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The kinetics of the reactive diffusion between solid Fe and liquid Sn was experimentally examined using Fe/Sn diffusion couples. The diffusion couples were prepared by an isothermal bonding technique and then immediately annealed in the temperature range of T = 703773 K for various times up to t = 90 ks (25 h). During annealing, a compound layer of FeSn 2 is formed at the initial Fe/Sn interface in the diffusion couple and grows mainly into the liquid Sn specimen. At T = 703 K, the compound layer indicates a needle-like microstructure in the early stages but a columnar microstructure in the late stages. On the other hand, at T = 723773 K, only the columnar microstructure is realized in the compound layer within the experimental annealing times. The mean thickness of the compound layer is proportional to a power function of the annealing time. The exponent n of the power function is mostly close to 0.6. In the early stages at T = 703 K, however, n is equal to unity within experimental uncertainty. If growth of a compound layer with a uniform thickness is controlled by volume diffusion, n is equivalent to 0.5. In contrast, n is equal to unity for longitudinal growth of a needle-like grain, even though volume diffusion is the rate-controlling process. This is the case for the layer growth in the early stages at T = 703 K. Since independent longitudinal growth of each columnar grain contributes to the layer growth governed by volume diffusion, n is slightly greater than 0.5 for the compound layer with the columnar microstructure. Consequently, the discontinuous variation of n corresponds to the transition from the needle-like microstructure to the columnar microstructure.

The effects of Fe, W and Mo on the kinetics of discontinuous precipitation in the NiCr system were experimentally examined using a binary Ni38Cr alloy and ternary Ni38Cr0.9Fe, Ni38Cr3.2W and Ni38Cr1.7Mo alloys. These alloys were homogenized at 1423 K for 3 h, solution treated at 1423 K for 1 h, and then isothermally annealed in the temperature range of 8731023 K for various times up to 2300 h. Due to the solution heat treatment, all the alloys show the polycrystalline single-phase microstructure of the Ni-rich solid-solution (£) phase with the face-centered cubic structure. During isothermal annealing, however, the cell of the lamellar microstructure consisting of the £ phase and the Cr-rich solid-solution (¡) phase with the body-centered cubic structure is formed along the grain boundary of the £ matrix and then grows into the £ matrix. At each annealing temperature, the migration distance of the moving cell boundary is proportional to the annealing time. Hence, the growth rate of the cell is constant independent of the annealing time. The growth rate and the interlamellar spacing of the cell almost monotonically increase with increasing annealing temperature at 8731023 K. The cell growth is decelerated by W and Mo but not by Fe. The deceleration is more remarkable for Mo than for W. A kinetic model for the binary discontinuous precipitation controlled by boundary diffusion was used to analyze quantitatively the experimental result. According to the analysis, W and Mo retard the boundary diffusion of Cr along the moving cell boundary, but Fe slightly hastens that. Such influence of the alloying component on the boundary diffusion is ascribable to the thermodynamic interaction between Cr and the alloying component in the moving cell boundary. As a consequence, it is concluded that the retardation of the Cr boundary diffusion causes the deceleration of the cell growth.

Mg2ࢪ -Έ၁ࢪǨᲡȪɳɧɒḠȡ᧸ǓǮəɳɁƑҽᠭệǺȗȚ ȽɞɧȷɵɻȪɳɧɒɃɱɧɋȷɁǽΰ⓯ ‫-‬ᨀଛগɿᮧᗱʶଉ ɿ୮‫ۑ‬ሳᘢɿᑴᨀ◯ʶɿ‫ل‬࡞ছЈఎগકకકȤᢼకકᲇࢫ380-8553 ϗʅ᭔ϗʅల⇙ʃ 4-17-1 ଉ ϗʅ᭔కᏡཪ⒈ἕ۰Ƀɻɇʀࢫ380-0928 ϗʅ᭔ϗʅల⇙ʃ 1-18-1Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering PECS using Mg 2ࢪ -doped transition alumina powders at 1200-1350c C under a uniaxial pressure of 40 or 80 MPa. The Mg 2ࢪ -doped transition alumina powders 0-0.50 massಚ MgO base were prepared through a new sol-gel route using high-purity polyhydroxoaluminum PHA and MgCl 2 solutions as starting materials. The composite gels obtained were calcined at 900c C and ground by planetary ball-milling. Upon heating, the composite gels transformed into a single-phase g-alumina or mixed phase of g-and x-aluminas, depending on the MgO content. The resultant transition alumina powders were solid solutions, in which Mg 2ࢪ cations were substituted into the crystal lattice. The powders were re-calcined to increase the content of a-alumina particles, which act as seeding for lowtemperature densification. Densification depended on the MgO content and loading pressure. The critical Mg 2ࢪ -doping for suppressing grain growth was found to be 0.10 massಚ MgO. Higher loading pressures led to full densification at lower temperatures, resulting in a more uniform and finer microstructure. Thus, dense alumina ceramics relative densityĽ99.6ಚ with a uniform microstructure composed of fine grains with an average size of 0.47 mm could be obtained by PECS at 1250c C under 80 MPa.

The effects of Co, Cu and V on the kinetics of discontinuous precipitation in the Ni-Cr system were experimentally examined using ternary Ni-39Cr-1.0Co, Ni-39Cr-1.2Cu and Ni-39Cr-0.89V alloys. The alloys were homogenized at 1 423 K for 3 h, solution treated at 1 423 K for 1 h, and then isothermally annealed in the temperature range of 873-1 023 K for various times up to 2 300 h. Due to the solution heat treatment, all the alloys show the polycrystalline single-phase microstructure of the Ni-rich solid-solution (γ ) phase with the face-centered cubic structure. During isothermal annealing, however, the cell of the lamellar microstructure composed of the γ phase and the Cr-rich solid-solution (α) phase with the bodycentered cubic structure is produced along the grain boundary of the γ matrix and then grows into the γ matrix. At each annealing temperature, the migration distance of the moving cell boundary increases in proportion to the annealing time. Thus, the growth rate of the cell is constant independent of the annealing time. The growth rate and the interlamellar spacing of the cell almost monotonically increase with increasing annealing temperature at 873-1 023 K. The cell growth is slightly accelerated by Cu but not by Co and V. However, the acceleration is less remarkable at 1 023 K. A kinetic model for the binary discontinuous precipitation controlled by boundary diffusion was used to analyze quantitatively the experimental result. In the moving cell boundary, the thermodynamic interaction is attractive between Cr and V but repulsive between Cr and Cu. Therefore, it is anticipated that the boundary diffusion of Cr along the moving cell boundary may be expedited by V but retarded by Cu. According to the analysis, however, the Cr boundary diffusion is hardly affected by Cu and V as well as Co. On the other hand, the boundary diffusion is lightly faster for Cu than for Co but slower for V than for Co. Consequently, for Cu and V, the thermodynamic and diffusional contributions to the Cr boundary diffusion are compensated each other.