The Influence of High-Temperature Creep on the Ultrasonic Velocity in Alloy 800H

The occurrence of creep damage limits the lifetime of component that are exposed to stresses at temperatures higher than approximately half the melting temperature. Such conditions are generally met by a lot of structural components especially in power plants (pipes, turbines, etc.). According to conventional safety rules critical parts are usually exchanged long before any failure has to be expected. This procedure is based on statistics drawn from material tests by standardized methods rather than on the actual state of the component concerned. During the last years an increasing need can be stated to develop NDE methods for the detection of early damage stages in order to improve the reliability and safety of components. Basically, techniques are required which are sensitive to either small strains or, better, to small concentrations of micropores and microcracks, respectively. With regard to in-field applications, only replica techniques are used successfully for that purpose up to now [1,2]. These metallographic techniques are restricted to surfaces where appropriate spots have to be selected and to be prepared carefully. In this work the influence of creep damage on the ultrasonic velocity has been investigated on a representative high-temperature alloy for tube components, i. e., Alloy 800 H (X10 NiCrAlTi 32 20)

Million-atom molecular dynamics simulation by order-N electronic structure theory and parallel computation

Parallelism of tight-binding molecular dynamics simulations is presented by means of the order-N electronic structure theory with the Wannier states, recently developed (J. Phys. Soc. Jpn. 69,3773 (2000)). An application is tested for silicon nanocrystals of more than millions atoms with the transferable tight-binding Hamiltonian. The efficiency of parallelism is perfect, 98.8 %, and the method is the most suitable to parallel computation. The elapse time for a system of $2\times 10^6$ atoms is 3.0 minutes by a computer system of 64 processors of SGI Origin 3800. The calculated results are in good agreement with the results of the exact diagonalization, with an error of 2 % for the lattice constant and errors less than 10 % for elastic constants.Comment: 5 pages, 3 figure

Dislocation density and graphitization of diamond crystals

Two sets of diamond specimens compressed at 2 GPa at temperatures varying between 1060 K and 1760 K were prepared; one in which graphitization was promoted by the presence of water and another in which graphitization of diamond was practically absent. X-ray diffraction peak profiles of both sets were analyzed for the microstructure by using the modified Williamson-Hall method and by fitting the Fourier coefficients of the measured profiles by theoretical functions for crystallite size and lattice strain. The procedures determined mean size and size distribution of crystallites as well as the density and the character of the dislocations. The same experimental conditions resulted in different microstructures for the two sets of samples. They were explained in terms of hydrostatic conditions present in the graphitized samples

Cross‐scale seismic anisotropy analysis in metamorphic rocks from the COSC‐1 borehole in the Scandinavian Caledonides

Metamorphic and deformed rocks in thrust zones show particularly high seismic anisotropy causing challenges for seismic imaging and interpretation. A good example is the Seve Nappe Complex in central Sweden, an old exhumed orogenic thrust zone that is characterized by a strong but incoherent seismic reflectivity and considerable seismic anisotropy. However, only little is known about their origin in relation to composition and structural influences on measurements at different seismic scales. Here, we present a new integrative study of cross‐scale seismic anisotropy analyses combining mineralogical composition, microstructural analyses and seismic laboratory experiments from the COSC‐1 borehole, which sampled a 2.5 km‐deep section of metamorphic rocks deformed in an orogenic root now preserved in the Lower Seve Nappe. While there is strong crystallographic preferred orientation in most samples in general, variations in anisotropy depend mostly on bulk mineral composition and dominant core lithology as shown by a strong correlation between these. This relationship enables to identify three distinct seismic anisotropy facies providing a continuous anisotropy profile along the borehole. Moreover, comparison of laboratory seismic measurements and electron‐backscatter diffraction data reveals a strong scale‐dependence, which is more pronounced in the highly deformed, heterogeneous samples. This highlights the need for comprehensive cross‐validation of microscale anisotropy analyses with additional lithological data when integrating seismic anisotropy over seismic scales

Digital Measurement of Ultrasonic Velocity

The ultrasonic material evaluation has been applied to composite materials and nonhomogeneous materials. In quantitative evaluation of these materials the ultrasonic velocity and attenuation are widely used. In addition acoustoelastic stress measurement requires high precision measurement of the ultrasonic velocity

Raman spectroscopy on carbon nanotubes at high pressure

Raman spectroscopy has been the most extensively employed method to study carbon nanotubes at high pressures. This review covers reversible pressure-induced changes of the lattice dynamics and structure of single- and multi-wall carbon nanotubes as well as irreversible transformations induced by high pressures. The interplay of covalent and van-der-Waals bonding in single-wall nanotube bundles and a structural distortion near 2 GPa are discussed in detail. Attempts of transforming carbon nanotubes into diamond and other "superhard" phases are reviewed critically.Comment: 33 pages, 20 figures, review article, to appear in J. Raman Spectroscop

Transient Lamb Wave Velocity Determination Using Holographic Mapping of Spatial Feautres of Propagating Waves

Measurement of surface displacements resulting from acoustic wave propagation in solids has been used extensively in determining elastic properties of materials [1],[2]. Additionally, examination of acoustic wave propagation in materials has been used as a nondestructive tool in testing the integrity of structures, evaluating the size and position of bulk material defects, determining material dimensions, and in general, characterizing a number of material or structural parameters [3]</p