Transformation Induced Toughening in Pure Nanocrystalline Aluminum
Sandeep Kumar (UC,Riverside), Aman Haque ()
Mechanics of Phase Transforming and Multifunctional Materials
Wed 10:45 - 12:15
CIT 219
Pure metals do not exhibit solid-solid phase transformation since they deform and fail far below the theoretically predicted stress levels, for phase transformation, exceeding hundreds of GPa. We propose that by controlling grain size and thickness, classical deformation modes can be suppressed to induce phase transformation in pure metal films at stresses few orders of magnitude lower than theoretical values. For the first time, we present in-situ transmission electron diffraction evidence of face-centered cubic (FCC) to hexagonal phase transformation in 99.99% pure nanocrystalline aluminum at room temperature and only 2.5 GPa of tensile stress. For 60 nm average grain size, the aluminum films did not show any appreciable diffusion-based processes such as grain growth, rotation and sliding. At the same time, the 200 nm thick specimens are thin enough for the dislocations to escape through the surface. With no active dislocation sources, in-situ microscopy did not show any dislocation-based deformation either. Facilitated by the absence of dislocation and diffusion based processes, the uniaxial nature of specimen loading results in phase transformation at stresses two orders of magnitude lower than that predicted for aluminum. We also observed that phase transformation can result in a very flaw tolerant material