Title: A Novel Computational Thermodynamics Framework with Intrinsic Chemical Short-Range Order
Abstract: Exploiting chemical short-range order (SRO) is a promising new avenue for manipulating the properties of alloys. However, existing computational thermodynamic modeling frameworks are not sufficient to understand and predict SRO in multicomponent (>3) alloys. CALPHAD is a leading method for modeling and calculations of phase equilibria in materials. However the prevailing solution model used in CALPHAD, the sublattice model, is an empirical mean-field model based on Bragg-Williams (ideal entropy of mixing) approximation. This makes CALPHAD inadequate for properly describing order-disorder transformations or chemical short-range order(SRO) in alloys, such as the Guinier-Preston zones or nanoscale clusters, which are critical for alloy mechanical properties. First-principles calculations of phase diagrams, using the cluster variation method (CVM) or cluster expansion method, can describe SRO but are generally limited to binary or ternary systems due to the large number of configuration variables.
Here we propose to develop a hybrid computational thermodynamics framework by marrying unique advantages from CVM and CALPHAD through incorporating chemical SRO into CALPHAD using a cluster-based solution model. The most important technique here is the Fowler-Yang-Li transform to decompose the cumbersome cluster probabilities in CVM into fewer site/point probabilities of the basis cluster, thereby considerably reducing the number of variables that must be minimized for multicomponent (>3) systems. Modern, efficient algorithms will be employed to minimize the non-linear cluster-based free energy functions.
This study aims to put more physics, primarily intrinsic SRO, into CALPHAD, while maintaining its practicality and efficiency. It will leverage statistical mechanics to yield a more physical description of configurational entropy and open the door to cluster-based CALPHAD database development. The configurational and non-configurational (vibrational, elastic) contributions to free energy will be modeled separately, gaining insights into their respective effects on phase stability. The interplay between magnetic SRO and chemical SRO for the phase transformation also has the potential to be depicted under the same formalism.
Sean Agnew, MSE, Chair
Bi-Cheng Zhou, Advisor
Leonid Zhigilei, MSE
Tao Sun, MSE
Eugene Kolomeisky, Physics
Mingda Li, MIT
Zoom link: https://virginia.zoom.us/j/6243385391
All interested persons are invited to attend.