Kinetics of martensite decomposition in Ti–6Al–4V–xH alloys

The kinetics of martensite decomposition in hydrogenated Ti–6Al–4V alloy samples was studied. To produce the martensite structure, the samples containing 0, 10, 20 and 30 at.% H were annealed in the β phase field and water quenched. Optical microscopy, transmission electron microscopy, X-ray diffraction analysis and microhardness testing techniques were utilized to study the phases and phase transformations and to obtain time–temperature-transformation (TTT) diagrams for the martensite decomposition. The martensite structure of the hydrogenated samples consisted of a mixture of hexagonal close packed (hcp) α′ and orthorhombic α″ martensites. The amount of the orthorhombic α″ martensite increased from 0 to ∼80 vol.% when the hydrogen content of the alloy was increased from 0 to 30 at.%. During aging at temperatures below the β transus temperature and above the martensite start temperature (Ms), the martensite structure transformed into a mixture of hexagonal close-packed (hcp) α and body-centered-cubic (bcc) β phases. At aging temperatures below the Ms, on the other hand, the martensite first transformed partially into a metastable β phase, and then equilibrium α and β phases were formed. On quenching after aging, in both these cases, the β transformed into martensite plus residual β, with the amount of the latter increasing with an increase in the hydrogen concentration and a decrease in the aging temperature. Hydrogen additions lowered the Ms of the Ti–6Al–4V alloy, and for samples containing 30 at.% H the Ms was below 500 °C. In the alloys containing 20 and 30 at.% H, a hydride phase was also detected. Complete decomposition of the martensite structure in the samples containing 30 at.% H and aged at 530 °C resulted in a fine and homogenous equiaxed microstructure consisting of a mixture of α, β and hydride phases. The nose temperature for the start of the martensite decomposition decreased from 800 to 625 °C when the hydrogen concentration increased from 0 to 30 at.%. The nose time for the start of the martensite decomposition increased from 6 s to 10 min when the hydrogen concentration increased from 0 to 10 at.% and did not change significantly with further increase in the hydrogen concentration.