Mechanism of pressure-induced martensitic phase transformations: A molecular-dynamics study

Publisher Website
Google Scholar
Abstract
The pressure-induced martensitic transformation from bcc to hcp iron has been studied in great detail with synchrotron radiation and diamond-anvil cells. The mechanism of the transformation has been postulated to be associated with a sliding of parallel bcc (110) planes combined with a contraction along the [001] direction; the (110) planes becoming the close-packed planes of the hcp structure. The synchrotron experiments support this picture of the transformation. We carry out the computer experiment of pressure-loading a bcc crystal to induce this transformation using molecular dynamics. We present the detailed microscopic motion of the particles during the transformation and show that the mechanism mentioned above for the transformation is confirmed for two different potentials and for two different particle numbers. The reverse transformation shows hysteresis, as is experimentally observed. For the two potentials which give the transformation we find that the final structure is often a close-packed polytype. We never observed a fcc crystal as the final structure. For another potential the final state was a defective crystal with no obvious resemblance to a close-packed structure. Most of our simulations were started by applying pressure to a low-temperature bcc crystal. A study of the sliding of the bcc (110) planes relative to each other suggests that the hcp structure is the preferred structure at low enough temperatures in pressure-induced bcc-to-close-packed structural transformations. For both potentials that give successful transformations the final structure is hcp at low enough temperature. In the phase diagram for iron the hcp structure is the low-temperature structure obtained by compressing bcc iron.