Terminal solid solubility determinations in the H–Ti system

Abstract

The terminal solid solubility of hydrogen in titanium was measured by differential scanning calorimetry in the concentration range of 0.3–4.1 at.% which practically corresponds to the whole solubility range of hydrogen in α-Ti. The solvus enthalpy obtained in this range from the overall data set was 22.8 ± 0.5 kJ/molH. However, a more careful analysis of the experimental results shows that the solubility curve has two different behaviors as a function of concentration. In the high concentration range 1.4–4.1 at.% a solvus enthalpy of 29.0 ± 1.5 kJ/molH was obtained representing the α/α + δ equilibrium boundary. In the low concentration range, 0.3 at.% to 1.4 at.%, the slope was noticeably lower with 24.2 ± 1.5 kJ/molH for the solvus enthalpy. This last value should correspond to the [α]/[α + γ] equilibrium curve. Although it is possible this value might be influenced by the presence of tiny amounts of the now metastable δ phase–as its presence is revealed by X-ray diffraction analysis – anyway it is consistent with a α + δ ↔ γ peritectoid reaction temperature of 168 °C obtained from the literature. The eutectoid α + δ ↔ β decomposition temperature was determined using samples of high hydrogen contents, ranging from 9 to 11.0 at.%. This temperature was determined to be 319.9 ± 1 °C from the analysis of the DSC diagrams. The solubility limit [α]/[α + δ] at this eutectoid reaction was estimated to be 5.44 ± 0.27 at.%. The present results are believe to provide a closer approximation to the solubility values of H in α-Ti as presently reported in the literature.