ABOUT THE PRESENTER

Dr. Aaronson Dr. Aaronson is R. F. Mehl University Professor Emeritus in the Dept. of Materials Science and Engineering at Carnegie Mellon University. He is also a Visiting Professor in the School of Physics and Materials Engineering at Monash University and an Adjunct Professor in the Dept. of Materials Science and Engineering at the University of Virginia. He received his B.S., M.S. and Ph.D. degrees from Carnegie Institute of Technology (now Carnegie Mellon Univ.). His specialty is the morphology, mechanisms and kinetics of diffusional phase transformations. Special emphasis has been placed throughout his research on the migration of partly coherent interphase boundaries by the ledge mechanism. He has published about 340 papers. He is a Member of NAE, Fellow of TMS, Fellow of ASM and an Honorary Member of the Japan Institute of Metals. His awards include the C. H. Mathewson Gold Medal (TMS), Albert Sauveur Achievement Award (ASM), Institute of Metals Lecture and R. F. Mehl Medal (TMS) and the TMS Educator Award. Dr. Aaronson has been active on both TMS and ASM committees and boards (especially the ASM-TMS Phase Transformations Committee) and has been an organizer or co-organizer of 15 published symposia.

Date: Monday, March 15, 2004
Time: 8:30AM
Location: Charlotte Convention Center
Room: 208A

“Low Energy and Low Mobility Structures at Irrational Interphase Boundaries, and Compliance With Nucleation Theory”

Presented by:
Hubert I. Aaronson, Carnegie Mellon University

Sponsored by:
Electronic, Magnetic & Photonic Materials Division, Structural Materials Division, Jt. EMPMD/SMD - Alloy Phases Committee and Phase Transformation Committee (Jt. ASM – MSCTS)

About the topic:
Nucleation theory has led to predictions that partially coherent interfacial structures should develop during diffusional growth even at irrational planar boundaries (ALK, 1968). This prediction is now tested in alloy systems that appear to be successively less metallic in character. In both a massively transformed Ag-26% Al alloy and during precipitation at grain boundaries in Ti-Cr and Ni-Cr, nearly all interphase boundaries are indeed partially coherent. However in the massive transformations in near-TiAl and in MnAl(+2% C) alloys partially coherent structures are largely absent (Vasudevan et al, Sofa et al., 2002). Nie and Muddle (NM, 2002) and Howe, Reynolds and Vasudevan (HRV, 2002) reported a preference for edge-to-edge (Kelly and Zhang, 1999) rather than plane-to-plane matching of certain low index planes in both lattices at facets in a Ti-46.54% alloy. Recently Reynolds et al (2003) have shown with 3-D, NCDS modeling that planar facets whose highly irrational crystallography was accurately determined by NM and HRV correspond physically to dense parallel rows of atoms intermingled in no obvious sequence but perhaps displaceable only with Moire ledges, Li et al will report in this symposium planar ZrN:alpha Zr-N facets whose structures cannot be rationalized by edge-to-edge matching. This implies that other type(s) of irrational interfacial structure capable of impending growth remain to be discovered. Al:Xe(solid) interfaces, at which {111} Al and {100} Al facets are present despite a misfi t of ca. 50% (Howe), suggest shallow minima in the polar gamma-plot that affect only nucleation. This spectrum of observations indicates that a marked difference in the bond strength and character in the matrix and the product phases can cause the two phases to play unequal roles in determining the crystallography and structure of low energy and low mobility boundaries.