An electron microprobe analysis of solute segregation near grain boundaries in an Al-Zn-Mg alloy

The concentration of zinc and magnesium across grain boundaries was measured by means of an electron probe microanalyzer for an Ag-Zn-Mg alloy after different quenching (brine, water, oil, and air) and aging heat treatments (room temperature, 165°, and 200°C). Significant solute segregation was detected in quenched specimens and also in specimens that were aged at room temperature. While no solute enrichment was measured in specimens that were aged at elevated temperatures, solute depletion was observed in a considerable proportion of the examined boundaries. It was concluded that solute segregation to grain boundaries occurred during quenching and was relieved during aging. C. R. SHASTRY, Formerly Graduate Research Assistant at Rens-selaer Polytechnic Institute, Troy, N. Y.     

A study of grain boundary precipitate-free zone formation in an Al−Zn−Mg alloy

The effect of quench rate and boundary type on the width of precipitate-free zones in an Al-6.86 wt pct Zn-2.35 wt pct Mg alloy that was aged at two different temperatures was studied by transmission electron microscopy. The width of the precipitate-free zones increased with decreasing quench rate, but by a considerably smaller extent than that predicted from calculations using previous models based upon quenched vacancy profiles. The simple tilt-type boundaries were associated, in general, with narrower precipitate-free zones than those occurring at complex-type boundaries. The width of precipitate-free zones was also observed to decrease with increasing precipitate coverage in the boundary. A mechanism of precipitate-free zone formation based on the modification of the as-quenched vacancy profile by the vacancies generated due to grain boundary precipitation was proposed to explain the observed results. C. R. SHASTRY, formerly Graduate Student, Materials Division, Rensselaer Polytechnic Institute, Troy. N. Y.     

Slow relaxation in spin glasses

The remarkable remanence effects observed in spin glasses are discussed. Some theoretical approaches and results are reviewed. Invited talk given at the Indian Academy of Sciences, Bangalore, Academy discussion meeting on phase transitions, June 21st, 1978.     

Studies on stress concentration in tensile strips with large circular holes

The stress concentration factor K in a tensile strip with large circular hole is accurately determined using a high precision, plane stress, triangular element. The values of a/w where a is the diameter of the hole and w is the width of the strip, considered range from 0.99 to 0.999. The values of K obtained found to be very near to but greater than 2.0, when linear analysis is performed. Some discrepancies in the stress values at the outer free edge observed in the present linear analysis compared to the well documented experimental values prompted the authors to go for geometrically nonlinear analysis gave a K value which is a function of the load applied, less than 2.0 and for sufficiently large loads this value is very near but slightly greater than 1.0.     

Effect of Heat Treatment on Solute Concentration at Grain Boundaries in 7075 Aluminum Alloy

The effect of heat treatment on the relative amounts of magnesium, zinc, copper, iron and silicon in grain boundaries of aluminum 7075 alloy was investigated with Auger electron spectroscopy in a scanning Auger microprobe. It was observed that both aging and solution treatments affect the distribution of these elements at grain boundaries. If identical solution heat treatments were employed, the aging treatment determined the solute distribution at the grain boundaries, and for fixed final aging treatment, the temperature of solution anneal determined the solute distribution. Among specimens given a T6 final aging treatment (24 h at 394 K (250 °F)) with prior solution treatment temperatures in the 666 K (740 °F) to 800 K (980 °F) range, 711 K (820 °F) solution treated specimen was found to have the least solute concentration at grain boundaries. The observed behavior is explained on the basis of equilibrium and nonequilibrium segregation of solute elements to grain boundaries occurring on quenching and during aging.     

Distribution of aluminum in hot-dip galvanized coatings

Hot-dip galvanized panels of low-carbon (LC) and interstitial-free (IF) steels were produced in a laboratory simulator with an average coating mass of 60 g/m². Three pot aluminum levels were used, viz., 0.10 pct (by wt), 0.15 pct, and 0.18 pct. Metallography, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to characterize coating and base steel microstructures. Wet chemical analysis and scanning transmission electron microscopy (STEM) were employed for compositional analyses. The aluminum content of the melt was found to be the predominant factor influencing the distribution of Al in the coating. At 0.18 pct melt aluminum, Al is partitioned between the aluminide inhibition layer at the coating-steel interface (∼80 pct) and the zinc overlay (∼20 pct). At 0.15 pct, it is partitioned among the aluminide layer (∼75 pct to 80 pct), zinc-iron (FeZn₁₃, ζ) intermetallic layer (∼5 pct to 15 pct), and the coating overlay (∼10 pct). At 0.10 pct, the aluminum is divided almost equally between the overlay and the zinc-iron intermetallics. At the two lower aluminum levels is the distribution marginally influenced by the steel grade. The ζ was found to not preferentially nucleate at the ferrite grain boundaries. When both the aluminide and ζ occurred at the coating-steel interface, the ζ particles appeared near discontinuities and thinner regions in the aluminide layer. The coating, relative to the melt, is enriched in aluminum because of its concentration in the aluminide and in the zinc-iron intermetallics. This enrichment increases with melt aluminum through an increase in the aluminum content of the aluminide layer and not of its thickness. In addition, a few tens-of-nanometers-thick layer enriched in aluminum, oxygen, and iron is observed on the outer surface of all coatings. The aluminum content in this layer also increases with an increase in the melt aluminum, but it contributes negligibly to the coatings’s content because of its extreme thinness.     

Comments on `The impact of NRZ data asymmetry on the performance ofa space telemetry system' by T.M. Nguyen

The commenters point out an error in the continuous power density spectrum given by the terms indicated in the above-titled paper (see ibid., vol.33, no.4, p.343-50, 1991). The continuous component of the spectrum computed directly from an equation given in the paper is shown and differs substantially from that shown in the original figure. Some repercussions as regards other results in the paper are indicated