Hydrogen partitioning in pure cast aluminum as determined by dynamic evolution rate measurements
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IT is well known that hydrogen can be found in pure aluminum in solid solution and in gas-filled pores resulting from a supersaturated state. Numerous studies have been conducted to determine the solubility and diffusion coefficients of hydrogen in solid and liquid aluminum and to determine the characteristics of the pores; 1-6however, little information exists on the fraction of gas partitioned in these states or on the kinetics of the gas evolution. The reason for this is primarily because the results of past studies are based on cumulative equilibrium measurements and not continuous real time H 2 evolution. Further, ultrahigh vacuum (UHV) conditions which would assist in the separation of bulk and surface effects and in minimizing numerous contaminating sources have not been used. The results presented here were generated by employing a dynamic technique under U H V conditions to measure the evolution of hydrogen from commercially available samples of polycrystalline pure aluminum and by then comparing these data with a statistical analysis of the porosity in the cast aluminum. R. A. OUTLAW is Research Scientist, NASA, Langley Research Center, Hampton, VA 23665. D. T. PETERSON and F. A. SCHMIDT, Metallurgist, are with 222 Metals Development, Ames Laboratory, Ames, IA 5001 I. Manuscript submitted October 6, 1980. METALLURGICAL TRANSACTIONS A
POROSITY IN CAST A L U M I N U M The samples were obtained from an Aluminum Company of America casting which was open-mold formed into a 1 l-kg pig with trapezoidal cross section (base = 17.8 cm, top = 10 cm, height = 9.5 cm) with the following impurities: 0.003 wt pct Si; 0.0015 wt pct Fe; and 0.003 wt pct Cu. The samples were taken from five different places in the casting, polished in the usual sequential way to 15-/zm silicon carbide paper and then ultimately to a final finish with 0.05-~m silica paste. Figure l(a) shows a representative sample magnified 20 times. Also shown are greater magnifications of observed pores (Figs. l(b), (c), (d)). The porosity observed ranged in size from an average dimension of less than 1 /xm to greater than 400 ~tm. The larger pores (Fig. l(c)) varied in geometrical shape from spherical to ellipsoidal (some having major axes as much as 10 times greater than the minor axes) and the smaller pores were quite dense and mostly spherical (Fig. l(d)). Figure 2 represents the porosity in cast vacuum remelted aluminum. The aluminum was heated to 1000 ~ for 72 h within a vacuum of 10-4 Pa ( ~ ' 1 0 -6 Torr). The samples were examined under the optical microscope at 320 times and an areal analysis conducted in order to determine pore volume fraction and size distribution. The resulting data were then processed according to the JohnsonSaltykov linear logarithmic classification method# This
U.S. GOVERNMENT WORK NOT PROTECTED BY U.S. COPYRIGHT
VOLUME 12A, OCTOBER 1981--1809
Fig. I--SEM micrographs of large porosity. Note in (a) the stain that occurred during polishing which may be due t o H 2 evolution from a newly opened pore. Also shown are two po
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