Prof. William A. Curtin, Jr.
École Polytechnique Fédérale de Lausanne
A Screwy Theory on the Edge: Origins of High-Temperature Strength Retention in BCC High Entropy Alloys
The refractory BCC High Entropy Alloys (HEAs) in the class of Mo-Nb-Ta-V-W are very strong (1-1.5 GPa) at room temperature and have exceptional retained strengths of 400-500 MPa at 1600C. The mechanistic origin of these outstanding properties is not yet understood, in spite of extensive experimental studies of these and related alloys. Strengthening in BCC metals and alloys has long been understood, and observed experimentally, to be entirely controlled by the motion of screw dislocations. Here, we present a parameter-free theory for the strength versus temperature in this class of BCC HEAs based on the motion of edge dislocations through the random energy landscape created by the solute fluctuations. The heresy of claiming that motion of the edge dislocation is supported by a careful study of the literature in complex BCC alloys and by predictions for both screw and edge motion in complex alloys. Application of the edge theory is shown to be in excellent agreement with results from direct large-scale atomistic simulations at T=0K for a range of alloys. Moreover, very good agreement with experiments is achieved for the same alloys at T=0C, and for the two alloys MoNbTaW and MoNbTaVW that have been studied up T=1600C. The fundamental origins of the high retained strength are achieved because of large barriers to edge motion. The edge theory then enables computationally-guided design of new alloy compositions aiming for the highest retained strengths and strength-to-weight ratios in this family of alloys, and several new compositions are proposed.
BIO: Professor Curtin earned a 4 yr. ScB/ScM degree in Physics from Brown University in 1981 and a PhD in theoretical physics from Cornell University in 1986. He worked as staff researcher at British Petroleum until 1993 and then joined Virginia Tech as an Associate Professor in both Engineering Mechanics and Materials Science. In 1998 he returned to Brown as Full Professor of Engineering in the Solid Mechanics group, where he was appointed Elisha Benjamin Andrews Professor in 2006. He joined Ecole Polytechnique Federale de Lausanne as the Director of the Institute of Mechanical Engineering in 2011 and as Full Professor in 2012. His research successes include predictive theories of optical properties of nanoparticles, statistical mechanics of freezing, hydrogen storage in amorphous metals, strength and toughness of fiber composites, dynamic strain aging and ductility in lightweight Al and Mg metal alloys, solute strengthening of metal alloys including high entropy alloys, and hydrogen embrittlement of metals, along with innovative multiscale modeling methods to tackle many of these problems. Professor Curtin was a Guggenheim Fellow in 2005-06, was Editor-in-Chief of “Modeling and Simulation in Materials Science and Engineering” from 2006-2016, has published over 275 journal papers that have received over 11000 citations with h-index of 57 (Scopus), and has been the Principal Investigator on over $35M of funded research.