Sn₅₂ and Ni₂SnP were found for the first time in a solder joint. Spherical voids are formed at the interface between Ni₄₈Sn₅₂ and Ni₂SnP, and columnar voids are formed at the interface between Ni₂SnP and Ni–20 maas% P. From the analysis of the migration of the respective interfaces observed during in situ heatingexperiments, it is concluded that these voids are Kirkendall voids formed due to the difference in diffusivity of Ni across the interfaces. Fracture takes place at either of those interfaces during a dropping test.     

Effect of rhenium on solidification of Inconel 718 alloy

Abstract

The effect of rhenium (Re) on the solidification of standard Inconel 718 (St-In718) has been investigated by using experimental alloys containing 2·4, 3·5, and 6·0%Re. Rhenium is one of the most powerful refractory elements that improve the high temperature mechanical properties of Ni base superalloys. Results indicate that solidification starts with precipitation of primary γ phase, which is followed by (γ+ NbC) eutectic and (γ+ Ni₂Nb)eutectic. The solidification temperature of the St-In718 is increased by 30 K with the addition of 6%Re. Moreover, the volume fraction of the primary γ is increased, while the volume fraction of eutectic (γ+ NbC) and (γ+ Ni₂Nb) are decreased by the addition of Re. Also, increasing Re content enlarges the secondary dendrite arm spacing (SDAS). Finally, the effects of Re on the partition coefficients k of alloying elements to primary γ and to eutectic γ+ NbC were evaluated and Re was found to segregate preferentially to the primary γ.     

Directional solidification and growth characteristics of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary eutectic ceramic by laser floating zone melting

Directionally solidified Al₂O₃/Er₃Al₅O₁₂/ZrO₂ ternary eutectic ceramic in situ composite rods with length of 110 mm have been fabricated by laser floating zone melting. The microstructural characteristics of steady growth zone, initial growth zone and solid/liquid interface are investigated under high temperature gradient. In the steady growth zone, the eutectic spacing (λ) is rapidly decreased as increasing the growth rate (V), and the corresponding relationship between growth rate and eutectic spacing is determined to be λ = 11.14 × V ^?1/2. The temperature gradient has been measured to be about 5.3 × 10³ K/cm. In the initial growth zone, the melting process and temperature distribution are recorded by infrared thermal imager, and several unstable complex microstructures are observed. In the quenched zone, the regular eutectics with minimum eutectic spacing of 200 nm are obtained. Moreover, the solid/liquid interface during solidification shows convex interface morphology and the interface height is gradually decreased as increasing the growth rate. The eutectic growth behaviors at the center and edge of the as-grown rod are compared and discussed.     

Carbide morphology in bulk and unidirectionally solidified Fe-C-V eutectics

Abstract

Investigations on the development of eutectic structures in Fe-C-V are of considerable interest as a result of the unique properties exhibited by these alloys. In this work a series of Fe-C-V alloys were prepared in order to investigate the solidification conditions and sequence, eutectic morphology, and the development of in situ eutectics. It was found that near perfect eutectic structures can be achieved by slow cooling of an Fe-C-V alloy. The eutectic obtained consisted of semispherical grains with fibrelike VC_1–x carbides. These fibres were found to develop appreciable ramifications during the solidification process. Assuming a critical area of liquid per fibre at the solidification front, as needed for stable eutectic growth, yielded a parabolic type of expression N_b ∝ t² for the number of branches (N_b) developed as afunction of the solidification time t. A qualitative model is proposed in this work to explain the different stages involved during the solidification of alloys of hypo-, hyper-, and eutectic composition. In addition, unidirectional solidification gave rise to in situ eutectics with aligned fibres. Finally, a solidification constant (u_eλ² = K) of 202 μm³ s^?1 for stable eutectic growth was experimentally determined.