Phase transformation analysis in titanium at nanosecond time resolution

Polycrystalline and mosaic bicrystalline titanium films were subjected to steep heating/cooling rate of 10(11)/10(8) K s(-1) by laser pulsing. The induced phase transformations were followed by imaging and diffraction with a dynamic transmission electron microscope on the time scale of nanoseconds. On heating the film up to near the melting point with a 6-ns laser pulse, the low-temperature hcp phase transformed to the high-temperature bcc phase, with a nucleation rate of 10(25) m(-3) s(-1) and a crystal-growth velocity of about 1000 m s(-1). Quenching of molten Ti first produced the bcc phase, which in turn transformed to the hcp phase within a few microseconds. Thus, hcp-bcc transformations occur in Ti, even at the above high thermal rates. They are martensitic and not diffusion limited as claimed for transformations at low thermal rates. (c) 2005 American Institute of Physics.