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The effect of cooling rate on eutectoid decomposition in a near eutectoid (Ti-5.5 at.% Cu) alloy has been investigated in a systematic manner by coupling scanning electron microscopy, transmission electron microscopy and atom probe tomography studies. Thus, the competition between nucleation and growth of proeutectoidαplates from pre-existingβgrain boundaries, and eutectoid decomposition (α + Ti2Cu) via a pearlitic mechanism has been studied as a function of cooling rate, using a Jominy-end quenched sample that was cooled from the high-temperature singleβphase. When the alloy was subjected to very fast cooling (160 K/s), proeutectoidαplates, supersaturated in Cu, are formed along with a highly refined lamellar eutectoid product between theseαplates. In contrast, intermediate (9 K/s) and slow (2 K/s) cooling results in considerably coarser proeutectoidαplates as well as lamellar eutectoid products. With the decrease in the cooling rate, there was a substantial increase in the volume fraction of the lamellar eutectoid product and the composition of all decomposition products approached their equilibrium values. Also, the slowest cooled sample (2 K/s) exhibited substantially rougher and irregular interfaces between the proeutectoidαand the lamellar eutectoid product, which seems to promote the cooperative growth of lamellarα + Ti2Cu. Irrespective of the cooling rate, nucleation of the lamellar eutectoid (α + Ti2Cu) product appears to only occur at the interface between the proeutectoidαplates and theβmatrix. [ABSTRACT FROM AUTHOR]

In the present work the fluxing technique was applied to the Sn–51 wt.% Se hypoeutectic alloy. The aim was to study the influence of the undercooling on the microstructure of this alloy, particularly concerning the formation of oriented microstructure and even some metastability occurrence. High undercooling (63 °C) was achieved for the primary SnSe phase leading to an oriented microstructure in the firstly solidified region. Although the undercooling for the eutectic was not so high (19 °C) morphological changes were observed. The rapid movement of the interface interfered with cooperative eutectic growth producing an irregular eutectic in the first region to solidify followed by a fine lamellar and directional eutectic. In order to detect some supersaturated solid solution in the SnSe phase a precise lattice parameter measurement was done from the accurate performed X-ray diffraction (XRD) on the firstly solidified region of the sample. [Copyright &y& Elsevier]

As the Sn-Se eutectic solidification produces a lamellar structure, formed by SnSe and SnSe2 compound, which are p and n semiconducting type, respectively, the SnSe-SnSe2 in situ composite is a promising material to be used in photovoltaic device manufacturing. In this work, the Sn-Se alloys corresponding to the eutectic composition as well as to SnSe and SnSe2 composition were processed by direction solidification at several solidification rates in a vertical Bridgman-Stockbarger crystal growth unit. The aim of the experiments was to evaluate the eutectic microstructure behavior as a function of directional solidification parameters. The obtained microstructures were analyzed by using X-ray diffraction and scanning electron and optical microscopies. The results obtained show that a very regular and aligned structure formed by the SnSe and SnSe2 solid phase can be achieved. It was found that the presence of imperfections in the eutectic microstructure depends on the growth rate, and mainly, on the alloy homogenization process. [ABSTRACT FROM AUTHOR]

The effect of the growth condition on the microstructure and mechanical behaviour of the NiAl–Mo eutectic alloy was investigated. Samples of the eutectic alloy were prepared using an arc furnace under argon atmosphere and then directionally solidified. The samples directionally solidified showed regular and aligned eutectic microstructure. Compression tests were performed at room temperature and 1173 K. Vickers hardness was determined at several temperatures, from room temperature to 1150 K. Mechanical properties of NiAl were also determined. Results showed that when the growth rate is gradually increased from 8 to 50 mm/h the aligned microstructure presents higher fibre density, which leads to higher strength level at room temperature, while further increase in the growth rate causes the microstructure the loose regularity. NiAl presents a well defined mechanical behaviour transition, while the NiAl–Mo eutectic alloy softens almost linearly. The highest yield strength value of NiAl–Mo was found to be about 1200 and 300 MPa at room temperature and 1173 K, respectively. [Copyright &y& Elsevier]

