Partial Fe-Ti alloy phase diagrams at high pressure
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Fig. 1—Iron concentration profiles as a function of Boltzmann parameter X/t1/2 in Fe/Ti react diffusion couples annealed at 1173 K and 2.3 GPa for 14.4, 28.8, and 57.6 ks.
Fig. 2—Titanium concentration profiles in Fe/Ti react diffusion couples annealed at 1373 K for 14.4 ks at 0, 2.3, and 2.7 GPa.
were converted to the concentrations of titanium and iron with the aid of prepared alloy standards. Figure 1 shows the diffusion profiles of iron concentrations in the diffusion couples annealed at 1173 K and 2.3 GPa for the various diffusion times of 14.4, 28.8, and 57.6 ks against the Boltzmann parameter, X/t1/2, where X is the distance from Matano interface and t the diffusion time. As a result of react diffusion, the b Ti, Fe2Ti, FeTi, g Fe, and a Fe phases appear in the react diffusion zones. All of the diffusion profiles are on the same curve against the Boltzmann parameter. This means that the diffusion phenomena are kept in partial equilibrium and the phase interface concentrations in the diffusion zones should be equal to the equilibrium concentrations in the Fe-Ti phase diagram. Therefore, the react diffusion couple method enables us to clarify equilibrium for constructing the phase diagrams. Namely, the phase diagrams can be established from the interface concentrations between the adjacent phases in the diffusion zone. Figures 2 and 3 show, as an example, the diffusion profiles of titanium concentrations in the iron-rich part of the diffusion couples annealed at 1373 K for 14.4 ks under 0, 2.3, and 2.7 GPa, and those annealed at 1323, 1273, and 1223 K for 28.8 ks under 2.7 GPa, respectively. From the interface concentrations, the iron-rich Fe-Ti phase diagrams are assessed as shown in Figures 4 through 6. As shown in Figures 2 and 3, the diffusion distance in the a Fe phase is VOLUME 30A, NOVEMBER 1999—3009
Fig. 3—Titanium concentration profiles in Fe/Ti react diffusion couples annealed at 1323, 1273, and 1223 K for 28.8 ks under 2.7 GPa.
Fig. 5—Iron-rich Fe-Ti phase diagram at 2.3 GPa.
Fig. 4—Iron-rich Fe-Ti phase diagram at 0 GPa.
long and the titanium concentration varies gently, while the diffusion distance in the g Fe region is quite short and its concentration gradients are considerably steep, because titanium diffuses fairly faster in the a Fe phase than the g phase. Therefore, the concentrations at the a/(a 1 g) and a/(a 1 Fe2Ti) are not distinct, because the diffusion distances in the g Fe region are about 4 mm, which is near the resolution area (about 1 or 2 mm in diameter) for the concentration measurement by electron probe microanalysis. Therefore, The Cg/(a 1 g)’s are drawn by the bars in the phase diagrams. Both interface concentrations increase with increasing pressures. As shown in Figure 3, the a and g Fe phases are observed in the diffusion zones and interface concentrations are almost equal at both 1324 and 1273 K under the same pressure of 2.7 GPa. However, it is noticed that the a Fe disappears in the diffusion zone at 1223 K and 2.7 GPa, and thereby, the g Fe region phase is adjace
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