Terminal solid solubility determinations in the H-Ti system

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 alpha-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 alpha/alpha + delta 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 sohrus enthalpy. This last value should correspond to the [alpha]/[alpha+gamma] equilibrium curve. Although it is possible this value might be influenced by the presence of tiny amounts of the now metastable delta phase-as its presence is revealed by Xray diffraction analysis anyway it is consistent with a alpha + delta <-> gamma peritectoid reaction temperature of 168 degrees C obtained from the literature. The eutectoid alpha+delta <-> beta 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 degrees C from the analysis of the DSC diagrams. The solubility limit [alpha]/[alpha+delta] 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 alpha-Ti as presently reported in the literature. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.