Plan to attend the Thermal Processing Technology Center Seminar on Friday, March 30 at 10 a.m. in the John T. Rettaliata Engineering Center, Room 122.  Ricardo B. Schwarz, MST Division, Los Alamos National Laboratory, will discuss “Atomistic Modeling of the Pd-H System.”

Hydrogen incorporates readily into the fcc Pd lattice, occupying interstitial octahedral-symmetry sites. Below approximately 570K, the Pd-H system decomposes into two fcc phases. Although this is perhaps the simplest solid solution one can study, several characteristics of this system are not yet understood. One of them is explaining why, within the coexistence region, the chemical potentials needed to load and unload the H from the Pd lattice at a constant T has hysteresis. Several theories have attributed the hysteresis to dissipative mechanisms such as the generation and annihilation of dislocations. These models were questioned by Schwarz and Khachaturyan [SK], who proposed instead that the hysteresis is caused by a macroscopic thermodynamic barrier generated by the coherency strains between the coexisting phases. We have used Molecular Dynamics and Monte Carlo atomistic simulations to study the thermodynamic properties of the Pd-H system over a range of compositions and temperatures. The simulations reproduce both our measurements and the SK model. Further, they provide simple explanations for the
observed hysteresis and its T-dependence.