This award supports an ongoing international collaboration in materials modeling between researchers in the US and in Germany. The aim of this work is to model and understand the evolution of characteristic nanoclusters, so-called precipitates, in alloys based on first-principles input and large-scale simulations. The research group of S. Mueller in Erlangen, Germany, is noted for its interest in low-dimensional aspects of materials and developing a cluster expansion (CE) methodology for surfaces. The research group of G. Hart has experience in studying vacancy-ordering compounds, extensive CE methodology development, and a successful undergraduate research program. The project builds on the international collaboration between the PIs that started with their previous award, NSF-0244183 and DFG-MU1648/3. Under that project the PIs developed an evolutionary approach (i.e., genetic algorithm) for constructing model Hamiltonians; modeled the difficult one-dimensional and two-dimensional superstructures of the Cu-Pd system with remarkable accuracy; studied vacancy-ordering systems in transition-metal (TM) carbides and nitrides, finding a surprising connection to TM chalcogenides; studied refractory bcc compounds. The complementary strengths of the PIs respective groups create a synergy which has been essential to the success of the project so far. Additionally, the project and its collaboration have supported several graduate students and leveraged G. Hart's strong commitment to undergraduate research, providing opportunities for almost a dozen undergraduate students, including several participants from underrepresented groups.
Building on the first two years' work, the PIs will extend their methodology to large-scale modeling of precipitate evolution; modeling precipitate nucleation, growth, and ripening; modeling vacancy ordering. During this project, they will: 1. Develop further advanced kinetic and thermodynamic Monte Carlo codes and study precipitation in fcc-, bcc-, and hcp-based metal alloys. This includes an extension to non-Bravais crystals (see point 3.). 2. Extend existing genetic algorithm approaches for optimizing alloy configurations (e.g., ground state searches) to simulation cell-independent models that can explore larger search spaces. 3. Develop new codes built on an extended cluster expansion formalism and study non- Bravais crystal systems (e.g., hexagonal-close-packed systems), including an explicit treatment of long-range strain effects for the first time in these systems.
The broader impacts of the project include: 1. For the first time, a large range of important alloy types can be explored, not just cubic systems. The ability to treat hcp systems opens the door to Mg-based and Ti-based intermetallic systems and the large class of wurtzite-based III-V and II-IV semiconductor materials. 2. These extended methods and codes will be freely available to other researchers, including the genetic algorithms for model Hamiltonian construction and searching crystal structures. 3. Continue to provide undergraduate students, especially those from underrepresented groups, with research experience and one-to-one faculty mentoring. The US side of the collaboration has very successfully integrated undergraduate research experiences into the project and the collaboration. Students are involved long-term, from project inception to final publication.
This award is co-funded by the Europe program of the NSF Office of International Science and Engineering.
