Atomic-scale Analysis of the Mechanical Behavior of Ultrathin Films of Face-centered Cubic Metals

Special seminar
Monday, Nov. 24 at 10:30am in 13-2137 (Von Hippel room)

Kedarnath Kolluri
University of Massachusetts, Amherst MA 01003

Nanometer-scale-thick metallic films are used increasingly in modern technologies, from microelectronics to various areas of nanofabrication. The mechanical behavior of these small-volume structures is very different from that of bulk metals. Processing of such ultrathin-film materials generates structural defects including voids and cracks and may induce structural transformations. Improvement of the reliability, functionality, and performance of nano-scale devices requires a fundamental understanding of the atomistic mechanisms that govern the response to mechanical loading in the thin films in order to establish links between their structural evolution and their mechanical behavior.
This presentation is focused on atomic-scale mechanisms of plastic deformation in freestanding ultrathin films of face-centered cubic (fcc) metals that are subjected to biaxial tensile strain, and is based on large-scale molecular-dynamics simulations. Elementary mechanisms of nucleation of dislocations are studied and several problems involving structural evolution of the thin films due to the glide of and interactions between dislocations are discussed. These problems include void nucleation, martensitic transformation, and the role of stacking faults in facilitating dislocation depletion in small-volume structures of fcc metals. Void nucleation is analyzed as a mechanism of strain relaxation in copper thin films. The glide of multiple dislocations causes shearing of atomic planes and leads to formation of surface pits, while vacancies are generated due to the glide of jogged dislocations. Coalescence of vacancy clusters with surface pits leads to formation of voids. In addition, a transformation of fcc to hexagonal-close packed copper films is studied. The resulting martensite phase nucleates at the surface and grows into the bulk of the film due to dislocation glide. The role of surface orientation in the strain relaxation of these biaxially tensile strained thin films is discussed and the stability of the fcc crystal is analyzed. Finally, the mechanical response during dynamical tensile loading of dislocation-rich thin films of fcc metals with varying propensity for the formation of stacking faults is analyzed. Dislocation-stacking-fault interactions play a significant role in the cross-slip and eventual annihilation of dislocations in fcc metals with low to medium values of stable-to-unstable stacking fault energy ratio, γs/γu. Stacking-fault-mediated mechanisms of dislocation depletion in these small-volume structures are explored.

Innovation in Materials

in connection with MIT Entrepreneurship Week
Nov 19 at 4.270:

2pm Dr. Desh Deshpande
Conviction, perseverance and team work required to commercialize an idea

2:30pm Professor Gene Fitzgerald 
Innovation in Materials and Paths to Commercialization

3:00pm: Panel discussion with Dr. Deshpande, Prof. Fitzgerald, Professor Angela Belcher, and Dr. Dave Danielson; led by Professor Ned Thomas.

From Fiber Optic Surgical Scalpels to Fabrics That See: How Materials Scientists are Shaping the Future

Prof. Fink has been announced as the Fall 2008 Wulff Lecturer, on Monday, Nov. 17, at 4:30, in 10-250. Prof. Fink’s research in photonics led to the development of the “BeamPath fiber system,” a means of delivering CO₂ laser energy to a surgical site, a site that may be inaccessible through other conventional surgical methods.

The Wulff Lecture is named for Prof. John Wulff, a skilled, provocative, and entertaining teacher who inaugurated a new approach to teaching the popular freshman subject: 3.091 Introduction to Solid State Chemistry. Wulff Lecturers are asked to educate, inspire, and encourage MIT undergraduates in the field of materials science and engineering and related fields. The entire MIT community is invited, and freshmen and undesignated sophomores are particularly encouraged to attend.  A reception will follow the talk.

 Development and Testing of (M)EAM Potentials for Elements and Alloys

Bohumir Jelinek

Department of Physics and Astronomy

Mississippi State Univeristy

 Monday, Nov. 17 at 10:30am in 24-213

 To produce an alloy with desired properties, it is important to understand the effects of alloying elements and the phenomena involved in the alloy production process. Computational modeling and simulation provide cost effective methods of exploring interactions between individual alloying elements and studying the phenomena in alloy production.

An accurate modeling of the structure and properties of an atomic system can be performed by first-principle methods, possibly involving calculation of spatial electron density in the simulated system.  However, first-principle simulations of the phenomena of interest may require a number of atoms that exceeds the available computational resources.

An alternative to first-principle methods is to use semi-empirical interatomic potentials. The effective evaluation of such potentials can allow modeling of systems with more than a million atoms. This presentation will introduce the work on the construction and testing of interatomic potentials for metallic alloys done at the Center for Advanced Vehicular System, Mississippi State University.

EXPERIMENTAL ARCHAEOLOGY AND ROMAN HARBOR TECHNOLOGY:
Building a Roman Pila in Brindisi, Italy

A Lecture by

Robert Hohlfelder
Professor of History
University of Colorado, Boulder

Thursday, 20 November
The Chipman Room (6-104)
3-4 pm

Sponsored by The Center for Materials Research in Archaeology and Ethnology (CMRAE)