Yuying Wu

Morphological evolution of TiC from octahedron to cube induced by elemental nickel

TiC particles with various morphologies from octahedron to cube were synthesized and investigated by Field emission scanning electron microscopy (FESEM) in three-dimensional space. The morphological evolution of TiC grain and its growth mechanism were also discussed. TiC particles prefer an octahedral morphology (equilibrium shape) enclosed by eight {111} facets with minimized total surface free energy in Al–Ti–C alloy, while they tend to form a cube enclosed by six {100} facets under the influence of Ni in Al–Ni–Ti–C alloy. Due to the strong interaction between Ni–3d and C–2p orbitals, Ni atoms in the melt selectively absorb on {100} faces of the growing TiC crystal rather than on the polar {111} faces and reduce the specific surface energy of {100}. According to Wuffs theorem, the growth rate of {100} is lowered correspondingly, while the relative growth rate of {111} is accelerated. Thus, the higher growth rate along direction will lead to the shrinkage of {111} faces gradually, while six {100} faces are reserved to form a TiC cube because of their lower growth rates. Furthermore, a similar morphology evolution to TiC crystals can also be found in Fe- and Co-containing melts. It is revealed that the crystal growth of TiC follows the same model under the effect of group VIII elements (Fe, Co and Ni).

Unveiling the Semicoherent Interface with Definite Orientation Relationships between Reinforcements and Matrix in Novel Al3BC/Al Composites.

High-strength lightweight Al-based composites are promising materials for a wide range of applications. To provide high performance, a strong bonding interface for effective load transfer from the matrix to the reinforcement is essential. In this work, the novel Al3BC reinforced Al composites have been in situ fabricated through a liquid-solid reaction method and the bonding interface between Al3BC and Al matrix has been unveiled. The HRTEM characterizations on the Al3BC/Al interface verify it to be a semicoherent bonding structure with definite orientation relationships: (0001)Al3BC//(11̅1)Al;[112̅0]Al3BC//[011]Al. Periodic arrays of geometrical misfit dislocations are also observed along the interface at each (0001)Al3BC plane or every five (11̅1)Al planes. This kind of interface between the reinforcement and the matrix is strong enough for effective load transfer, which would lead to the evidently improved strength and stiffness of the introduced new Al3BC/Al composites.

In-situ formation of Al-CaB6 composites with low resistivity

In this work, in-situ CaB6 reinforced aluminum matrix composites were fabricated, and the microstructure, resistivity, microhardness and coefficient of thermal expansion (CTE) of Al-CaB6 composites were studied. It is found that CaB6 compounds can be formed by reducing reaction occurred in the Al melt: AlB2 + Ca → CaB6 + [Al]. CaB6 exhibits a hexahedron morphology and distributes uniformly in the Al alloy matrix. The resistivity of Al-CaB6 composites is 3.02×10−8 Ω·m, which is close to that of pure Al and lower than that of 6063Al/Ga composites. The average microhardness of Al-CaB6 composites can reach 1270 MPa, 259.8% higher than that of pure Al. Compared to pure Al, the CTE of Al-CaB6 composites is much lower.

The dispersive orientated-precipitation of AlP on alumina film and its effect on the primary Si gathering behavior in the Al–Si alloy surface layer

In this paper, the dispersive orientated-precipitation of AlP on γ-Al2O3 film and its effect on the primary Si gathering behavior in the near-eutectic Al–Si alloy surface layer was investigated. Experimental results show that (111) orientated AlP can nucleate on the γ-Al2O3 film, which is attributed to the high tendency for P to be adsorbed on γ-Al2O3 and the good lattice matching relationship between AlP and γ-Al2O3. It was also found that the pre-precipitated AlP particles on the γ-Al2O3 film could induce Si atoms to precipitate into numerous primary Si particles, leading to an extraordinary high volume fraction of primary Si in the near-eutectic Al–Si alloy surface layer. What is more, the X-ray diffraction experiment shows that the intensity of the Si (111) peak increases significantly and other Si peaks decrease to a very low level, which means that the primary Si particles inherited the (111) orientation characteristic from the orientated AlP grains.

Growth mechanisms of alpha-boron and beta-boron in a copper melt at ambient pressure and its stabilities

Relying on copper melt as a solvent, we successfully synthesize alpha-boron (α-B) and beta-boron (β-B) at ambient pressure. Distinct obtained allotropes ascribe to different fabrication processes and growth mechanisms. Coralloid eutectic boron exposes the messages of α-B, and primary boron has the typical traits of β-B. Considering experimental conditions and morphologies, constituent supercooling with appropriate temperature gradients is chiefly responsible for the growth of α-B. Moreover, a proper eutectic temperature and restricted growth space are provided by copper melt for α-B. For the formation of β-B, cyclic-twinning, screw dislocation-driven and the twin plane re-entrant edge (TPRE) growth mechanisms work together. Borons growth models were put forward for vivid elucidation of these mechanisms. Thereafter, the stabilities of α-B and β-B were compared based on their forming conditions and sizes. Synthesis of α-B and β-B in copper melt at ambient pressure are of great significance for their practical applications in future.

Growth of single crystalline boron nanotubes in a Cu alloy

Herein, we report the successful synthesis of single crystalline boron nanotubes in a Cu alloy via a novel and simple direct melt reaction process. The boron nanotubes are perfect single crystals with a β-rhombohedral structure, whose size can be controlled and changed. The combined experimental and dynamic simulation results lead to the conclusion that the boron nanotubes in the Cu alloy can be considered as a crystal skeleton, closely related to the liquid structure and solidification characteristics of the Cu–B alloy, which is significantly affected by the cooling rate and boron concentration. In addition, the stable morphology of the boron nanotubes, which minimizes the energy content, is not a round tube but a parallelogram. Thus, boron tubes with large size range scale can be prepared in Cu alloys.

Relationship of Ca, B, and AlP in Al–12.6Si alloy

P modification has been widely used in Al–Si piston industry, but trace of Ca element has great influence on the P modification efficiency. In this work, it is found that primary Si can be heterogeneously nucleated by AlP in near eutectic Al–12.6Si alloy, but Ca element may destroy the P modification efficiency, whereas the addition of B can recover the P modification efficiency in near eutectic Al–12.6Si alloy with high Ca containing. The microstructure transformation was related to the reaction of Ca, B, and AlP. According to the thermodynamic calculation, Ca may react with AlP and form Ca3P2 compound in Al–Si alloy, whereas, when B was added into the melt, AlP could be reformed. The reaction of Ca, B, and AlP can be shown as follows:

Morphological evolution and growth mechanism of primary Mg2Si phase in Al–Mg2Si alloys

2{\text{AlP}} + 3{\text{Ca}} \Rightarrow {\text{Ca}}_{3} {\text{P}}{}_{2}^{{}} + 2{\text{Al}}