Transformation induced toughening and flaw tolerance in pure nanocrystalline aluminum

Pure bulk metals do not exhibit solid–solid phase transformation since they deform and fail far below the required stress levels for phase transformation, which exceeds hundreds of GPa. We propose that for certain grain size, thickness and notch geometry, classical deformation modes can be suppressed to induce phase transformation in pure metal films at stresses few orders of magnitude lower than the theoretical values. For the first time, we present in situ transmission electron diffraction evidence of face-centered cubic (FCC) to hexagonal ω 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 propose that phase transformation can result in a flaw tolerance in the material.

We demonstrate transformation in 99.99% pure aluminum from face centered cubic (left) to a hexagonal ω phase (right) at room temperature and only 2.5 GPa tensile stress. While there is no theoretical or experimental evidence of ω phase transformation in pure aluminum, theory suggests about 300 GPa stress for any solid–solid phase transformation in this material.Figure optionsDownload as PowerPoint slideHighlights
► We report face centered cubic to hexagonal omega phase transformation in pure aluminum.
► In contrast to theory (300 GPa), the transformation took place at 2.5 GPa stress.
► The transformation occurs if dislocation or diffusion based processes are suppressed.
► The transformation aids in flaw tolerance at the nanoscale.