Yunhu Zhang

Heterogeneous nucleation of pure gold on highly smooth ceramic substrates and the influence of lattice misfit and cooling rate

Abstract The present paper considers the heterogeneous nucleation of pure Au droplet solidified on a single crystal substrate plate under the influence of lattice misfit and cooling rate. The undercooling of droplet was measured by DSC. The experimental results present that the undercooling of Au droplet is directly proportional to both the lattice misfit and the cooling rate. It means that the heterogeneous nucleation can be significantly driven by lattice misfit and cooling rate. In addition, the related results show that the relationship between the undercooling and the cooling rate of Au droplet is independent on the lattice misfit. The results with respect to the interface of solidified Au and substrate analyzed by HRTEM prove that the orientation relationship between the substrate and the solidified Au matches with the minimum lattice misfit.

Relevance of electrical current distribution to the forced flow and grain refinement in solidified Al-Si hypoeutectic alloy

Significant grain refinement in cast metals can be achieved through the application of electric currents during the solidification process. The present paper investigates the distribution of electric currents on the grain size of solidified Al-7wt.%Si alloy under the application of electric current with constant parameters flowing through two parallel electrodes into the melt within a cylindrical mould. The distribution of electric current was controlled by applying an electrical insulation material coating, boron nitride (NB), to the sidewall of the electrodes. Experimental results showed that the employment of these insulated electrodes can reduce grain size in comparison with the reference case of electrodes without BN coating. Flow measurements were performed in Ga-20wt.%In-12wt.%Sn liquid metal. Higher intensity forced flow occurred when the sidewall of the electrodes was insulated. In order to understand the underlying mechanism behind the stronger forced flow, corresponding numerical simulations were performed to reveal the distributions of the electric current, magnetic field, Lorentz force, and the resultant forced flow. The results achieved indicate that the mechanism of grain refinement driven by electric current is dendrite fragmentation induced by forced flow. In addition, a novel approach to enhance the grain refinement without additional input of current energy was developed.

Microstructure Evolution in Solidified Al–Si Hypereutectic Alloys Under the Impact of an Electric Current Pulse

Investigating the structure evolution of silicon phase in Al–Si alloys in the extra energy field is of high importance to understand and control the growth behavior of faceting phases. The present paper focuses on the influence of an electric current pulse (ECP) on the structure of directionally solidified Al–20.5 wt%Si hypereutectic alloy. Experimental results showed that ECP had a great impact on the structures of silicon phase. The interconnected, porous primary silicon structure was observed in the initial growth period, accompanied by a small quantity of eutectic silicon directly growing from the primary silicon. In the following growth period, it was surprisingly found that the numerous complex regular silicon and eutectic silicon structures appear instead of the primary silicon. On the other hand, the reference sample without ECP showeds that the structures were composed of several coarse plate-like primary silicon and eutectic structures. The variation of silicon structures indicated that the solidification behavior of faceting phases was remarkably modified by ECP, which may be due to the forced melt flow generated by the electromagnetic force.

Structures and Properties of Near-Rapidly Solidified Fe–12Mn–9Al–1.2C Dual-Phase Lightweight Steel Containing 3 wt% Ni

Fe–Mn–Al–C lightweight steel possesses not only excellent comprehensive mechanical properties, but also a low density, which is attractive to the automotive industry. In the present paper, an austenite stabilizer Ni element which usually increases the austenite content was added and its effects on the structure and mechanical properties of a near-rapidly solidified Fe–12Mn–9Al–1.2C dual-phase lightweight steel strip were studied. It was found that the addition of 3% Ni did not show the potential to increase the austenite content or enhance the mechanical properties of Fe–12Mn–9Al–1.2C dual-phase lightweight steel strip. Moreover, the addition of 3% Ni deteriorated the thermal stability of Fe–12Mn–9Al–1.2C steel. In the strip with the addition of Ni, the formed metastable austenite almost fully transformed into ferrite + κ-carbide at a lower annealing temperature. The formation process of constituent phase in this near-rapidly solidified dual-phase lightweight steel was analyzed. The related results suggested that the constituent phases of the near-rapidly solidified dual-phase lightweight steel depended on the liquid/solid transition, which was controlled by both thermodynamics and kinetics factors.