Zhi Qiang Cao

Formation rules of single phase solid solution in high entropy alloys

A new empirical rule to form single phase solid solution structured high entropy alloys was proposed by integrating three dependent parameters, i.e. atomic size difference δ, mixing enthalpy ΔHmix and valence electron concentration VEC. It was found that the single phase face centred cubic solid solution will form if δu2009<u20094.27%, −u20097.27u200akJu2009mol−u20091u2009<u2009ΔHmixu2009<u20094u200akJu2009mol−u20091 and VEC>8, while the single phase body centred cubic solid solution will form in the case of δu2009<u20094.27%, −u20097.27u200akJu2009mol−u20091u2009<u2009ΔHmixu2009<u20094u200akJu2009mol−u20091 and VECu2009<u20096.87. This empirical rule was verified by typical high entropy alloys that have been reported.

Simulation study on horizontal continuous casting process of copper hollow billet under rotating electromagnetic stirring Part 1—model description and primary results

In this paper, a comprehensive three-dimensional mathematical model is built to investigate the effect of rotating electromagnetic stirring on solidification process of copper hollow billet during horizontal continuous casting. The finite element package ANSYS and the commercial finite volume package FLUENT are employed for simulating electromagnetic field and thermal flow field respectively. The Lorenz force and Joule heat are transferred between them with a special data communication method. The model involves the solution of Maxwell equations, Navier—Stokes equations and energy equation for the turbulence characteristics κ and ϵ. The measured magnetic flux density is used for validating the proposed model. The simulated results show that rotating electromagnetic stirring causes a swirling motion in the hollow billet and makes the temperature field uniform. In the present study, the temperature gradient decreases from 748 to 196 K m−1, and the sump depth is reduced from 48·8 to 29·0 mm.

Microstructure and Mechanical Properties of the W-Ni-Co System Refractory High-Entropy Alloys

The elements Mo, Cr and V were added to the W-Ni-Co system high entropy alloys, the effects of these added elements on microstructure and mechanical properties of these alloys were studied. The alloys were produced by vacuum arc melting. The compositions were W0.5Ni2Co2VMo0.5, W0.5Ni2Co2VCr0.5 and W0.5Ni2Co2CrMo0.5 (denoted as Alloy 1, Alloy 2 and Alloy 3) respectively. The theoretical melting temperatures were higher than 2000 K. X-ray diffraction, SEM and energy dispersive spectroscopy (EDS) results indicated that the matrix of the alloys is face-centered cubic (FCC) solid-solution, the alloys showed dendrite crystal structure. Ni, Co elements were enriched in the dendrite areas, the W, Mo were enriched in the inter-dendrite regions ,while V, Cr elements were uniform distribution. The Vickers hardness of these alloys was 376.1 HV, 255.88 HV and 306.8 HV, respectively. The yield strength values (σ0.2) of Alloy 1, Alloy 2 and Alloy 3 were approximately 1000MPa, 750MPa, 250MPa, respectively. The alloys show good compression plasticity deformation capacity at RT.

Simulation study on horizontal continuous casting process of copper hollow billet under rotating electromagnetic stirring Part 2 - effects of electromagnetic and casting parameters on solidification process

The comprehensive three-dimensional mathematical model proposed in Part 1 is used to investigate the effect of rotating electromagnetic stirring on the solidification process of copper hollow billet during horizontal continuous casting. In this part, the model is used to investigate the effects of electromagnetic parameters and casting speed on the electromagnetic field, temperature field, fluid flow and solidification during the horizontal continuous casting with rotating electromagnetic stirring. The results show that electromagnetic frequency, current intensity and casting speed have significant influence on the tangential velocity, temperature gradient, liquid fraction and sump depth.

Numerical simulation and fabrication for bimetal clad slab under direct current magnetic field

Abstract In this paper, a new fabrication process of clad slab by semicontinuous casting under direct current magnetic field is presented. The influence of casting temperature and casting speed on temperature field as well as the distribution of liquid fraction during the fabrication of clad slab are numerically studied with the engineering software FLUENT. The simulation results show that the casting speed has more influence on producing well clad slab than casting temperature. The casting temperature should be controlled to be as low as possible. When the casting speed is higher than 0·002 m s−1, it will make the two kinds of alloy melts mix badly. According to the optimal processing parameters obtained from the simulation results, the clad slab of 3003 and 4004 wrought aluminium alloys, which have different properties on both sides, is successfully produced. Therefore, the simulation model is well validated.

