Hongchao Kou

Microstructure and Tribological Properties of AlCoCrFeNiTi0.5 High-Entropy Alloy in Hydrogen Peroxide Solution

Microstructure and tribological properties of an AlCoCrFeNiTi0.5 high-entropy alloy in high-concentration hydrogen peroxide solution were investigated in this work. The results show that the sigma phase precipitates and the content of bcc2 decrease during the annealing process. Meanwhile, the complex construction of the interdendrite region changes into simple isolated-island shape, and much more spherical precipitates are formed. Those changes of microstructure during the annealing process lead to the increase of hardness of this alloy. In the testing conditions, the AlCoCrFeNiTi0.5 alloy shows smoother worn surfaces and steadier coefficient of friction curves than does the 1Cr18Ni9Ti stainless steel, and SiC ceramic preserves better wear resistance than ZrO2 ceramic. After annealing, the wear resistance of the AlCoCrFeNiTi0.5 alloy increases coupled with SiC counterface but decreases with ZrO2 counterface.

Static recrystallization simulations by coupling cellular automata and crystal plasticity finite element method using a physically based model for nucleation

This study presents a 2D cellular automata simulation of static recrystallization (SRX) arising from the subgrain growth in single-phase material following cold deformation by coupling with a crystal plasticity finite element (CPFE) method. The spatial distribution of the stored deformation energy was obtained by CPFE simulation, based on which the initial deformed microstructure consisting of nonuniformly distributed subgrains was predicted. To simulate grain/subgrain growth during annealing, a curvature-driven mechanism was used, in which the grain/subgrain boundary energy and mobility were misorientation-dependent. On the SRX nucleation, a physically based model using critical subgrain size as criterion was adopted, which could provide better insight into the recrystallization nucleation mechanism involving grain boundary bulging. Simulations under different pre-deformation conditions were performed, and the influence of strain rate and strain on the SRX microstructure evolution and the transformation kinetics were investigated. Results show that deformation at higher strain rate can accelerate the SRX kinetics, and the SRX behavior depends more on the deformation state of individual grain than the nominal strain due to the relatively small computational domain.

Mechanical properties of porous titanium with different distributions of pore size

Abstract To satisfy the mechanical and biological requirement of porous bone substitutes, porous Ti with two different pore sizes designed in advance was fabricated by the space-holder sintering process. Mechanical properties of the porous Ti were explored via room temperature compressive~tests. The pore sizes and shapes are uniform throughout the specimens with porosities ranging from 36% to 63%. The compression strength and the elastic modulus are in the range from 94.05 to 468.57 MPa and 2.662 to 18 GPa, respectively. It is worth noting that the relationship between the compressive strength and the porosities is completely linear relation beyond the effect of pore size distributions on the mechanical properties. The value of the constant C achieved from the Gibson-Ashby model suggests that the pore sizes affect the yield strength of the porous Ti and the values of density exponent (n) for porous Ti with two different pore sizes are higher than 2, which suggests that the deformation mode of the porous Ti with a porosity ranging from 36% to 63% is mainly buckling of the cell struts.

Fabrication, pore structure and compressive behavior of anisotropic porous titanium for human trabecular bone implant applications

Porous titanium with average pore size of 100-650 μm and porosity of 30-70% was fabricated by diffusion bonding of titanium meshes. Pore structure was characterized by Micro-CT scan and SEM. Compressive behavior of porous titanium in the out-of-plane direction was studied. The effect of porosity and pore size on the compressive properties was also discussed based on the deformation mode. The results reveal that the fabrication process can control the porosity precisely. The average pore size of porous titanium can be tailored by adjusting the pore size of titanium meshes. The fabricated porous titanium possesses an anisotropic structure with square pores in the in-plane direction and elongated pores in the out-of-plane direction. The compressive Youngs modulus and yield stress are in the range of 1-7.5 GPa and 10-110 MPa, respectively. The dominant compressive deformation mode is buckling of mesh wires, but some uncoordinated buckling is present in porous titanium with lower porosity. Relationship between compressive properties and porosity conforms well to the Gibson-Ashby model. The effect of pore size on compressive properties is fundamentally ascribed to the aspect ratio of titanium meshes. Porous titanium with 60-70% porosity has potential for trabecular bone implant applications.

