The strength, ductility, and forming ability of metallic alloys are all related to plastic deformation, which arises from the motion of dislocations. While dislocation processes are well understood, many important materials (including oxide glasses, metallic glasses, many polymers, and some semiconductors) are non-crystalline and undergo plastic deformation by mechanisms that do not involve dislocations. A better understanding of these mechanisms would aid the development of new materials with high strength and improved resistance to fracture and fatigue. This collaborative international research effort uses advanced experimental and computational techniques to characterize, quantify, and predict fundamental mechanisms of plastic deformation in amorphous materials. The experimental and computational portions of the programs are linked by a continuum non-equilibrium thermodynamic framework based on the shear transformation zone (STZ) theory of deformation of amorphous materials. To accomplish this, the effort brings together international experts in electron microscopy (W. Chen, Tohoku University, Japan), x-ray scattering (T. Hufnagel, Johns Hopkins U.), micromechanical testing (L. Greer, Cambridge U., UK), and computational materials science (M. Falk, Johns Hopkins U.). Structural characterization efforts focus on quantifying the effects of deformation on the nanoscale structure of amorphous materials; these include aberration-corrected transmission electron microscopy, fluctuation electron microscopy, and coherent x-ray scattering. The nanoscale mechanical properties are measured by instrumented nanoindentation and micropillar compression over a range of temperatures. In order to place the work in the broadest possible context, materials at two extremes of structure and behavior are studied: metallic glasses, with largely non-directional bonding and substantial ability to support plastic deformation; and amorphous silicon, with highly directional covalent bonding and limited plastic deformation. The experimental efforts are complemented by atomistic simulations and continuum modeling. The atomistic simulations allow detailed investigation of fundamental mechanisms of deformation and the accompanying structural evolution, and are critical for interpreting the experimental results. Continuum numerical solutions of well-posed boundary value problems are compared to carefully chosen experiments to test the validity of the STZ theory and place the experimental observations in the context of a predictive constitutive theory.
The effort advances the education and training of graduate students, undergraduate students, and post-doctoral scholars by integrating them into an international research team that includes researchers from the United States, United Kingdom, and Japan. A central feature of the effort is the opportunity for the junior members of the team to make extended visits to the other institutions to foster the collaboration and benefit from extended exposure to different cultures. The collaboration is also fostered by regular web conferences and annual meetings of the entire team. Another focus is the use of peer instruction techniques in undergraduate materials science and engineering education. Content appropriate for a sophomore-level Structure of Materials course is developed and the effectiveness of this teaching methodology is being evaluated. Results of the assessments will be disseminated via journal articles and conference presentations, and the teaching content produced will be made freely available for other instructors to use.