Effect of Ta Addition on Structural Evolution and Mechanical Properties of the CoFeNi2W0.5 High Entropy Alloy

A series of CoFeNi2W0.5Tax (x = 0-0.6) high entropy alloys (HEAs) were synthesized by arc melting to investigate the alloying effect of Ta element on the microstructure and mechanical properties of the CoFeNi2W0.5 alloy system. Phase constitution, microstructure and mechanical properties of the alloys were analyzed by X-ray diffraction (XRD), scanning electron microscopes (SEM), Vickers hardness and compressive test. It was found that when x = 0, the alloy consists of a single-phase face-centered cubic (FCC) solid solution structure and exhibit excellent ductility, the compressive plastic elongation of which can reach 80% without fracture. While with increasing Ta content, the brittle Co2Ta-type Laves phase appears which leads to a decrease of the plastic strain and an increase of the yield strength, and the Vickers hardness shows an obvious increase from HV 179.5 to HV 753.2.

Effect of Annealing Time on the Microstructure and Mechanical Properties of the AlCrFeNi2Ti0.5 High Entropy Alloys

As-cast AlCrFeNi2Ti0.5 high entropy alloy was annealed at 600 °C and 800 °C for 6 h, 12 h and 24 h, respectively. The effect of annealing time on the microstructure and mechanical properties were investigated intensively. The as-cast sample showed a fine and homogeneous microstructure, and owned an excellent ductility of 16.3%. While the 600 °C, 12 h annealed sample showed the highest compressive strength of 2658 MPa and yield strength of 2046 MPa. The mechanical properties of the samples after 800 °C annealed for 12 h and 24 h deteriorated obviously. High annealing temperature promotes the growth of the crystal grains, and the new generated FeCr-type intermetallic phase leads to the embrittlement of the alloy.

Numerical simulation on direct chilled continuous casting process for preparing clad slab of aluminium alloys

Abstract The numerical simulation is used to investigate the influence of processing parameters on direct chilled continuous casting process for preparing clad slab of Al–1Mn and Al–10Si alloys. In order to get a clear bonding interface avoiding unexpected mixing, a special dividing plate is set between molten clad materials. The effects of cooling intensity and cooled part height of dividing plate, casting speed and pouring temperature on the temperature field, liquid fraction, and solidification shell of clad slab have been studied in detail. A practical experiment has been done based on the simulation studies. It was found that a clad slab with excellent metallurgical bonding can be obtained when the cooling water flow of dividing plate is 250 L h−1, the cooled part height of dividing plate is 20 mm, the casting speed is 80 mm min−1 and the pouring temperatures of Al–1Mn and Al–10Si alloys are 710 and 670°C respectively.

Fabrication of MgAl2O4 Reinforced Aluminum Matrix Gradient Composites under High Frequency Magnetic Field

Two kinds of gradient composites, whose compositions were Al-20%Mg2Si-5%Si (sample-1) and Al-14.9%Mg2Si-10.3%Si-11.8% MgAl2O4 (sample-2), were fabricated based on the electromagnetic separation under the high frequency magnetic field. The microcosmic structures of two composites were analyzed and the effect of the addition of SiO2 on the microcosmic structure and the mechanical property were discussed. It is found that the particles MgAl2O4 accumulated in periphery of specimen of sample-2, which come from the diffusion reaction between SiO2 and aluminum and magnesium in the melt and was proved by the result of X-ray analysis. Besides the MgAl2O4, the primary phases of Mg2Si, Si also emerged in periphery of specimens. At last, the hardness was given along the radial direction. The result shows that the hardness of the two materials exhibit graded distribution in radial direction and the hardness of sample-2 is higher than that of sample-1 due to the formation of MgAl2O4 particles. Both the two materials meet the requirements of gradient composite material whose outer area is in high intensity and center area is in flexile.