Effect of reactor irradiation on properties of a nongalling alloy

Effect of reactor irradiation on properties of nongalling alloy

Flat growth of 7075, 7475, 7050 and 7049 aluminum alloy plate in stress corrosion environments: 2-year marine atmosphere results

Marine atmospheric exposure of smooth and precracked specimens from 7075, 7475, 7050 and 7049 plates support the conclusion that for a given strength level, the short transverse stress corrosion resistance of 7050-T7X and 7049-T7X is superior to that of 7075-T7X. The threshold stress intensity (K sub Iscc) for these alloys is about 25 MPa square root m at a yield strength of about 460 MPa; the corresponding yield strength level for 7075-T7X at this SCR level is about 425 MPa. Additional tests on two lots of high-toughness 7475 plate indicate that this alloy is capable of achieving K sub Iscc values of about 35 MPa square root m at yield strengths of 400-450 MPa. Precracked specimens from all these 7XXX-series alloys are subject to self loading from corrosion product wedging. This effect causes stress corrosion cracks to continue growing at very low apparent stress intensities, and should therefore be considered a potential driving force for stress corrosion in design and materials selection

Flaw growth of 7075, 7475, 7050 and 7049 aluminum alloy plate in stress corrosion environments: 4-year marine atmosphere results

After nearly 53 months of exposure to marine atmosphere, crack growth in SL DCB specimens from 7075, 7475, 7050, and 7049-T7X plate has slowed to the arbitrary 10 to the -10 power m/sec used to define threshold stress intensity. Because some specimens appear to be approaching crack arrest, the importance of self-loading from corrosion product wedging as a significant driving force for crack propagation in overaged materials is questioned. Crack length-time data were analyzed using a computer curve fitting program which minimized the effects of normal data scatter, and provided a clearer picture of material performance. Precracked specimen data are supported by the results of smooth specimen tests. Transgranular stress corrosion cracking was observed in TL DCB specimens from all four alloys. This process is extremely slow and is characterized by a striated surface morphology

Flaw growth of 7075, 7475, 7050 and 7049 aluminum alloy plate in stress corrosion environments

Marine atmosphere and laboratory stress corrosion test results on smooth and precracked specimens from 7075, 7475, 7050, and 7049 alloy plates (1.25 and 3.0-in. thick) are presented. It is shown that for a given strength level, alloys 7050-T7X and 7049-T7X have superior short-transverse stress corrosion resistance (SCR) to 7X75-T7X. At typical strength levels above the minimum, for example, SCR of these alloys is considerably better than that of 7075-T76, and approaches that of 7075-T73. Alloy 7475 maintains an advantage in the area of fracture toughness, however, because it can be thermally processed to give particularly clean microstructures. Results from precracked specimens are in good qualitative agreement with those obtained from smooth specimens. Although both specimen types are capable of distinguishing between -T6, -T76 and -T73 tempers in relatively short time periods the precracked specimen provides more information about crack growth rates

pp-adic Zeros of Systems of Quadratic Forms

This survey describes work on the number of variables required to ensure that a system of r quadratic forms over the p-adics has a non-trivial common zero

Energy dependence of nucleus-nucleus potential close to the Coulomb barrier

The nucleus-nucleus interaction potentials in heavy-ion fusion reactions are extracted from the microscopic time-dependent Hartree-Fock theory for mass symmetric reactions $^{16}$O${}+^{16}$O, $^{40}$Ca${}+^{40}$Ca, $^{48}$Ca${}+^{48}$Ca and mass asymmetric reactions $^{16}$O$+^{40,48}$Ca, $^{40}$Ca${}+^{48}$Ca, $^{16}$O+$^{208}$Pb, $^{40}$Ca+$^{90}$Zr. When the center-of-mass energy is much higher than the Coulomb barrier energy, potentials deduced with the microscopic theory identify with the frozen density approximation. As the center-of-mass energy decreases and approaches the Coulomb barrier, potentials become energy dependent. This dependence signs dynamical reorganization of internal degrees of freedom and leads to a reduction of the "apparent" barrier felt by the two nuclei during fusion of the order of $2-3$ compared to the frozen density case. Several examples illustrate that the potential landscape changes rapidly when the center-of-mass energy is in the vicinity of the Coulomb barrier energy. The energy dependence is expected to have a significant role on fusion around the Coulomb barrier.Comment: 11 pages, 13 figures, 1 table, discussion of effects of coordinate-dependent mass added, accepted for publication in Phys. Rev.

Stochastic Semi-Classical Description of Fusion at Near-Barrier Energies

Fusion reactions of heavy ions are investigated by employing a simple stochastic semi-classical model which includes the coupling between relative motion and low frequency collective surface modes of colliding ions similarly to the quantal coupled-channels description. The quantal effect enters into the calculation through the initial zero-point fluctuations of the surface vibrations. Good agreement with the result of coupled-channels calculations as well as data is obtained for the fusion cross sections of nickel isotopes. The internal excitations in non-fusing events as well as the fusion time are investigated.Comment: 8 pages, 8 figures, Published in Phys. Rev.

Origin of the neutron skin thickness of 208Pb in nuclear mean-field models

We study whether the neutron skin thickness (NST) of 208Pb originates from the bulk or from the surface of the nucleon density distributions, according to the mean-field models of nuclear structure, and find that it depends on the stiffness of the nuclear symmetry energy. The bulk contribution to NST arises from an extended sharp radius of neutrons, whereas the surface contribution arises from different widths of the neutron and proton surfaces. Nuclear models where the symmetry energy is stiff, as typical relativistic models, predict a bulk contribution in NST of 208Pb about twice as large as the surface contribution. In contrast, models with a soft symmetry energy like common nonrelativistic models predict that NST of 208Pb is divided similarly into bulk and surface parts. Indeed, if the symmetry energy is supersoft, the surface contribution becomes dominant. We note that the linear correlation of NST of 208Pb with the density derivative of the nuclear symmetry energy arises from the bulk part of NST. We also note that most models predict a mixed-type (between halo and skin) neutron distribution for 208Pb. Although the halo-type limit is actually found in the models with a supersoft symmetry energy, the skin-type limit is not supported by any mean-field model. Finally, we compute parity-violating electron scattering in the conditions of the 208Pb parity radius experiment (PREX) and obtain a pocket formula for the parity-violating asymmetry in terms of the parameters that characterize the shape of the 208Pb nucleon densities.Comment: 11 pages, 4 figures; minor stylistic changes in text, new Ref. [56] added (new measurement of the neutron skin thickness of 208Pb

Cluster formations in deformed states for 28^{28}Si and 32^{32}S

We study cluster formation in strongly deformed states for $^{28}$Si and $^{32}$S using a macroscopic-microscopic model. The study is based on calculated total-energy surfaces, which are the sums of deformation-dependent macroscopic-microscopic potential-energy surfaces and rotational-energy contributions. We analyze the angular-momentum-dependent total-energy surfaces and identify the normal- and super-deformed states in $^{28}$Si and $^{32}$S, respectively. We show that at sufficiently high angular momenta strongly deformed minima appear. The corresponding microscopic density distributions show cluster structure that closely resemble the $^{16}$O+$^{12}$C and $^{16}$O+$^{16}$O configurations. At still higher deformations, beyond the minima, valleys develop in the calculated surfaces. These valleys lead to mass divisions that correspond to the target-projectile configurations for which molecular resonance states have been observed. We discuss the relation between the one-body deformed minima and the two-body molecular-resonance states.Comment: 6 pages, 7 figure