Creep of CaO-Stabilized ZrO2

The compressive creep of 18 mol% CaO-stabilized ZrO₂ was studied at 1200° to 1400°C and 500 to 4000 psi. The specimens were polycrystalline with grain diameters from 7 to 29 μm. The activation energy for creep is 94 kcal/mol, and the creep rates are linearly proportional to the stress and to the inverse of the grain size. These results lead to the conclusion that creep in 18 mol% CaO-stabilized ZrO₂ may be controlled by cation diffusion associated with grain-boundary sliding.     

Mechanical Properties of Hot-Pressed B-B4C Materials

Elastic modulus and flexural strength were measured at room temperature for solid pieces hot-pressed at 1900 to 2600 K from mixtures of B₄C powder and ≤15 wt% B powder. Regression analysis showed that elastic modulus and flexural strength are not significantly affected by these boron additions. Elastic modulus is related to porosity and flexural strength to porosity and grain size. The fracture surface energy of boron carbide was evaluated. X-ray diffraction and chemical analyses showed that the specimens were composed entirely of boron carbide after hot-pressing. Lattice constants increased with the initial boron content.     

Tests for non-correlation of two cointegrated ARMA time series

In multivariate time series modelling, we are often led to investigate the existence of a relationship between two time series. Here, we generalize the procedure proposed by Haugh (1976 ) and extended by El Himdi and Roy (1997 ) for multivariate stationary ARMA time series to the case of cointegrated (or partially nonstationary) ARMA series. The main contribution consists in showing that, in the case of two uncorrelated cointegrated time series, an arbitrary vector of residual cross-correlation matrices asymptotically follows the same distribution as the corresponding vector of cross-correlation matrices between the two innovation series. The estimation method from which the residuals are obtained can be the conditional maximum likelihood method as discussed in Yap and Reinsel (1995 ) or some other which has the same convergence rate. From this result, it follows that the considered test statistics, which are based on residual cross-correlation matrices, asymptotically follow χ² distributions. The finite sample properties, under the null hypothesis, of the test statistics are studied by simulation.     

Surface composition of scratched and sputtered Al–Cu ribbons prepared by melt-spinning

In this study, aluminium–copper ribbons of various compositions are prepared by melt-spinning and analysed by Auger electron spectroscopy. Rapidly solidified ribbons usually show a microcrystalline or featureless microstructure, especially in zones located near the surface solidified in direct contact with the rotating wheel. The composition of that surface of the ribbons was analysed by Auger electron spectroscopy. These measurements were performed in both scratched and also ion sputtered zones. After sputtering of the ribbon surface with argon ions, a surface enrichment in copper was observed. However scratching the surface of the ribbons produced a surface composition nearly identical to the bulk. The interest of rapidly solidified alloys in calibration applications is discussed.