Study of strain softening behavior of Al–Al3Sc multilayers using microcompression testing

Multilayer thin films with bilayer thicknesses in the nanometer range have been reported to have very high strengths. A previous study has shown that Al–Al3Sc multilayers, with bilayer thicknesses as small as 6 nm, have hardnesses as high as ∼3 GPa as measured by sharp tip nanoindentation. In the present study, we have avoided some of the complications associated with sharp tip nanoindentation by directly measuring the yield strengths and strain hardening/softening properties of Al–Al3Sc multilayers using microcompression testing methods with a nanoindenter. The results show the expected trend of increasing yield strength with decreasing bilayer thickness, and compare favorably with estimates of the yield strengths based on sharp tip nanoindentation. During deformation, the Al–Al3Sc multilayer pillars with smaller bilayer spacings experience considerable strain softening, resulting in a “flat-top mushroom” shape after deformation. We have developed a numerical model to account for this inhomogeneous deformation behavior and to calculate stress–strain relationships during strain softening. A new transmission electron microscopy study of a deformed pillar shows that the softening is a result of destruction of the layered structure due to shearing and rotation.