Zhi-Sheng Nong

First principles calculation of intermetallic compounds in FeTiCoNiVCrMnCuAl system high entropy alloy

The structural, electronic and elastic properties of common intermetallic compounds in FeTiCoNiVCrMnCuAl system high entropy alloy were investigated by the first principles calculation. The calculation results of formation enthalpy and cohesive energy show that FeTi, Fe2Ti, AlCrFe2, Co2Ti, AlMn2V and Mn2Ti phases may form in the formation process of the alloy. Further studies show that FeTi, Fe2Ti, AlCrFe2, Co2Ti and AlMn2V phases with higher shear modulus and elastic modulus would be excellent strengthening phases in high entropy alloy and would improve the hardness of the alloy. In addition, the partial density of states was investigated for revealing the bonding mode, and the analyses on the strength of p-d hybridization also reveal the underlying mechanism for the elastic properties of these compounds.

Prediction of mechanical properties of A357 alloy using artificial neural network

Abstract The workpieces of A357 alloy were routinely heat treated to the T6 state in order to gain an adequate mechanical property. The mechanical properties of these workpieces depend mainly on solid-solution temperature, solid-solution time, artificial aging temperature and artificial aging time. An artificial neural network (ANN) model with a back-propagation (BP) algorithm was used to predict mechanical properties of A357 alloy, and the effects of heat treatment processes on mechanical behavior of this alloy were studied. The results show that this BP model is able to predict the mechanical properties with a high accuracy. This model was used to reflect the influence of heat treatments on the mechanical properties of A357 alloy. Isograms of ultimate tensile strength and elongation were drawn in the same picture, which are very helpful to understand the relationship among aging parameters, ultimate tensile strength and elongation.

Residual elastic stress−strain field and geometrically necessary dislocation density distribution around nano-indentation in TA15 titanium alloy

Abstract Nanoindentation and high resolution electron backscatter diffraction (EBSD) were combined to examine the elastic modulus and hardness of α and β phases, anisotropy in residual elastic stress–strain fields and distributions of geometrically necessary dislocation (GND) density around the indentations within TA15 titanium alloy. The nano-indention tests were conducted on α and β phases, respectively. The residual stress–strain fields surrounding the indentation were calculated through cross-correlation method from recorded patterns. The GND density distribution around the indentation was calculated based on the strain gradient theories to reveal the micro-mechanism of plastic deformation. The results indicate that the elastic modulus and hardness for α phase are 129.05 GPas and 6.44 GPa, while for β phase, their values are 109.80 GPa and 4.29 GPa, respectively. The residual Mises stress distribution around the indentation is relatively heterogeneous and significantly influenced by neighboring soft β phase. The region with low residual stress around the indentation is accompanied with markedly high 〈 a 〉 type and prismatic-GND density.

Finite element analysis of quenching temperature field, residual stress and distortion in A357 aluminum alloy large complicated thin-wall workpieces

Abstract The quenching processes of A357 aluminum alloy large complicated thin-wall workpieces were investigated by finite element method (FEM) simulation based on ABAQUS software. Heat transfer coefficients of different quenchants were accurately calculated by a traditional method of inverse heat transfer. The accurate heat transfer coefficients of different quenchants can ensure accuracy FEM results of temperature field of A357 alloy large complicated thin-wall workpieces during quenching. The quenchants of water, machine oil and 5%-UCON quenchant A were used. Quenching residual stress and distortion were investigated by considering the influence of quenchant and quenchant temperature. The maximum residual stress and distortion of quenched workpieces were predicted by using FEM simulation based on ABAQUS software.

Optimum design of flow distribution in quenching tank for heat treatment of A357 aluminum alloy large complicated thin-wall workpieces by CFD simulation and ANN approach

Abstract Based on the computational fluid dynamics (CFD) method, a quenching tank with two agitator systems and two flow-equilibrating devices was selected to simulate flow distribution using Fluent software. A numerical example was used to testify the validity of the quenching tank model. In order to take tank parameters (agitation speed, position of directional flow baffle and coordinate position in quench zone) into account, an approach that combines the artificial neural network (ANN) with CFD method was developed to study the flow distribution in the quenching tank. The flow rate of the quenching medium shows a very good agreement between the ANN predicted results and the Fluent simulated data. Methods for the optimal design of the quenching tank can be used as technical support for industrial production.

CONSTITUTIVE BEHAVIOR OF AS-QUENCHED Al-Cu-Mn ALLOY

The hot flow stress of as-quenched Al-Cu-Mn alloy was modeled using the constitutive equations. The as-quenched Al-Cu-Mn alloy were treated with isothermal hot compression tests in the temperature range of 350–500°C, the strain rate range of 0.001–1 s-1. The hyperbolic sine equation was found to be appropriate for flow stress modeling and prediction. Based on the hyperbolic sine equation, a constitutive equation is a relation between 0.2 pct yield stress and deformation conditions (strain rate and deformation temperature) was established. The corresponding hot deformation activation energy (Q) for as-quenched Al-Cu-Mn alloy was determined to be 251.314 kJ/mol. Parameters of constitutive equation of as-quenched Al-Cu-Mn alloy were calculated at different small strains (≤ 0.01). The calculated flow stresses from the constitutive equation are in good agreement with the experimental results. Therefore, this constitutive equation can be used as an accurate temperature-stress model to solve the problems of quench distortion of Al-Cu-Mn alloy parts.

Effects of Annealing on Microstructure, Mechanical and Electrical Properties of AlCrCuFeMnTi High Entropy Alloy

The multi-component AlCrCuFeMnTi high entropy alloy was prepared using a vacuum arc melting process. Serial annealing processes were subsequently performed at 590 °C, 750 °C, 955 °C and 1 100 °C respectively with a holding time of 4 h at each temperature. The effects of annealing on microstructure, mechanical and electrical properties of as-cast alloy were investigated by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results show that two C14 hexagonal structures remain unchanged after annealing the as-cast AlCrCuFeMnTi alloy specimens being heated to 1 100 °C. Both annealed and as-cast microstructures show typical cast-dendrite morphology and similar elemental segregation. The hardness of alloys declines as the annealing temperature increases while the strength of as-cast alloy improves obviously by the annealing treatment. The electrical conductivities of annealed and as-cast alloys are influenced by the distribution of interdendrite regions which is rich in Cu element.

CALCULATIONS OF SELF-DIFFUSION PROCESS AND COEFFICIENT IN ORDERED B2 AlCo

The self-diffusion process in B2 type intermetallic compound AlCo has been investigated by the first-principles calculations within the frame work of density functional theory (DFT). The obtained mono-vacancy formation, migration and activation energies for four self-diffusion mechanisms, the next-nearest-neighbor (NNN) jump, [110] six-jump cycle (6JC), straight [100] 6JC and bent [100] 6JC diffusion show that the NNN jump mechanism of Co vacancy requires the lowest activation energy (Q = 6.835 eV) in these diffusion mechanisms, which indicates that it is the main way of self-diffusion in AlCo. The electronic structure including the electron density difference on (-1 1 0) plane as well as atomic Mulliken populations were calculated, and the change of bonding behavior during the [110] 6JC process was discussed in detail. Finally, the self-diffusion coefficients of NNN jump and 6JC mechanisms for AlCo were also studied via the first-principles calculations and semi-empirical predictions, which indicates that the self-diffusion coefficients for NNN jump of Co vacancy show the highest value than the others.