Tribological properties of AlCoCrFeNiCu high-entropy alloy in hydrogen peroxide solution and in oil lubricant

The tribological properties of AlCoCrFeNiCu high entropy alloy sliding against GCr15 in hydrogen peroxide with different concentrations were evaluated using a ring-on-block wear test machine.The microstructure and morphology of the worn surfaces were analyzed using scanning electron microscopy(SEM) and the surface components of the tested samples were measured by energy dispersive spectrometer(EDS).The results indicated that the friction coefficient of AlCoCrFeNiCu/GCr15 tended to decrease with increasing concentration of hydrogen peroxide.The wear volume of AlCoCrFeNiCu alloy in hydrogen peroxide was much smaller than that in deionized water.The main wear mechanism was adhesive wear in deionized water,while the friction pair was dominated by a mixture of oxidative wear,abrasive wear and adhesive wear in 30% and 60% H2O2.In contrast with that in other concentrations,the wear resistance of AlCoCrFeNiCu alloy in 90% H2O2 increased significantly due to the formation of a compact oxide film.

Influences of material parameters on deep drawing of thin-walled hemispheric surface part

During deep drawing process, the material parameters of blank have a significant effect on the quality of the drawn part and the determination of process parameters. Here, a 3D finite element model is developed for the deep drawing process of a thin-walled hemispheric surface part. Then the influences of material parameters including hardening exponent n, yield stress σs and elastic modulus E on the process are investigated by simulation. The results show that the effects of n and σs on punch force, thickness variation and equivalent strain are more notable. The maximum equivalent plastic strain occurs outside the die corner. However, when the value of n is 0.03 or σs is smaller than 120 MPa, higher equivalent plastic strain occurs at ball top.

Cellular automata modeling of static recrystallization based on the curvature driven subgrain growth mechanism

A two-dimensional cellular automata model was developed to describe the static recrystallization (SRX) arising from the subgrain growth, the driving force of which is dependent on boundary energy and local curvature. At the same time, the subgrain boundary energy and mobility rely on the boundary misorientation angle. On the basis, a deterministic switch rule was adopted to simulate the subgrain growth and kinetics of recrystallization quantitatively to provide an insight into the grain boundary bulging nucleation mechanism. Microstructure evolutions during SRX in different cases were simulated by the developed model. At the beginning of the simulation, the initial polycrystalline microstructure which contains large number of uniformly distributed subgrains in every pre-existing grain was prepared using simple assumption based on experimental observations. Then, both homogeneous and inhomogeneous subgrain growth phenomena were captured by the simulation with different inter-subgrain misorientation, which showed continuous and discontinuous recrystallization, respectively. The effects of initial mean subgrain radius, distribution of initial subgrains, distribution of inter-subgrain misorientations, and annealing temperature on the recrystallization kinetics were also investigated.

Solidification characteristics of high Nb-containing γ-TiAl-based alloys with different aluminum contents

The effect of Al content on the microstructure and solidification characteristics of Ti–Al–Nb–V–Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive spectroscope (EDS), and transmission electron microscopy (TEM). The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47Al7Nb2.5V1.0Cr (at%) alloy solidifying through the peritectic reaction. The β-solidifying Ti46Al7Nb2.5V1.0Cr (at%) alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous fine-grained microstructure. However, the crystallization temperature range of Ti48Al7Nb2.5V1.0Cr (at%) alloy is equivalent to that of Ti46Al7Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.

Microstructure and texture of commercially pure titanium in cold deep drawing

The development of microstructure and texture during cold deep drawing of commercially pure titanium (CP-Ti) was investigated. Three parts, stretching region, drawing region and flange region, were sequentially formed in the deep drawing process of the hemispheric surface part, with reference to deformation modes and strain regimes. Results show that the plastic strain is accommodated by dislocation slip and deformation twinning in the whole deep drawing process. The texture of the CP-Ti sheet and its drawn part consists of rolling texture component and recrystallization texture component. The intensity and type of the initial texture varied during the drawing process are related to the production of deformation twinning and dislocation slip. Twinning weakens the initial texture by randomizing the orientations of crystals, especially for the recrystallization texture. The recrystallization texture in the drawing region disappears due to the significant forming of twinning. Furthermore, over drawing would result in the predominance of dislocation slip and the texture is strengthened.

Mechanical properties and pore structure deformation behaviour of biomedical porous titanium

Abstract Porous titanium has been shown to exhibit desirable properties as biomedical materials. In view of the load-bearing situation, the mechanical properties and pore structure deformation behaviour of porous titanium were studied. Porous titanium with porosities varying from 36%–66% and average pore size of 230 μm was fabricated by powder sintering. Microstructural features were characterized using scanning electron microscopy. Uniaxial compression tests were used to probe the mechanical response in terms of elastic modulus and compressive strength. The mechanical properties of porous titanium were found to be close to the those of human bone, with stiffness values ranging from 1.86 to 14.7 GPa and compressive strength values of 85.16–461.94 MPa. The relationships between mechanical properties and relative densities were established, and the increase in relative density showed significant effects on mechanical properties and deformations of porous titanium. In a lower relative density, the microscopic deformation mechanism of porous titanium was yielding, bending and buckling of cell walls, while the deformation of yielding and bending of cell walls was observed in the porous titanium with higher relative density.