Advanced Materials Research Vols. 97-101

Abstract: As one of the key factors to induce crack closure, crack surface roughness plays a dominated role in evaluating the characteristics of short fatigue cracks. The crack retardation induced by the crack surface roughness was investigated by numerical simulation in this paper. The influence of crack surface roughness on short fatigue cracks was directly applied into the numerical simulation procedure with the help of friction coefficients on the contacted crack surfaces. The driving forces of cracks represented by the two indicators such as crack tip displacement and plastic strain range were studied and used to characterize the retardation induced by crack surface roughness. It is shown that the evidence of crack surface roughness affecting on crack retardation is obvious and the influence of crack surface roughness on the characteristics of short fatigue cracks must account for in evaluating the performance of short fatigue cracks.

Abstract: MgxZn1-xO:Al ceramics with low resistivity using ZnO, MgO and Al2O3 nano-powders as raw materials were obtained by unpressurized sintering. The influence of Mg content on structure and electric properties of MgxZn1-xO:Al ceramics were studied. The results indicate that there is a single phase of wurtzite structure of ZnO in MgxZn1-xO:Al ceramics at x ≤ 0.3, and when the value of x was 0.3, phase separation was observed. The grains become smaller and more homogeneous with increasing Mg content to above 0.3. The resistivity of MgxZn1-xO:Al ceramics obviously increases with increasing Mg concentration due to the decrease of the carrier concentration and the mobility deriving from the decrease of Al-doping efficiency. The electrical conduction of MgxZn1-xO:Al ceramics can be markedly improved by increasing the Al-doping level and the lowest resistivity of 8.3×10-4Ω•cm can be obtained when Mg/(Mg+Zn)=0.2 and Al/(Mg+Zn+Al)=0.03.

Abstract: The model of the generalized linear micropolar magneto-thermoviscoelastic theory is presented in this paper. It can be applied to the coupled theory as well as to five generalizations, such as L-S theory, G-L theory, H-I theory, G-N theory of type (II) and C-T theory. If some parameters in this model are taken as given values, we can easily deduce the known models of Kelvin-Voigt model and other generalized micropolar/magneto/thermo/viscoelastic theory model. The magneto-thermo- viscoelastic waves at an interface between two micropolar viscoelastic solid mediums are discussed with the model. Using continuous conditions, the general frequency equations are obtained. Also as a validation, if one media is replaced by vacuum, we have deduced the frequency equation of Rayleigh wave. And if the problem is reduced to elastic case, it coincides with the results of Ewing et al.

Abstract: Fatigue crack growth rate (FCGR) test of B grade steel was performed to clarify the probabilistic behavior of crack propagation in near-threshold region and steady-state region. As a result, a crack propagation law considering influence of threshold value and average stress can be well proposed to estimate crack propagation behavior of B grade steel. The estimated value of threshold value ΔKth is 293.65MPamm1/2, approaching the experimental value of ΔKth, about 280.39MPamm1/2. Furthermore, the probabilistic FCGR curves of B grade steel are established from the viewpoint of survival probability and confidence degree, which reflects the influence of scatter regularity of data and sample size on reliability assessment.

Abstract: The microstructure and mechanical properties of a cast near β type titanium alloy, Ti-25Nb-3Zr-3Mo-2Sn, have been investigated in this paper. In the as cast condition, the alloy possesses coarse, equiaxed β grains and Transmission Electron Microscopy(TEM) observations showed the ω phase is also present in this condition. The alloy in the as cast condition has low strength and is highly ductile. Direct ageing at 450°C for 4 hours leads to the formation of a large number of fine scale α phase precipitates. The alloy in the aged condition is less ductile but exhibits significantly enhanced strength.

Abstract: Constant stress tensile creep tests were conducted on an AZ 91–25 vol.% aluminum silicate short fiber composite and on an unreinforced AZ 91 matrix alloy. The creep resistance of the composite is shown to be considerably improved compared with the matrix alloy, and the resistance effect is better with the increase of temperature. The steady-state creep rate of the composite is 4.54% of matrix alloy at 473K, and 2.56% of matrix alloy at 573K. The creep strengthening arises mainly from the effective load transfer between plastic flow in the matrix and the fibers. Microstructural investigations by SEM revealed good fiber–matrix interface bonding during creep exposure. Short fibers have a great function in load bearing and transmission load, and greatly hinder the dislocation movement, thus enhancing the creep resistance of the composite. The creep mechanism of the composite is dislocation and grain boundary sliding control.

Abstract: The critical resolved stresses (CRSS) of two kinds of Al-Li alloys 2090 and 2090+Ce in which contained trace addition of cerium are examined. The three heat treatment conditions (solution, peak aging and reversion after peak aging) are used in order to identify the influence of precipitates to CRSS. The experimental results show that the strengthening contribution of T1 Phase is greatly different at various orientation with respect to rolling direction, and the precipitation of T1 phase can weaken the anisotropy of mechanical properties. The T1 phase precipitated in subgain boundaries in peak aged and reverted specimens produce a stronger tendency of delaminating along grain boundary, which is a direct reason for the decrease of anisotropic yield strength.

Abstract: Magnetic behavior of ferromagnetic materials has been using to detect defects of materials. To evaluate the stress states of the components by the magnetic memory signal, Q235 defect asymmetrical samples were made. The characteristics of magnetic memory of Q235 have been studied in the three different testing environments which are online-loading, online-unloading and offline-unloading under cycle tensile stress. The results show that magnetic memory signals have different characteristics in different testing environment. It is feasible to evaluate preliminarily the stress state by the magnetic memory signals.

Abstract: Barium strontium titanate Ba0.6Sr0.4TiO3 (BST) thin films were fabricated by pulsed laser deposition (PLD) technique on Pt/TiO2/SiO2/Si substrate with and without high (100) oriented LSCO buffer layer. Crystal structure and surface morphology of BST thin films with and without LSCO buffer layer were characterized by X-ray diffraction (XRD) and atom force microscope (AFM). The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequency from 100 Hz to 1M Hz and at room temperature. It was found that the LSCO buffer layer was beneficial for BST films to decrease surface roughness, dielectric loss and to increase grain size, dielectric constant, tunability and figure of merit (FOM). The influence of LSCO buffer layer on the microstructure, dielectric and tunable properties of BST thin films were analyzed. At 1M Hz, the dielectric constants of BST films with and without LSCO buffer layer are 684 and 592, respectively. The tunability of BST thin film with LSCO buffer layer was about 65.48%, which was higher than that (about 41.84%) of BST thin film without LSCO buffer layer.

Abstract: Activated carbon fibers(ACFs) were prepared from general pitch-based carbon fiber by steam activation and catalytic activation method, respectively. The surface area and pore structure of the resultant ACFs were analyzed by N2 adsorption, and electrochemical performances as electrodes of super capacitors were characterized by galvanostatic, cyclic voltammograms and AC impedance spectrum analysis. The results show that ACFs prepared by both methods have similar BET surface area, while their pore size and distribution are different. Compared with steam activation, catalytic activation results in ACFs with high mesoporosity of 40%. The electrode performances show that the specific capacitances of ACFs prepared by catalytic activation method could be 213 F/g, two times of that of ACFs prepared by steam activation method, and more mesopores are the reason for the good capacitance performance.