Fatigue behavior of SiC reinforced titanium composites

The low cycle axial fatigue properties of 25 and 44 fiber volume percent SiC/Ti(6Al-4V) composites were measured at room temperature and at 650 deg C. The S-N curves for the composites showed no anticipated improvement over bulk matrix behavior at room temperature. Although axial and transverse tensile strength results suggest a degradation in SiC fiber strength during composite fabrication, it appears that the poor fatigue life of the composites was caused by a reduced fatigue resistance of the reinforced Ti(6Al-4V) matrix. The reduced matrix behavior was due, to the presence of flawed and fractured fibers created near the specimen surfaces by preparation techniques and to the large residual tensile stresses that can exist in fiber reinforced matrices. The effects of fatigue testing at high temperature are discussed

Impact of germanium on vacancy clustering in germanium-doped silicon

Recent density functional theory calculations by Chen et al. [J. Appl. Phys. 103, 123519 (2008)] revealed that vacancies (V) tend to accumulate around germanium (Ge) atoms in Ge-doped silicon (Si) to form GeVn clusters. In the present study, we employ similar electronic structure calculations to predict the binding energies of GeVn and Vn clusters containing up to four V. It is verified that V are strongly attracted to pre-existing GeVn clusters. Nevertheless, by comparing with the stability of Vn clusters, we predict that the Ge contribution to the binding energy of the GeVn clusters is limited. We use mass action analysis to quantify the relative concentrations of GeVn and Vn clusters over a wide temperature range: Vn clusters dominate in Ge-doped Si under realistic conditions

Radiation effects on beta 10.6 of pure and europium doped KCl

Changes in the optical absorption coefficient as a result of X-ray and electron bombardment of pure KCl (monocrystalline and polycrystalline), and divalent europium doped polycrystalline KCl were determined. The optical absorption coefficients were measured by a constant heat flow calorimetric method. Both 300 KV X-irradiation and 2 MeV electron irradiation produced significant increases in beta 10.6, measured at room temperature. The X-irradiation of pure moncrystalline KCl increased beta 10.6 by 0.005/cm for a 113 MR dose. For an equivalent dose, 2 MeV electrons were found less efficient in changing beta 10.6. However, electron irradiation of pure and Eu-doped polycrystalline KCl produced marked increases in adsorption. Beta increased to over 0.25/cm in Eu-doped material for a 30 x 10 to the 14th power electrons/sq cm dose, a factor of 20 increase over unirradiated material. Moreover, bleaching the electron irradiated doped KCl with 649 m light produced and additional factor of 1.5 increase. These findings will be discussed in light of known defect-center properties in KCl

Thermal degradation of the tensile strength of unidirectional boron/aluminum composites

The variation of ultimate tensile strength with thermal treatment of B-Al composite materials and of boron fibers chemically removed from these composites in an attempt to determine the mechanism of the resulting strength degradation was studied. Findings indicate that thermally cycling B-Al represents a more severe condition than equivalent time at temperature. Degradation of composite tensile strength from about 1.3 GN/m squared to as low as 0.34 GN/m squared was observed after 3,000 cycles to 420 C for 203 micrometers B-1100 Al composite. In general, the 1100 Al matrix composites degraded somewhat more than the 6061 matrix material studied. Measurement of fiber strengths confirmed a composite strength loss due to the degradation of fiber strength. Microscopy indicated a highly flawed fiber surface

Thermal environment effects on strength and impact properties of boron-aluminum composites

Thermal effects on fracture strength and impact energy were studied in 50 volume percent unidirectional composites of 143 and 203 micron boron fibers in 6061 and 1100 aluminum matrices. For 6061 matrix composites, strength was maintained to approximately 400 C in the cyclic tests and higher than 400 C in the static tests. For the 1100 matrix composites, strength degradation appeared near 260 C after cycling and higher than 260 C in static heating. This composite strength degradation is explained by a fiber degradation mechanism resulting from a boron-aluminum interface reaction. The impact energy absorption degraded significantly only above 400 C for both matrix alloys. Thus, while impact loss for the 6061 composite correlates with the fiber strength loss, other energy absorption processes appear to extend the impact resistance of the 1100 matrix composites to temperatures beyond where its strength is degraded. Interrupted impact tests on as-received and thermally cycled composites define the range of load over which the fibers break in the impact event

Defect interactions in Sn_1-<i>x</i>Ge_<i>x</i> random alloys

Sn1-xGex alloys are candidates for buffer layers to match the lattices of III-V or II-VI compounds with Si or Ge for microelectronic or optoelectronic applications. In the present work electronic structure calculations are used to study relative energies of clusters formed between Sn atoms and lattice vacancies in Ge that relate to alloys of low Sn content. We also establish that the special quasirandom structure approach correctly describes the random alloy nature of Sn1-xGex with higher Sn content. In particular, the calculated deviations of the lattice parameters from Vegard's Law are consistent with experimental results

Internal friction peaks observed in explosively deformed polycrystalline Mo, Nb, and Cu

Explosive deformation (50 kbar range) induced, in Cu, Mo and Nb, internal friction peaks identical to those observed after large normal deformation. The variation of the peaks with pressure for Mo and Nb lead to an explanation of these processes in terms of double kink generation in screw and edge dislocations

<i>E</i> centers in ternary Si_1-<i>x-y</i>Ge_<i>x</i>Sn_<i>y</i> random alloys

Density functional theory calculations are used to study the association of arsenic (As) atoms to lattice vacancies and the formation of As-vacancy pairs, known as E centers, in the random Si0.375Ge0.5Sn0.125 alloy. The local environments are described by 32-atom special quasirandom structures that represent random Si1-x-yGexSny alloys. It is predicted that the nearest-neighbor environment will exert a strong influence on the stability of E centers in ternary Si0.375Ge0.5Sn0.125