1,084 research outputs found
Stress corrosion cracking in Al-Zn-Mg-Cu aluminum alloys in saline environments
Copyright 2013 ASM International. This paper was published in Metallurgical and Materials Transactions A, 44A(3), 1230 - 1253, and is made
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modification of the content of this paper are prohibited.Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (E a) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys (~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor
Thermoelectric effects in superconducting proximity structures
Attaching a superconductor in good contact with a normal metal makes rise to
a proximity effect where the superconducting correlations leak into the normal
metal. An additional contact close to the first one makes it possible to carry
a supercurrent through the metal. Forcing this supercurrent flow along with an
additional quasiparticle current from one or many normal-metal reservoirs makes
rise to many interesting effects. The supercurrent can be used to tune the
local energy distribution function of the electrons. This mechanism also leads
to finite thermoelectric effects even in the presence of electron-hole
symmetry. Here we review these effects and discuss to which extent the existing
observations of thermoelectric effects in metallic samples can be explained
through the use of the dirty-limit quasiclassical theory.Comment: 14 pages, 10 figures. 374th WE-Heraus seminar: Spin physics of
superconducting heterostructures, Bad Honnef, 200
Observation of the Decays B0->K+pi-pi0 and B0->rho-K+
We report the observation of B^0 decays to the K^+pi^-pi^0 final state using
a data sample of 78 fb^-1 collected by the Belle detector at the KEKB e^+e^-
collider. With no assumptions about intermediate states in the decay, the
branching fraction is measured to be (36.6^{+4.2}_{-4.3}+- 3.0)*10^-6.We also
search for B decays to intermediate two-body states with the same K^+pi^-pi^0
final state. Significant B signals are observed in the rho(770)^- K^+ and
K^*(892)^+pi^- channels, with branching fractions of
(15.1^{+3.4+1.4+2.0}_{-3.3-1.5-2.1})* 10^-6 and
(14.8^{+4.6+1.5+2.4}_{-4.4-1.0-0.9})* 10^-6, respectively. The first error is
statistical, the second is systematic and the third is due to the largest
possible interference. Contributions from other possible two-body states will
be discussed. No CP asymmetry is found in the inclusive K^+pi^-pi^0 or rho^-K^+
modes, and we set 90% confidence level bounds on the asymmetry of
-0.12<A_{CP}<0.26 and -0.18<A_{CP}<0.64, respectively.Comment: 18 pages, 7 figure
Protons in near earth orbit
The proton spectrum in the kinetic energy range 0.1 to 200 GeV was measured
by the Alpha Magnetic Spectrometer (AMS) during space shuttle flight STS-91 at
an altitude of 380 km. Above the geomagnetic cutoff the observed spectrum is
parameterized by a power law. Below the geomagnetic cutoff a substantial second
spectrum was observed concentrated at equatorial latitudes with a flux ~ 70
m^-2 sec^-1 sr^-1. Most of these second spectrum protons follow a complicated
trajectory and originate from a restricted geographic region.Comment: 19 pages, Latex, 7 .eps figure
Search for antihelium in cosmic rays
The Alpha Magnetic Spectrometer (AMS) was flown on the space shuttle
Discovery during flight STS-91 in a 51.7 degree orbit at altitudes between 320
and 390 km. A total of 2.86 * 10^6 helium nuclei were observed in the rigidity
range 1 to 140 GV. No antihelium nuclei were detected at any rigidity. An upper
limit on the flux ratio of antihelium to helium of < 1.1 * 10^-6 is obtained.Comment: 18 pages, Latex, 9 .eps figure
A Study of Cosmic Ray Secondaries Induced by the Mir Space Station Using AMS-01
The Alpha Magnetic Spectrometer (AMS-02) is a high energy particle physics
experiment that will study cosmic rays in the to range and will be installed on the International Space Station
(ISS) for at least 3 years. A first version of AMS-02, AMS-01, flew aboard the
space shuttle \emph{Discovery} from June 2 to June 12, 1998, and collected
cosmic ray triggers. Part of the \emph{Mir} space station was within the
AMS-01 field of view during the four day \emph{Mir} docking phase of this
flight. We have reconstructed an image of this part of the \emph{Mir} space
station using secondary and emissions from primary cosmic rays
interacting with \emph{Mir}. This is the first time this reconstruction was
performed in AMS-01, and it is important for understanding potential
backgrounds during the 3 year AMS-02 mission.Comment: To be submitted to NIM B Added material requested by referee. Minor
stylistic and grammer change
Measurement of the inclusive semileptonic branching fraction of B mesons and |Vcb|
We present a measurement of the electron spectrum from inclusive semileptonic
{\it B} decay, using 5.1 fb of data collected with the
Belle detector. A high-momentum lepton tag was used to separate the
semileptonic {\it B} decay electrons from secondary decay electrons. We
obtained the branching fraction, , with minimal model dependence.
From this measurement, we derive a value for the Cabibbo-Kobayashi-Maskawa
matrix element .Comment: 16 pages, 3 figures, 3 table
Direct Observations of Sigma Phase Formation in Duplex Stainless Steels using In Situ Synchrotron X-Ray Diffraction
The formation and growth of sigma phase in 2205 duplex stainless steel was observed and measured in real time using synchrotron radiation during 10 hr isothermal heat treatments at temperatures between 700 C and 850 C. Sigma formed in near-equilibrium quantities during the isothermal holds, starting from a microstructure which contained a balanced mixture of metastable ferrite and austenite. In situ synchrotron diffraction continuously monitored the transformation, and these results were compared to those predicted by thermodynamic calculations. Differences between the calculated and measured amounts of sigma, ferrite and austenite suggest that the thermodynamic calculations underpredict the sigma dissolution temperature by approximately 50 C. The data were further analyzed using a modified Johnson-Mehl-Avrami (JMA) approach to determine kinetic parameters for sigma formation over this temperature range. The initial JMA exponent, n, at low fractions of sigma was found to be approximately 7.0, however, towards the end of the transformation, n decreased to values of approximately 0.75. The change in the JMA exponent was attributed to a change in the transformation mechanism from discontinuous precipitation with increasing nucleation rate, to growth of the existing sigma phase after nucleation site saturation occurred. Because of this change in mechanism, it was not possible to determine reliable values for the activation energy and pre-exponential terms for the JMA equation. While cooling back to room temperature, the partial transformation of austenite resulted in a substantial increase in the ferrite content, but sigma retained its high temperature value to room temperature
Observation of Cabibbo-suppressed and W-exchange Lambda_c^+ baryon decays
We present measurements of the Cabibbo-suppressed decays Lambda_c^+ -->
Lambda0 K+ and Lambda_c^+ --> Sigma0 K+ (both first observations), Lambda_c^+
--> Sigma+ K+ pi- (seen with large statistics for the first time), Lambda_c^+
--> p K+ K- and Lambda_c^+ --> p phi (measured with improved accuracy).
Improved branching ratio measurements for the decays Lambda_c^+ --> Sigma+ K+
K- and Lambda_c^+ --> Sigma+ phi, which are attributed to W-exchange diagrams,
are shown. We also present the first evidence for Lambda_c^+ --> Xi(1690)^0 K+
and set an upper limit on the non-resonant decay Lambda_c^+ --> Sigma+ K+ K-.
This analysis was performed using 32.6 fb^{-1} of data collected by the Belle
detector at the asymmetric e+ e- collider KEKB.Comment: Submitted to Phys. Lett. B. v2: A small correction to the Authorlist
was made. An earlier version of this analysis was released as
BELLE-CONF-0130, hep-ex/010800
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