The thermodynamics and kinetics of the β → α < inf> m transformation in three Ti-X systems

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Department of Materials Science and Engineering


Continuous cooling experiments were used to study the β → αm transformation in three titanium base eutectoid systems: Ti-Ag, Ti-Au and Ti-Si. Using thermal arrest temperature vs cooling rate data, the T0 temperature and the enthalpy of transformation. ΔHTβ → αm, were determined for eight alloys in these sytems. Subsequent microstructural analysis yielded growth rates as a function of reaction temperature from which an estimation of the activation enthalpy for trans-interphase boundary diffusion, ΔHDb, was made. The phase boundaries of the eutectoid region for each system were re-evaluated and found to agree well with some previous investigations and to differ markedly with others. Values of ΔHTβ → αm range from -2470 ± 500 to -3270 ± 460 J/mol (-590 ± 120 to -790 ± 110 cal/mol). Most T0 temperatures agree well with that of the bisector of the (α + β) region. Values of ΔHDb vary from 50 to 93 kJ/mol (11.7-22.2 kcal/mol). The growth distance of αm in one second is calculated to be ca. 6 × 10-5 m at 7 K undercooling as compared to the equilibrium precipitate growth distance (when volume diffusion-controlled) of ca. 1 × 10-7 m at the same time and temperature. These results further confirm the identification of the massive transformation in the alloys investigated.

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Acta Metallurgica