Phase Transformation Pathways at Speed

Graeme J. Ackland^1*, Con Healy¹ , Long Zhao² and HongXiang Zong^1,2

¹ School of Physics and Astronomy, University of Edinburgh, EH9 3FD Scotland, UK.

² State Key Laboratory for Mechanical Behavior, Xi’an Jiaotong University, Xi’an, Shanxi 710049, China

ABSTRACT: Under extremely high pressure, materials can undergo phase transformations to a more stable phase with a lower Gibbs Free Energy. Phase transformations typically occur within picoseconds, so following the trajectories of the atoms is nearly impossible experimentally. However, molecular dynamics simulations give us a computational model of the process, giving atomic-level details which can be validated when that process replicates the experiment observables. When phase transformations are driven by a slow process, such as heating or cooling, it is generally assumed that the pathway between these phases is one of low energy: the atoms rearrange themselves via the lowest barrier between one structure and the next. However, when the transformation is a fast process, such as shock beyond the plastic limit, the energy available is not the constraining factor, and the transformation occurs via the fastest route available, typically a high entropy intermediate structure. In such cases, the observed phase sequence may bypass some phases altogether, if there is no convenient rapid pathway. We demonstrate this by simulations of two different materials under shock: zirconium and potassium. In zirconium, the thermodynamic sequence of phases is hcp – omega – bcc. The transition pathway from hcp to omega is complex, and under shock the omega phase is bypassed altogether, transforming direct from hcp-bcc. In potassium, the intermediate phase is a complicated “host-guest” structure whose crystallography is described in four dimensions. The low energy pathway to this phase is known, but we find that under shock the transformation only occurs once the fcc structure is unstable to amorphization. An exactly similar situation is observed with plasticity under high shear rates, such as machining. If dislocation production and movement is too slow, plasticity occurs by twinning: a fast but more energetically dissipative process. In titanium, dislocations are immobile, twins have high energy, and the melting point is high. Under extreme shear, no mechanism is fast enough to allow plastic fracture, and so titanium is notoriously hard to machine.  In simulations, fast enough shear rates can give plastic yield via shear melting, if the melting point is suitable lowered.  Alloying Ti with rare earth elements lowers the melting point facilitating this mechanism, and creating an easily-machinable titanium alloy.      

Keywords：shock; machining; phase transformations;