The change in the hydrogen-trapping behavior of a TiC particle accompanying its coherent to incoherent interfacial-character transition in a 0.05C-0.20Ti-2.0Ni steel that was quenched and tempered in a partially protective argon atmosphere and in ultrahigh vacuum (UHV) has been studied by thermal desorption spectrometry (TDS). The results indicated that (semi)coherent TiC precipitates demonstrate distinctly different hydrogen-trapping features from that of incoherent TiC particles with respect to hydrogen capacity, interaction energy with hydrogen, locations available for hydrogen occupation, and the capability of hydrogen absorption from the environment. The broad (semi)coherent interface of the disc-shaped (semi)coherent TiC precipitate does not trap hydrogen during tempering in a partially protected argon atmosphere, but traps hydrogen during cathodic charging at room temperature. The semicoherent interface traps 1.3 atoms/nm 2 of hydrogen at the core of the misfit dislocation with short-time charging (1 hour), which is characterized by a desorption activation energy of 55.8 kJ/mol. The side interface of the (semi)coherent TiC precipitate acts like the broad interface when the precipitate is small. As the precipitate grows, the side interface gradually loses its coherency and results in a simultaneous increase in the trapping activation energy and the binding energy. An increase in the trapping activation energy, i.e., the energy barrier for trapping, makes hydrogen trapping more difficult in cathodic charging at room temperature, while an increase in the binding energy enhances the capability of hydrogen absorption from the atmosphere during heat treatment. An incoherent TiC particle is not able to trap hydrogen during cathodic charging at room temperature due to its high energy barrier for trapping, but absorbs hydrogen during heat treatment at high temperatures. The amount of hydrogen that is trapped by incoherent TiC particles depends on their volume, which strongly indicates that incoherent TiC particles trap hydrogen within them rather than at the particle/matrix interface. Octahedral carbon vacancies are supposedly the hydrogen trap sites in incoherent TiC particles.

Critical assessment of the existing models for the desorption rate of hydrogen trapped in steel indicated that the desorption rate can be described by the kinetic formula dX/dt ϭ A(1 Ϫ X) exp (ϪE d /RT). Good fit of the formula has been found to the hydrogen released during thermal-desorption spectrometry (TDS) analysis from the coherent and incoherent TiC particles in 0.05C-0.22Ti-2.0Ni and 0.42C-0.30Ti steels. The activation energy (E d ) and the constant parameter A can be determined uniquely with high accuracy by a single spectrum simulation. The activation energy for hydrogen desorption from the incoherent TiC particle in the well-tempered 0.05C-0.22Ti-2.0Ni steel is 85.7 kJ/mol. In the 0.42C-0.30Ti steel, a higher activation energy of 116 kJ/mol was obtained for the coarse incoherent TiC when tempered at 650 °C and 700 °C. The activation energy decreased from 116 kJ/mol at 650 °C to 68 kJ/mol at 500°C. The nanosized TiC coherent precipitates in the 0.42C-0.30Ti steel were found to have an activation energy ranging from 46 to 59 kJ/mol, depending on the tempering temperature. A low value of much less than 10 4 s Ϫ1 was obtained for the constant parameter A for most cases, which suggested that the retrapping of the released hydrogen is not important in the thermal-desorption analysis.

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