Molecular-Dynamics Simulations of Molten Ni-Based Superalloys

Event Date: April 21, 2014
Speaker: Dr. Christopher Woodward
Speaker Affiliation: Principal Materials Research Engineer, Air Force Research Laboratory, Wright Patterson Air Force Base
Sponsor: Purdue School of Materials Engineering
Type: 69 Seminar Series
Time: 3:30pm refreshments, 3:45pm seminar
Location: ARMS 1010

Fundamental parameters of liquid metal alloys are calculated using a first principles approach, based on Density Functional Theory, with the goal of informing models of defect formation during solidification processing. Ab-initio molecular dynamic simulations (AIMD) are applied to liquid metal alloys ranging from simple metals to commercial Ni-based superalloys in order to predict molar volumes (density), diffusion rates, viscosities, and local ordering. As proof of concept elemental Ni, Ni-5.4X, Ni-20X, Ni-10Al-2.8X (X= W, Re, and Ta) alloys (at%) were studied at 1750 and 1830 K. Calculated kinetics and the atomic distribution in the liquids indicate that simulations of 500 atoms run for approximately 7 ps converge these time-averaged properties, including molar volume. Overall diffusion rates and molar volumes are in good agreement with available experimental measurements, though the AIMD predictions appear to systematically underestimate thermal expansion. The method is then used to predict density inversions and viscosities in five alloys (Ni-14Al-3W, Rene-N4, Rene-N5, CMSX-4, and a model Ni-superalloy; SX-1) for temperatures and compositions expected in the mushy zone during directional solidification. Density inversion is predicted for some of these alloys and ranking of the effect is consistent with previous, albeit empirical, studies. A new dimensionless parameter for casting defects (e.g. freckles) is defined that incorporates all the (predicted) temperature dependent materials-properties. Strengths and weaknesses of this approach will be discussed.

SHORT BIO
Christopher Woodward is a Principal Materials Research Engineer at the Air Force Research Laboratory. Dr. Woodward earned a Masters in Physics in 1983 and Ph.D. in Physics in 1986 from the University of Illinois, Champaign Urbana. Since receiving his PhD in solid state physics Dr. Woodward has applied a variety of computational methods to study the materials science of structural metal alloys. Before joining the AFRL in 2008 he held positions at the AERA at Harwell (UK), Argonne National Laboratory, Michigan Technological University, Northwestern University, and at private research firms. His areas of research include computational materials science, effects of chemistry on alloy thermodynamics and plasticity, electronic structure of complex systems, novel boundary condition methods, dislocation dynamics, and high-performance computing. He has authored and co-authored more than 100 peer-reviewed publications and has given more than 60 invited presentations at international conferences and workshops. Dr. Woodward is currently chair of the TMS committee on Computational Materials Science and Engineering.