Huijun Kang

A Promising New Class of High-Temperature Alloys: Eutectic High-Entropy Alloys

High-entropy alloys (HEAs) can have either high strength or high ductility, and a simultaneous achievement of both still constitutes a tough challenge. The inferior castability and compositional segregation of HEAs are also obstacles for their technological applications. To tackle these problems, here we proposed a novel strategy to design HEAs using the eutectic alloy concept, i.e. to achieve a microstructure composed of alternating soft fcc and hard bcc phases. As a manifestation of this concept, an AlCoCrFeNi2.1 (atomic portion) eutectic high-entropy alloy (EHEA) was designed. The as-cast EHEA possessed a fine lamellar fcc/B2 microstructure, and showed an unprecedented combination of high tensile ductility and high fracture strength at room temperature. The excellent mechanical properties could be kept up to 700°C. This new alloy design strategy can be readily adapted to large-scale industrial production of HEAs with simultaneous high fracture strength and high ductility.

A promising structure for fabricating high strength and high electrical conductivity copper alloys

To address the trade-off between strength and electrical conductivity, we propose a strategy: introducing precipitated particles into a structure composed of deformation twins. A Cu-0.3%Zr alloy was designed to verify our strategy. Zirconium was dissolved into a copper matrix by solution treatment prior to cryorolling and precipitated in the form of Cu5Zr from copper matrix via a subsequent aging treatment. The microstructure evolutions of the processed samples were investigated by transmission electron microscopy and X-ray diffraction analysis, and the mechanical and physical behaviours were evaluated through tensile and electrical conductivity tests. The results demonstrated that superior tensile strength (602.04 MPa) and electrical conductivity (81.4% IACS) was achieved. This strategy provides a new route for balancing the strength and electrical conductivity of copper alloys, which can be developed for large-scale industrial application.

Revealing extraordinary tensile plasticity in layered Ti-Al metal composite

Layered Ti-Al metal composite (LMC) fabricated by hot-pressing and hot-rolling process displays higher ductility than that of both components. In this paper, a combination of digital image correlation (DIC) and X-ray tomography revealed that strain delocalization and constrained crack distribution are the origin of extraordinary tensile ductility. Strain delocalization was derived from the transfer of strain partitioning between Ti and Al layer, which relieved effectively the strain localization of LMC. Furthermore, the extensive cracks of LMC were restricted in the interface due to constraint effect. Layered architecture constrained the distribution of cracks and significantly relieved the strain localization. Meanwhile, the transfer of strain partitioning and constrained crack distribution were believed to inhibit the strain localization of Ti and change the deformation mechanisms of Ti. Our finding enriches current understanding about simultaneously improving the strength and ductility by structural design.

Horizontal continuous casting process under electromagnetic field for preparing AA3003/AA4045 clad composite hollow billets

Abstract A modified horizontal continuous casting process under the electromagnetic field was proposed for preparing AA3003/AA4045 clad composite hollow billets. To investigate the effect of electromagnetic field on this process, a comprehensive three-dimensional model was developed. Two cases with and without electromagnetic field were compared using the simulations. When rotating electromagnetic stirring is applied, the flow pattern of fluid melt is greatly modified; the mushy zone becomes much wider, the temperature profile becomes more uniform, and the solid fraction decreases for both the external and internal alloy melt layers. These modifications are beneficial for the formation of a bimetal interface and fine and uniform grain structure of the clad composite hollow billet. Experiments conducted using the same electrical and casting parameters as the simulations verify that under the electromagnetic field the microstructure of the clad composite hollow billet becomes fine and the diffusion of the elements at the interface is promoted.

Influence of Cryorolling on the Precipitation of Cu–Ni–Si Alloys: An In Situ X-ray Diffraction Study

The effect of cryorolling on the precipitation process of deformed Cu–Ni–Si alloys was investigated through in situ synchrotron X-ray diffraction technique. The results demonstrate that the precipitation process is significantly accelerated by cryorolling. Cryorolling produces higher dislocation density, which provides more heterogeneous nucleation sites for Ni2Si precipitates, hence promotes precipitation. In the early stage of aging, the enhanced nucleation of precipitates accelerates the depletion of supersaturation, and finer precipitates are obtained. In addition, recrystallization is promoted as a result of high stored energy in the cryorolled Cu–Ni–Si alloys, which facilitates the formation of discontinuous precipitation in the late stage of aging.

