Tingju Li

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 Criterion for Topological Close-Packed Phase Formation in High Entropy Alloys

The stability of topological close-packed (TCP) phases were found to be well related to the average value of the d-orbital energy level ( overline{Md} ) for most reported high entropy alloys (HEAs). Excluding some HEAs that contain high levels of the elements aluminum and vanadium, the results of this study indicated that the TCP phases form at ( overline{Md} ) > 1.09. This criterion, as a semi-empirical method, can play a key role in designing and preparing HEAs with high amounts of transitional elements.

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 (602.04u2009MPa) 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.

Grain refining performance of Al-B master alloys with different microstructures on Al-7Si alloy

The improved halide salt route has been employed to produce Al-3B master alloys with different microstructures. The micro-structural features of AlB2 particles exhibited different impacts on the grain refining response of Al-7Si alloy, evolving with the holding time after inoculation. Large blocky AlB2 particles suffer from an intense fading effect while strip-like particles are not effective enough at the 0.005 wt% B addition level. Among the various Al-3B master alloys, sample 1 containing fine, blocky AlB2 particles was revealed to be the most efficient in refining Al-7Si alloy. With inoculation of only 0.005%B of such alloy, the final grain size of Al-7Si alloy can reach 238 μm. Its grain refining efficiency was more or less retained for holding times up to 60 min.

Three-Layer Al/Al–B4C Composite Material Prepared by Casting and Hot Rolling

Casting and hot rolling are proposed to verify a semi-continuous casting and hot rolling process to produce a three-layer Al/Al–B4C metal-cermet composite. The outer layer is pure Al, the inner layer is Al matrix composite reinforced with B4C particles (the mass fraction of Al and B4C are 62.36 and 37.64%, respectively). The results show that the reinforced particles are distributed in the inner layer homogeneously, and both layers are sintered together with no pores or flaws. The clad ratios of the composites of 26, 5, 3, and 1 mm are 50.28, 54.67, 61.23, and 57.46%, respectively. The Vickers hardness varies from 46 HV for the outer pure aluminum layer to 158 HV for the inner composite layer in 3 mm specimen, indicating totally different properties in different layers.

Effect of B4C particle size on the mechanical properties of B4C reinforced aluminum matrix layered composite

Abstract Reinforcement particle size is very important for the performance of metal ceramic composites. This work studied the influence of B4C particle size on the mechanical properties of Al matrix layered composites. These composites were fabricated using a simplified semicontinuous casting and hot-rolling process. To obtain an optimized filling structure of particles, Horsfield filling principle was applied to determine the size and mass fraction of B4C particles. Four sizes of B4C particles were used with various combinations. The results showed that with the increase of the B4C particle size and fine B4C mass fraction, the hardness of the composites decreases whereas the impact strength and ultimate tensile strength increase. The residual stress at interface should be responsible for the variation in properties. Besides, the interparticle distance also contributes to the change in impact strength and ultimate tensile strength.

Influence of casting speed on fabricating Al-1%Mn and Al-10%Si alloy clad slab

Abstract In this paper, the effect of casting speed on fabricating Al-1%Mn and Al-10%Si alloy clad slab was investigated. Before the trials with the semi-continuous casting process were conducted, a series of simulation with the engineering software FLUENT was made to forecast the influence of the casting speed on the temperature field and the liquid fraction of the clad slab. The simulation results indicated that the increase of the casting speed reduces the time in the cooling zone, thus the thickness of the Al-1%Mn solidification shell under the dividing plate gets thinner. Based on the simulation results, the Al-1%Mn alloy and Al-10%Si alloy clad slab was successfully produced by semi-continuous casting process. The interface of clad ingots was investigated by methods of metallographic and electron probe microanalysis. The tests on the interface confirmed the simulation results and showed that a clad slab of two different aluminum alloys with excellent metallurgical bonding was achieved by semi-continuous casting. According to the result of the tensile tests, the strength of the specimens remains at 110 MPa, and the fracture position located in the Al-1%Mn alloy indicates that the strength of the interface is higher than that of the Al-1%Mn alloy.