Is there a critical size in nano grained metals for ductile to brittle transition?

摘要

Nanoscale metal films and electrodes are extensively used in today's micro and nano electronics as well as nano mechanical systems. These metal structures are usually polycrystalline in nature with nano scale grains connected to each other by grain boundaries. The small size offers large grain boundary to volume ratio that is likely to affect the metal properties significantly. Here, we discuss the role of grain size and boundaries in determining the mechanical behavior of metals, such as elasticity and yielding. We first develop a novel micro apparatus that allows uniaxial tensile testing of free standing nano scale metal specimens as thin as 30nm. There are two unique features of the apparatus: (1) The specimen and the sensors to measure forces and displacements are co-fabricated - a paradigm shift in materials testing. Co-fabrication avoids the problem of handling the small sample, and suppresses any misalignment errors by five orders of magnitude, and (2) Testing can be carried out in-situ in SEM (Scanning Electron Microscope) or TEM (Transmission Electron Microscope). The latter allows visualization of the micro structure at sub-nanometer resolution while the stress-strain response is measured. Thus, the evolution of microstructure can be correlated with the stress-strain response to reveal the mechanism of deformation at nano scale. Figure 1 shows the apparatus with an aluminum sample, 100nm thick, 10 μm wide and 250 μm long.