Structural behavior of laser-irradiated γ-Fe2O3 nanocrystals dispersed in porous silica matrix : γ-Fe2O3 to α-Fe2O3 phase transition and formation of ε-Fe2O3

The effects of laser irradiation on γ-Fe₂O₃ 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-Fe₂O₃ phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-Fe₂O₃ nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-Fe₂O₃ is an intermediate phase between γ-Fe₂O₃ and α-Fe₂O₃ and a series of distinct Raman vibrational bands were identified with the ε-Fe₂O₃ phase. The structural transformation of γ-Fe₂O₃ into α-Fe₂O₃ occurs either directly or via ε-Fe₂O₃, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.     

Strengthening of Zirconia–Alumina During Cyclic Fatigue Testing

Recent fatigue results of advanced ceramics are reviewed. It is pointed out that current statements of fatigue properties often relate to specimens with long artificial cracks; in this study, a Y-ZrO₂+Al₂O₃ ceramic is tested under cyclic stresses using specimens with natural flaws. Cyclic fatigue is restricted to a narrow range of stresses near the ultimate strength. Survivor specimens reveal increased residual strength, possibly resulting from a higher ZrO₂ phase transformation encouraged by long-term cyclic stresses.     

Nucleation and growth of the γ′(AlAg2) precipitate in Al–Ag(–Cu) alloys

Precipitation of the γ′(AlAg₂) phase was investigated in Al–Ag(–Cu) alloys using high-resolution transmission electron microscopy and scanning transmission electron microscopy. Precipitation commenced with segregation of Ag to stacking faults, followed by thickening in steps corresponding to single unit cell height ledges. In conjunction with gradual segregation of Ag and Al into ordered layers, this yielded γ′ phase platelets with a thickness of either 2 or 3 × the AlAg₂c-lattice parameter. Plates with a thickness of 2c(AlAg₂) could not achieve self-accommodation of the shape strain for transformation. Further thickening of the precipitates was slow, despite considerable Ag segregation around the precipitates. Growth by the addition of single unit cell height ledges is expected to lead to an additional shear strain energy barrier to ledge nucleation and this may contribute to a process of nucleation-limited growth.     

Quasi phase transition model of shear bands in metallic glasses

A quasi phase transition model of shear bands in metallic glasses (MGs) is presented from the thermodynamic viewpoint. Energy changes during shear banding in a sample–machine system are analyzed following fundamental energy theorems. Three characteristic parameters, i.e. the critical initiation energy ΔG_c, the shear band stability index k₀, and the critical shear band length l_c, are derived to elucidate the initiation and propagation of shear bands. The criteria for good plasticity in MGs with predominant thermodynamic arrest of shear bands are proposed as low ΔG_c, large k₀, and small l_c. The model, combined with experimental results, is used to analyze some controversial phenomena of deformation behavior in MGs, such as the size effect, the effect of testing machine stiffness and the relationship between elastic modulus and plasticity. This study has important implications for a fundamental understanding of shear banding as well as deformation mechanisms in MGs and provides a theoretical basis for improving the ductility of MGs.     

Effects of replacing Ni by Co on the crystallization behaviors of Al–Ni–La amorphous alloys

Effects of replacing Ni by Co on the crystallization behaviors of three Al–Ni–La amorphous alloys, i.e. Al₈₅Ni₉La₆, Al₈₆Ni₉La₅ and Al₈₇Ni₈La₅ were investigated by X-ray diffraction and differential scanning calorimeter. The results show that the glass-forming ability decreases when Ni is replaced by excessive Co. Meanwhile replacing Ni by Co improves the thermal stability, enlarges the supercooled liquid region ΔT_x and promotes the precipitation of the metastable phase(s) as the primary phase. The apparent activation energy E_a1 of the first reaction changes complicatedly during the replacement and is strongly dependent on the type of the primary phase, i.e. diffusion of atoms.     

Testing for Unit Roots and Non-linear Transformations

It is well known that the augmented Dickey–Fuller (ADF) test of unit roots in univariate time series is sensitive to non-linear transformations: a common example is when variables expressed in logarithms are found to be stationary, whereas the same variables in levels are found to be non-stationary. In this paper, the effects of non-linear transformations on the ADF auxiliary regression are investigated within the class of the Box–Cox model, and a test of non-linear transformation is developed to assess the adequacy of the ADF regression. The proposed test is computationally simple and is calculated as the t ratio of an added variable in the ADF regression. Cointegration among series which are subject to non-linear transformations is also analysed, and a simple procedure is developed to test the non-linear transformation used in cointegration analysis. Several empirical examples taken from the Nelson–Plosser data set illustrate the practical relevance of the proposed test for univariate series, and a second empirical example is used to illustrate the test of non-linear transformation for cointegrated series     

SIMPLIFIED SCALING TRANSFORMATION FOR THE NUMERICAL SIMULATION OF MEMS DEVICES WITH THIN FILM STRUCTURES

Thin film is a widely used structure in the present microelectromechanical systems （MEMS） and plays a vital role in many functional devices. However, the great size difference between the film＇s thickness and its planar dimensions makes it difficult to study the thin film performance numerically. In this work, a scaling transformation was presented to make the different dimensional sizes equivalent, and thereby, to improve the grid quality considerably. Two numerical experiments were studied to validate the present scaling transformation method. The numerical results indicated that the largest grid size difference can be decreased to one to two orders of magnitude by using the present scaling transformation, and the memory required by the numerical simulation, i.e., the total grid number, could be reduced by about two to three orders of magnitude, while the numerical accuracies with and without this scaling transformation were nearly the same.     

Competing chemical transformations in the combustion wave in the Fe2O3-Cr2O3-Al mixture

Laws of combustion of a thermit-type mixture Fe₂O₃-2Al-γCr₂O₃ are considered. The limits of liquid-phase combustion and separation of the oxide and metal phases in the final products are determined. Chromium oxide is demonstrated to weakly compete with iron oxide in the oxidation-reduction reaction with aluminum and to participate mainly in oxide-phase formation. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 89–91, May–June, 2006.     

Shear-induced chemical disordering in Ni3Al at different temperatures

Molecular dynamics simulations have been used to study shear-induced chemical disordering in Ni₃Al lattices at different temperatures and strain rates. Shearing determines the formation of an amorphous layer, the thickness of which increases linearly with the square root of time. The rate at which the amorphous layer grows is both shearing rate- and temperature-dependent. A linear correlation between the amorphous layer growth rate and the shear modulus is found. This suggests that mechanical properties could play a central role in shear-induced disordering processes.     

加热速率对CuAlBe形状记忆合金相变的影响

通过差示扫描量热技术研究了加热速率对CuAlBe形状记忆合金的相变特征的影响。结果表明：在不同加热速率下的相变点在一个区域内变化。快速加热使两个相变峰靠近，随着加热速率的降低，两个相变峰逐渐分离。用超低降温速率法分离后的DSC曲线，右端的大峰变成一连串由许多微区依次发生马氏体逆相变所产生的许多小的放热峰。