MMC Lecture on
Atomic and Magnetic Ordering in Alloys

Professor David E. Laughlin
Materials Science and Engineering Department,
Electrical and Computer Engineering Department, and
Data Storage Systems Center
Carnegie Mellon University

Date: Friday, October 24, 2008
Time: 9:00 - 12:00 (w/ 15 min intermission)
Place: 7th floor small seminar room

Some of the most interesting and important phase transformations are the ones in which the configurational entropy of crystalline materials decrease. Such transformations are called Disorder-Order transformations. In these transformations the high temperature high entropy phase (the disordered phase) changes to a low temperature lower entropy phase (the ordered one). In the mid 1930s Landau proposed that in all such phase transformations have thermodynamic parameter called the order parameter which tracks the progress of the transformation. The order parameter (and entropy) may change either discontinuously or continuously at the equilibrium transformation temperature, demarking the first order and higher order phase transformations first proposed by Ehrenfest. In these lecture the fundamental crystallographic and thermodynamic aspects of atomic ordering will be presented. The L10 structure of FePt and CoPt, as well as the L21 and C1b and structures (Huesler and half-Huesler) will be discussed. Both structural and translational domains which arise from ordering will be investigated. We will then look into the microstructures which arise from the atomic ordering as well as the various kinds of phase equilibria and phase diagrams which arise. Magnetic ordering will be briefly discussed. Finally we will make brief remarks about the effect of grain size and particle size on the mechanism of ordering.

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Professor David Laughlin is the ALCOA Professor of Physical Metallurgy at Carnegie Mellon University and a Fellow of ASM and TMS and has been the Editor of the Metallurgical and Materials Transactions since 1988. He has studied various phase transformations such as spinodal decomposition and atomic ordering processes in several alloys and compounds by detailed analysis of their microstructure as well as electron diffraction patterns. For the past twenty years he has focused on the investigation of the magnetic properties and microstructure of soft magnets (HITPERM), hard magnets (FePt and CoPt) and magnetic thin films for recording media.

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