Jijia Xie

Growth Mechanism And Joint Structure Of Zno Tetrapods

Regular zinc oxide (ZnO) tetrapods with a flat plane have been obtained on Si(1 0 0) substrate via the chemical vapour deposition approach. The x-ray diffraction result suggests that these tetrapods are all single crystals with a wurtzite structure that grow along the (0 0 0 1) direction and corresponding electron backscatter diffraction analysis reveals the crystal orientation of growth and exposed surface. Furthermore, we find some ZnO tetrapods with some legs off and the angles between every two legs are measured with the aid of scanning electron microscopy and image analysis, which benefit to reveal the structure of ZnO tetrapods joint. The structure model and growth mechanism of ZnO tetrapods are proposed. Besides, the stable model of the interface was obtained through the density-functional theory calculation and the energy needed to break the twin plane junction was calculated as 5.651 J m(-2).Regular zinc oxide (ZnO) tetrapods with a flat plane have been obtained on Si(1?0?0) substrate via the chemical vapour deposition approach. The x-ray diffraction result suggests that these tetrapods are all single crystals with a wurtzite structure that grow along the (0?0?0?1) direction and corresponding electron backscatter diffraction analysis reveals the crystal orientation of growth and exposed surface. Furthermore, we find some ZnO tetrapods with some legs off and the angles between every two legs are measured with the aid of scanning electron microscopy and image analysis, which benefit to reveal the structure of ZnO tetrapods joint. The structure model and growth mechanism of ZnO tetrapods are proposed. Besides, the stable model of the interface was obtained through the density-functional theory calculation and the energy needed to break the twin plane junction was calculated as 5.651?J?m?2.

Very high cycle fatigue for GCr15 steel with smooth and hole-defect specimens

Very high cycle fatigue (VHCF) properties of a low temperature tempering bearing steel GCr15 with smooth and hole-defect specimens are studied by employing a rotary bending test machine with frequency of 52.5 Hz. Both smooth and hole-defect specimens break in VHCF regime with some difference in fatigue crack initiation. For smooth specimens, a fine granular area (FGA) is observed near the grain boundary in the fracture surface of the specimens broken after 10^7 cycles. But no FGA is observed in the hole-defect specimens broken in VHCF regime, and the VHCF crack does not initiate from the small hole at the surface as it does at low or high cycle fatigue regime. Internal stress is employed to explain the VHCF behavior of these two types of specimens. At last, an advanced dislocation model based on Tanaka and Mura model is proposed to illustrate the internal stress process and to predict fatigue crack initiation life with FGA observed in the fracture region.

Study on Mechanical Properties of Nanocrystal Surface Layer of an Aluminum Alloy

The nanocrystal surface layer of an aluminum alloy induced by High Speed Shot Peening (HSSP) was investigated in this paper. The results of nanoindentation experiment show that the elastic modulus and the hardness of nanocrystal surface layer increased,by 8% and 20%, respectively. The elastic modulus and the hardness appear to be independent of the distance from nanocrystalized surface and the process time.

Effects of Loading Frequency and Loading Type on High-Cycle and Very-High-Cycle Fatigue of a High-Strength Steel

High-cycle and very-high-cycle fatigue tests via rotary bending (52.5 Hz), electromagnetic resonance (120 Hz) axial cycling, and ultrasonic (20 kHz) axial cycling were performed for a high-strength steel with three heat treatment conditions, and the effects of loading frequency and loading type on fatigue strength and fatigue life were investigated. The results revealed that the loading frequency effect is caused by the combined response of strain rate increase and induced temperature rise. A parameter η was proposed to judge the occurrence of loading frequency effect, and the calculated results were in agreement with the experimental data. In addition, a statistical method based on the control volume was used to reconcile the effect of loading type, and the predicted data were consistent with the experimental results.

An energy-equilibrium model for complex stress effect on fatigue crack initiation

Based on Tanaka and Mura’s fatigue model and Griffith theory for fracture, an energy-equilibrium model was proposed to explain the complex stress effect on fatigue behavior. When the summation of the elastic strain energy release and the stored strain energy of accumulated dislocations reach the surface energy of a crack, the fatigue crack will initiate in materials. According to this model, for multiaxial stress condition, the orientation of the crack initiation and the initiation life can be deduced from the energy equilibrium equation. For the uniaxial fatigue loading with mean stress, the relation between the maximum stress or the minimum stress and the stress amplitude is in agreement with an ellipse equation on the constant life diagram. If the ratio of the mean stress to stress amplitude is less than a critical value −0.17, and the stress amplitude keeps constant, the fatigue crack initiation life will decrease with the increase of the compress mean stress. In this model, the mean stress does not cause damage accumulation with the fatigue cycles in crack initiation. For this reason, the loading sequence of different load levels would induce the cumulative damage to deviate from the Palmgren-Miner cumulative damage rule. The procedure of estimating the damage under random loading is also discussed.