Zhonghong Lai

Fabrication and microstructure of ZrO2-Ni functional gradient material by powder metallurgy

Functional gradient material (FGM) of the ZrO2-Ni system was developed by a powder metallurgical process, and investigated for its microstructure by means of X-ray diffractometry (XRD), electron probe microanalysis (EPMA), transmission electron microscopy (TEM) and optical microscopy. It was shown that the sintered body of ZrO2-Ni FGM is almost fully densified, and its chemical composition and microstructure have the expected gradient distribution. With composition variation, the microstructure changes gradually from zirconia particles dispersed in a nickel matrix to the converse with nickel particles dispersed in a zirconia matrix, with network structures in the intermediate composition range. Therefore, no distinct interfaces appear in the FGM due to the gradient change of components, that is, both zirconia and nickel are present everywhere in the microstructure. In phase composition, the sintered ZrO2-Ni FGM consists of nickel, tetragonal zirconia and a little monoclinic zirconia. No reaction between nickel and zirconia has been detected. The substructure of nickel and monoclinic zirconia are twins, and strain fringes can also be found in zirconia.

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.

Mechanical Performance of ZrO2-Ni Functionally Graded Material by Powder Metallurgy

ABSTRACT The mechanical performance of a ZrO2-Ni functionally graded material (FGM) developed by powder metallurgy was investigated by means of three-point bending test. It was shown that the mechanical behavior of ZrO2-Ni system strongly depends on constitutional variation, exhibiting typical behavior of elasto-plastic deformation similar to metallic material in the Ni-rich composition and typical behavior of elastic deformation and macroscopic brittle fracture in the ZrO2-rich composition. The mechanical properties of ZrO2-Ni system also display various gradient distributions corresponding to constitutional change. With the rise of Ni content, Vickers hardness decreases, but fracture toughness increases remarkably. Young’s modulus changes corresponding to the distribution of porosity. Bending strength is quite sensitive to microstructural factors, and its peak and valley point are relevant to the dispersion strengthening of ZrO2 particles and the distributional transition of components, respectively.

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.

Quantum confined two-dimensional electron/hole gas switching by facet orientation of perovskite oxides

The Polar discontinuity at heterointerface and the bare surface reconstructs the electronic phase of perovskite oxides. This gives rise to confined free electrons which intrinsically transit material from band insulator to metal. However, the insulator–metal transition induced by free holes has not been investigated so far due to the challenge in obtaining free hole state in oxides. Here, we propose a simple method whereby free holes can be obtained via polar facet reorientation. In the high polarity case, free holes can be supported by the lift up of O 2p subbands, which split into three independent subbands (one heavy hole subband and two light hole subbands) due to strong quantum confinement. Results show that both of the free electron and hole states are confined in a two dimensional quantum well, subjecting to the confined energy (E − EF) and occupied density of states around the Fermi level, indicating a finite thickness for preserving the metal states.

Surface plasmon resonances behavior in visible light of non-metal perovskite oxides AgNbO3

We investigate the surface plasmon resonances (SPRs) behavior of silver niobate (AgNbO3) experimentally and theoretically. Result shows that the localized SPRs (LSPRs) of AgNbO3 combining with its interband transitions enlarge the absorption band across the whole ultraviolet-visible range. The LSPRs behavior in visible-light is mainly ascribed to the metal-like state of silver ion and self-assembled microstructures of AgNbO3 microcrystal. The ab initio density functional theory calculations are carried out to obtain the further insight of the SPRs behaviors. Theoretical study indicates that the Ag atoms are weakly bound in the perovskite structure, leading to a metal-like state, which was the key factor to SPRs behavior of AgNbO3.