College of Engineering News • Iowa State University

Iowa State professor and Ph.D. student explore new phase transformation mechanism

Researchers focused on the effect of elastic energy on nucleation.

Valery Levitas
Valery Levitas

Valery Levitas, Schafer Professor and faculty member of aerospace engineering and of mechanical engineering, continues to work on the solid-solid phase transformation via intermediate melt (IM) research he developed a decade ago.

His original work (Levitas et al. Phys. Review Letters, 2004, 92, 235702) has received significant attention, and he recently collaborated with Kasra Momeni, aerospace engineering Ph.D. student, and James Warren, technical program director for materials genomics at the National Institute of Standards and Technology (NIST), to uncover a new phase transformation mechanism that occurs via IM.

The group published the paper “The strong influence of internal stresses on the nucleation of a nanosized, deeply undercooled melt at a solid-solid phase interface,” in Nano Letters, a highly ranked interdisciplinary journal.

In their work, the researchers developed a phase-field approach and found unexpected effects of mechanics, or the energy of internal elastic stresses due to lattice misfit at solid-solid interface, on nucleation. They observed the appearance of a nanometer-sized IM at the interface between two solid phases when temperatures were at 120K below melting temperature.

Levitas explains that internal stresses, in particular, allow for the IM to appear for solid-solid interface energies that are smaller than twice the energy of a solid-melt interface – a phenomenon that is thermodynamically impossible without mechanics.

He adds that even though this happens, the promoting effect of internal stresses on the thermodynamics and barrierless IM nucleation is still relatively modest. That is why the researchers weren’t expecting to see that the internal stresses significantly reduce the activation energy of the IM critical nucleus, which eventually led to satisfying a kinetic nucleation criterion and thermally activated melting at 120K below melting temperature.

The team says this reduction in energy of the critical nucleus, which was reduced as much as 16 times for HMX energetic crystals, can be traced to when the elastic energy makes a small change between energy of the ground state and the state within the critical nucleus.

Additionally, the researchers were able to clarify the mysterious behavior of why IM persists during solid-melt-solid interface propagation. Levitas explains that thermally activated resolidification is kinetically impossible, and that they were able to confirm that finding was consistent with experimental data for HMX.

“We expect similar effects to occur for other material systems where solid-solid phase transformations via IM take place, including electronic (Si and Ge), geological (ice, quartz, and coesite), pharmaceutical (Avandia, which is an important substance for diabetes treatment), ferroelectric (PbTiO3), colloidal and superhard (BN) materials,” Levitas added. “This developed method also has implications for grain-boundary melting, formation of interfacial and intergranular crystalline or amorphous phases (complexions) in ceramic and metallic systems, and developing corresponding interfacial phase diagrams.”