A long-standing problem in materials science is that of predicting mechanical properties from knowing the collective behavior of individual defects and material microstructure. The objective of this project is to develop a physical understanding of plasticity mechanisms operating in single-crystalline, amorphous, and nano-crystalline solids, when material dimensions are reduced to the nanometer scale and deformation is homogeneous.

Our in-situ tension/compression experiments are performed in a one-of-a-kind instrument "SEMentor" (SEM + Nanoindenter + Mentor = SEMentor), whose main capabilities are currently being developed by the group. The stress-strain data obtained by the SEMentor, combined with post-mortem TEM microstructural characterization and modeling efforts present a powerful method for gaining a more thorough understanding of plasticity at the nano-scale. Eventually they will provide us with critical information for the development of constitutive laws that govern flow stress as a function of size through an experimentally-based physical understanding of the underlying phenomena.

Above: the SEMentor

Some specific on-going efforts of this project are:

1. Investigation of the impact of surfaces and interfaces on strength increase and on defect nucleation. The need for understanding the effect of free surfaces on defect behavior is furthered by their weakening, as the ideal surface strength has been reported to be lower than the ideal bulk strength.
   
   
2. Assessment of mechanical response of nano-pillars through uniaxial compression and tension experiments. The development of the proposed one-of-a-kind tensile capability will enhance the current understanding of mechanical behavior of materials at the nano-scale by providing information on their ductility, fracture and ultimate tensile strengths. It will also allow for a direct comparison between material strengths in tension vs. compression, providing for the first time the necessary experimental foundation for the tension/compression asymmetry, predicted computationally.
   
   
3. Post-deformation cross-sectional TEM analysis of nano-scale specimens, which will shed light on dislocation activity (crystals) and on possible nano-crystallization (bulk metallic glasses) during and after the deformation. While the SEMentor is not capable of capturing the motion of individual mobile dislocations, a specific dislocation network (or lack thereof) evolved as a result of applied deformation can be visualized and analyzed via TEM. 

We are currently focusing on single nano-crystals like gold, aluminum, and copper (FCC); molybdenum and tungsten (BCC); and Indium (tetragonal), as well as on nano-crystalline metals (Ni-W), and bulk metallic glasses (BMGs).