Tensile failure by grain thinning in micromachined aluminum thin films

This article reports the results of microtensile tests and transmission electron microscopy (TEM) analyses of micron-scale free-standing aluminum thin films. We fabricated the free-standing aluminum beams using micromachining procedures and tested them in a piezo-actuator-driven test apparatus. Microtensile tests revealed that the mechanical characteristics of the free-standing beams are quite different from those of the bulk material. Some unique features of our free-standing Al beams are high yield strength, low ductility, and strain softening. While the high yield strength is attributed to the typically small grain size, the strain softening and the small ductility are explained by locally thinned grains found in TEM analyses. Further TEM analyses suggested that dislocations pass through the free surfaces in the interiors of some large grains and lead to strain softening, which, in turn, results in deformation concentration in such areas and hence localized thinning.