Abstract Evidences of both sluggish eutectoid and active eutectoid (not suppressible under rapid cooling) transformations have been found for the first time in a single hyper-eutectoid Ti-Cu alloy. Both of these types of eutectoid reactions have been investigated in detail, at different length scales, by coupling scanning electron microscopy (SEM),transmission electron microscopy (TEM) and atom probe tomography (APT). The unique three phase crystallographic relationship between the parent β (bcc) and the two product phases, α (hcp) and Ti 2 Cu, has been established. The extent of partitioning of the solute (Cu) between the two product phases has been determined by APT and is rationalised in terms of thermodynamic considerations. Based on the observed lattice site correspondence and the extent of solute partitioning, a possible mechanism of active eutectoid transformation is proposed. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract: Directional solidification of eutectic alloys attracts considerable attention, when in situ composites are concerned. The eutectic alloys are regarded as presenting regular morphology (lamellar and fibrous structures). Besides, when directionally solidified they show high microstructure stability at high temperatures. This work reports a morphological study of an Al–Cr–Nb eutectic alloy. The solidification morphology of the alloys was studied both in the as-cast and in the directionally solidified conditions. The samples were first obtained in an arc furnace and then directionally solidified using Bridgman equipment. During the directional solidification process, the growth rates utilized varied from 5.0 to 30.0mm/h. Optical (OM) and scanning electron microscopy (SEM) was used in order to determine the influence of the solidification conditions on the microstructure. The results obtained indicated that the eutectic transformation temperature is near 1347.9°C with formation of Al3(Nb,Cr) and Cr(Al,Nb) phases. Also, it was noted that the Cr(Al,Nb) phase undergoes a polymorphic transformation (∼892.3°C) forming the Al(Nb)Cr2 compound, followed by eutectoid decomposition Cr(Al,Nb)→Al(Nb)Cr2 +Al8Cr5. [Copyright &y& Elsevier]

Abstract: High temperature structural components demand materials that maintain satisfactory mechanical and chemical characteristics. These needs may be met by applying some eutectic alloys, including Ni-Ni3Si. This paper deals with the directional solidification of Ni-Ni3Si grown under several growth rates. The analysis of the eutectic microstructure was carried out using atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). The results obtained provided a precise analysis of the Ni3Si phase. It could be noticed that the solid/solid transformations by which Ni3Si phase goes through, deeply affects its morphology. In addition, quantitative information on the eutectic structure was obtained. It was confirmed that the growth rate variation deeply affects the final microstructure as it influences the efficiency of atomic diffusion along the solid/liquid interface. [Copyright &y& Elsevier]

Abstract: In situ composite materials obtained by directional growth of eutectic alloys usually show improved properties, that make them potential candidates for high temperature applications. The eutectic alloy found in the Al–Nb system is composed of the two intermetallic phases Al3Nb (D022) and Nb2Al (D8b). This paper describes the directional solidification of an Al–Nb eutectic alloy using a Bridgman type facility at growth rates varying from 1.0 to 2.9 cm/h. Longitudinal and transverse sections of grown samples were characterized regarding the solidification microstructure by using optical and scanning electron microscopy, energy dispersive spectroscopy (EDS) and X-ray diffraction. Despite both phases being intermetallic compounds, the eutectic microstructure obtained was very regular. The results obtained were discussed regarding the effect of the growth rate on the microstructure, lamellar–rod transition and variation of phase volume fraction. [Copyright &y& Elsevier]

Abstract: The exciting microstructures found in several eutectic alloys are a result of a cooperative growth, which is connected to the atomic transfer in the melt ahead the solid/liquid interface. In a eutectic system, the growth of solid phases depends on the simultaneous rejection of constituents to the liquid phase, which causes adjustments of the melt composition and hence, mass transport by diffusion normal to the growth direction. Generally, eutectic microstructures are examined by using optical (OM) and scanning electron microscopy (SEM). While OM may not provide the necessary resolution, the eutectic microstructure may present three-dimensional features, as a result of etching, which is not always possible to be observed by SEM. As an alternative, this paper describes the use of atomic force microscopy (AFM) in understanding micro-scale feature of a eutectic microstructure. For such a purpose, directionally solidified samples of a Ni–Al–V lamellar eutectic, a Ni–Al–Mo fibrous eutectic and a Ni–Al–Nb three-phase eutectic were examined. The results obtained provided a new picture of multi-phase microstructures and allows one to understand their new characteristics. [Copyright &y& Elsevier]