Relationship between Microstructure and Exfoliation Corrosion in Mg-14mass%Li-3mass%Al Cold-Rolled Alloy

Relationship between microstructure and exfoliation corrosion in Mg-14mass%Li-3mass%Al cold rolled material were investigated by controlling microstructure using heat treatment. The exfoliation corrosion was exhibited on the specimens with elongated microstructure by cold-rolling. Heat treatment at 200 C only recovery occurred. Recovery and recrystallization were caused by heat treatment at 300 C However, heat treated at 300 C decreased precipitate fraction. Corrosion rate of heat-treated at 200 C decreased by release the residual strain. However, heat treatment at 300 C has lower corrosion resistance than heat-treated at 200 C because it has less precipitate density than as-rolled. Exfoliation corrosion was shown in as-rolled and heat-treated at 200 C. In as-rolled exfoliation corrosion was shown more significant when reduction rate increase. In as-rolled, exfoliation corrosion was shown more significant than heat-treated at 200 C. After heat treatment to change the grain shape from elongated to equiatial, exfoliation was suppressed by the distribution of AlLi phase.

Vacancy Species Produced by Rapid Thermal Annealing of Silicon Wafers

Rapid thermal annealing (RTA) of Czochralski silicon wafers at around 1260°C installs a depth profile of some vacancy species. Subsequent oxygen precipitation in such wafers is vacancy-assisted. The data on RTA-installed vacancy profiles - and the corresponding precipitate density profiles - suggest that there is a slow-diffusing vacancy species (Vs) along with two fast-diffusing species: a Watkins vacancy (Vw) manifested in irradiation experiments and fast vacancy (Vf) responsible for the high-T vacancy contribution into self-diffusion. The Vs species are lost during cooling stage of RTA, and the loss seems to occur by conversion of Vs into Vf followed by a quick out-diffusion of Vf. A model based on this scenario provides a good fit to the reported profiles of oxide precipitate density in RTA wafers for different values of TRTA and different cooling rates.

Microstructure and Residual Stresses in Dissimilar Mg-Al-Zn-Alloy Single Overlap Laser Beam Welds

Microstructure, hardness and residual stresses of the laser beam overlap welds between AZ31B sheets and AZ31, AZ61 and AZ80 extruded profiles are investigated using microscopy and X-ray diffraction. The result of the investigations reveal that weld microstructure, the size of the HAZ, precipitate density and the maximum compressive residual stress values depend strongly on the Al content of the weld zone of two Mg-alloys.

Regular Dislocation Networks in Si. Part II: Luminescence

Regular dislocation networks formed as a result of the direct bonding of Cz-Si wafers with oxide remnants on the pre-bonding surfaces were investigated. Besides the dislocation network, oxide precipitates were detected at the bonding interface. The precipitate density across the network was ~5×1010 cm-2, except small irregularly distributed circular areas, several mm in diameter, where the density was remarkably lower (<5×108 cm-2). The dislocation network structure was not affected by the change in the precipitate density. Photoluminescence spectroscopy (PL) and light beam induced current (LBIC) mapping were applied for characterization of such dislocation networks. For the locations with high precipitate density, PL signal from dislocations and that from the band-to-band transitions were enhanced. On the other hand, the LBIC results indicated that oxide precipitates are active recombination centers and thus should suppress the observed radiative transitions. The controversy can be explained in the assumption that the D-band PL signal increases due to scattering of excitation light by the precipitates and due to related expansion of the excitation area of the dislocation network. The light reflection from the precipitate layer also enhances the detected band-to-band PL signal. The shape of PL spectra from the samples in the range of photon energies 0.75 – 1.15 eV was not influenced by the oxide precipitates.

Fatigue Crack Growth Behaviors of AISI Type 347 Nuclear Piping Material

The effect of the temperature and microstructure on the fatigue crack growth behaviors of two microstucturally different AISI type 347 stainless steel(SS)s are studied at reactor operating temperature conditions. From the results, it was observed that the fatigue crack growth rate(FCGR) of type 347 SS increased as the test temperature was increased. However, its temperature sensitivity was reduced when the ΔK was large. The measured FCGRs for both TP347-LP and TP347-HP at 25oC and 345oC were lower than those presented in the ASME code. Especially near the threshold region, the trend curve in ASME code overestimated the FCGR of type 347 SS when compared with the obtained data. Even though the effect of the microstructure on the macro-crack growth rate was not significant during stage 2, the differences between threshold values for the specimen with a high precipitate density and the one with a low precipitate density were observed. It is assumed that these differences in the precipitate density, which resulted in a difference in mean free path length of dislocation movement, are the source of the difference in ΔKth of type 347 steels.