Simulation Study of Al-1Mn/Al-10Si Circular Clad Ingots Prepared by Direct Chill Casting

A modified direct chill casting process based on Novelis FusionTM Technology co-casting process was used recently to prepare Al-1Mn/Al-10Si circular clad ingots. In the current study, a comprehensive simulation model was developed to investigate the direct chill casting process for preparing the Al-1Mn/Al-10Si circular clad ingots, and a parametric study and experimental research of the direct chill casting process was conducted to explore potential success and failure casting conditions. The simulation results revealed the bonding mechanism of the Al-1Mn/Al-10Si interface in the direct chill casting process and identified the effect of certain parameters on casting performance. The results indicated that the effect of casting speed and Al-1Mn casting temperature on the variations of the minimum solid fraction of Al-1Mn at the interface is stronger than that of cooling water flow rate in inner mold, while Al-10Si casting temperature is the weakest of the four casting parameters. The corresponding experimental results verified that Al-1Mn/Al-10Si circular clad ingot with acceptable metallurgical bonding can be successfully prepared by direct chill casting process under the proper casting parameters. The thickness of diffusion zone is about 40 μm, and the fractured position in tensile test was located in the Al-1Mn alloy side which indicated the strength of the interfacial region is higher than that of Al-1Mn alloy.

Optimizing the thermoelectric transport properties of BiCuSeO via doping with the rare-earth variable-valence element Yb

Herein, we propose for the first time a novel recipe to improve the thermoelectric properties of BiCuSeO by doping with variable valence elements. Taking Yb-doped BiCuSeO as a typical example, the dimensionless figure of merit (ZT) is optimized by simultaneously doping with Yb2+ to increase the carrier concentration and by introducing Yb3+ to increase the carrier mobility. The mechanisms of valence fluctuation, the increase in carrier concentration and the increase in mobility are investigated by combining experimental results with first-principles calculations. By coupling high electronic conductivity, medium Seebeck coefficient, and low thermal conductivity, a maximum ZT value of 0.62 is achieved at 823 K for Bi0.7Yb0.3CuSeO, which is 1.55 times higher than that of pristine BiCuSeO. These findings undoubtedly provide a new strategy for the rational design of high-ZT thermoelectric materials.

3D Morphology and Formation Process of the Icosahedral Quasicrystalline Phase in Rapidly Solidified Al–Mn Alloy

Three-dimensional morphology and formation process of icosahedral quasicrystal phase have been investigated in a melt-spun Al–18Mn alloy (in wt%). Three distinct layers corresponding to varying temperature gradient have been observed on the cross section of the ribbons. 3D morphologies of cellular and dendritic icosahedral phase have been obtained through electro-etching. A model has been proposed to describe the formation process of the icosahedral phase and α-Al during the rapid solidification. The icosahedral phases are primarily precipitated from the melt into fine cellular and dendritic particles, and subsequently engulfed by the α-Al which propagates in a planar morphology.

Application of Rotating Magnetic Field to Improve the Reinforcement Distribution, Electrical Conductivity and Mechanical Properties of Copper Matrix Composite

Particulate reinforced metal matrix composites are confronted with the problem of particle aggregation emerging in the process of solidification. It sharply deteriorates the mechanical properties of the composites. In order to improve the particle distribution, in situ TiB2/Cu composites were prepared using Ti and Cu-B master alloys in a vacuum medium frequency induction furnace equipped with a rotating magnetic field (RMF). The effect of RMF magnetic field intensity employed on the particles distribution, mechanical properties and electrical conductivity of the TiB2/Cu composites were investigated. The results show that with the applied RMF, TiB2 particles are homogeneously distributed in the copper matrix, which significantly improves the mechanical properties and electrical conductivity of TiB2/Cu composites. The mechanism of RMF may be ascribed to the following two aspects. On one hand, the electromagnetic body force generated by appropriate RMF drives forced convection in the equatorial plane of composite melt during solidification. On the other hand, a secondary flow in the meridional plane is engendered by a radial pressure gradient, thus making a strong agitation in the melt. These two effects result in a homogenous dispersion of TiB2 particles in the copper matrix, and hence excellent properties of TiB2/Cu composites were obtained.

In Situ Study on the Evolution of Dendrite Morphology Affected by Electric Field in Sn‐Bi Alloy

With high energy, high brilliance and eminent collimation, synchrotron X-ray imaging enables in situ observation of dendrite growth during the solidification of Sn-Bi alloy under an applied electric field such as direct current (DC) and electric current pulse (ECP). In present study, the application of DC leads to the suppression of dendrite growth and the degeneration of the secondary and higher order dendrites. Due to the induced Joule heat, the sharp dendrite tips are modified to be round or flat. By applying ECP to the sample during the whole solidification process, a significant grain refinement is observed.