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The strength and ductility trade‐off dilemma has limited the wide application of TiAl‐based alloys. Here, a new insight into the potential for increasing the strength and ductility of a hypoperitectic Ti–48 at% Al–8 at% Nb alloy is accomplished by the electromagnetic levitation (EML) technique. Moreover, a systematic analysis of the primary and subsequent peritectic solidification kinetics is conducted in the undercooling range of 308 K. Assisted by a high‐speed camera, in situ observation of the liquid–solid (primary β‐Ti phase and peritectic α‐Ti phase) interface migration is accomplished. When the alloy melt is undercooled to 240 K, high‐ordered nanotwins are observed in the Ti–48 at% Al–8 at% Nb alloy, which form a unique hierarchical microstructure. Upon further increasing the undercooling, the density of these nanotwins is significantly enhanced. The room‐temperature compression results reveal that the strength and ductility increase up to 140% and 150%, respectively. This is mainly ascribed to the remarkable grain refinement, formation of nanotwins with various orientations, accumulation of dislocations and stacking faults, and retention of the metastable γ‐phase. The superior combination of strength and ductility indicates the possibility to fabricate high‐ordered nanotwins via rapid solidification, thus improving the performance of γ‐TiAl‐based alloys. [ABSTRACT FROM AUTHOR]

Rapid solidification of highly undercooled Fe–Co–Ni alloys is realized by drop tube technique. The microstructures of Fe–10%Co–10%Ni, Fe–15.6%Co–12%Ni, and Fe–10%Co–20%Ni alloys are all composed of single α(Fe) solid solution phase, which are confirmed by the results of XRD and DSC. With the decrease of droplet diameters, the cooling rate and undercooling increase, meanwhile, the microstructural characteristic of α(Fe) phase transforms from coarse dendrites to equiaxed grains. The size of coarse dendrites decreases to one tenth for the refined equiaxed grains. Employing the Vickers hardness and nano indenter techniques, the mechanical properties of α(Fe) dendrites are investigated. The average Vickers microhardness of α(Fe) phase is remarkably enhanced with the decrease of the grain size. Due to the more homogenous solute distribution in the dendrite trunks, the nanohardness presents a gentle fluctuation. Once the droplet diameter exceeds a critical value, the solutes concentrate at the grain boundaries. As a result, the closer the indenters to the grain boundary, the larger the nanohardness. [ABSTRACT FROM AUTHOR]

Using liquid FeCu alloy as a model, the structure of liquid Fe-Cu alloy systems is investigated in the temperature range 1200-2200 K, covering a large metastable undercooled regime, to understand the phase separation of liquid Fe-Cu alloys on the atomic scale. The total pair distribution functions (PDFs) indicate that liquid FeCu alloy is ordered in the short range and disordered in the long range. If the atom types are ignored, the total atom number densities and PDFs demonstrate that the atoms are distributed homogenously in the liquid alloy. However, the segregation of Fe and Cu atoms is very obvious with decreasing temperature. The partial PDFs and coordination numbers show that the Cu and Fe atoms are not apt to get together on the atomic scale at low temperatures; this will lead to large fluctuations and phase separation in liquid Fe-Cu alloy. [ABSTRACT FROM AUTHOR]

High undercooling rapid solidification process has a significant impact on the solidified microstructure and mechanical properties of materials. In this study, particles with a diameter range between 100 and 1100 µm of ternary Zr 50 Cu 40 Al 10 and Zr 49 Cu 46 Al 5 alloys were prepared by a drop tube. The evolution process of particle-solidified microstructure with decreasing particle diameter was analyzed using XRD, SEM, and TEM. For Zr 50 Cu 40 Al 10 alloy, the solidified microstructure in the particle diameter ranges of 200–1100 µm is composed of an amorphous phase and a primary B2-ZrCu high-temperature metastable phase. As the particle diameter reduces, the retained crystalline phase in the solidified microstructure decreases progressively, and the dendrite grain length gradually becomes finer. When the particle diameter is reduced to below 200 µm, the crystalline phase has completely disappeared, and the solidified microstructure evolves into a completely amorphous structure. For Zr 49 Cu 46 Al 5 alloy, in the particle diameter range of 400–1100 µm, the solidified microstructure consists of primary B19'-ZrCu phase, a small amount of Cu 10 Zr 7 phase and Zr 14 Cu 51 phase. As the particle diameter reduces, a grain refinement phenomenon also occurs in the solidified microstructure and the dendritic morphology changes from equiaxed grain to rod-shaped dendrite. The amorphous phase appears when the particle diameter is lowered to 470 µm and solidified microstructure evolves into a completely amorphous structure as the particle diameter is reduced to 146 µm. The nanoindentation results show that the micromechanical properties of amorphous composite microstructures are superior to the single component composition of crystalline or amorphous phase for Zr 50 Cu 40 Al 10 and Zr 49 Cu 46 Al 5 alloys. The reinforcement effect on the amorphous phase is related to the content of the second crystalline phase. Moreover, under similar particle diameters, the mechanical properties of Zr 50 Cu 40 Al 10 particles are always better than those of Zr 49 Cu 46 Al 5 particles. • In-situ preparation of amorphous composite particles using containerless rapid solidification technique. • Comparison of the amorphous forming ability between the two Zr-Cu-Al alloys. • Characterization of the phase constitution and solidified microstructure evolution under different diameters. • Amorphous composite materials exhibit higher hardness and elastic modulus properties. [ABSTRACT FROM AUTHOR]

Abstract: Substantial undercooling up to 550 K (0.386T E, with T E as the melting point) was achieved in eutectic Ni78.6Si21.4 alloy melt using glass fluxing combined with cyclic superheating. Accordingly, a particular refined microcrystalline morphology is obtained in the as-solidified structure. The physical mechanism of the grain refinement subjected to high undercooling is interpreted in terms of the classical nucleation theory and LKT/BCT model. It was concluded that the above refinement can be ascribed to the substantially increased nucleation rate under high undercooling. [Copyright &y& Elsevier]

A large undercooling (250 K) was achieved in eutectic Ni78.6Si21.4 melt by the combination of molten-glass denucleation and cyclic superheating. The metastable phase formation process in the bulk undercooled eutectic Ni78.6Si21.4 melts was investigated. With the increase of undercooling, different metastable phases form in eutectic Ni78.6Si21.4 melts and part of these metastable phases can be kept at room temperature through slow post-solidification. Under large undercooling, the metastable phases β2-Ni3Si, Ni31Si12 and Ni3Si2 were identified. Especially, the Ni3Si2 phase was obtained in eutectic Ni78.6Si21.4 alloy for the first time. Based on the principle of free energy minimum and transient nucleation theory, the solidification behavior of melts was analyzed with regard to the metastable phase formation when the melts were in highly undercooled state. [ABSTRACT FROM AUTHOR]